Aristo Prime Mover Motor Blocks in Detail


This is a LONG page, but there is a wealth of information, so if you are repairing a faulty motor block, you NEED to read this.

I started this page, like most of my pages, as notes to myself. I already had issues with the Aristo "Prime Mover" hardware. Then, I had a custom built Northern locomotive bashed from two Aristo-Craft pacific locos. (Courtesy Rex Ammerman) This loco had two Pacific motor blocks grafted so it had 4 axles for drivers.

The first time I ran it there was smoke coming from it! Not the stack, but the connecting rod and pin on the first driver!

I reasoned that there was a power pickup/conduction problem. I was not really familiar with how the entire system worked, and with all the previous issues brought up in forums, I decided to investigate and learn myself. Thus I began to understand the "inside story" on the Aristo-Craft modular drive system, which Aristo calls the "prime mover" drive system.

Thus this is both not only an explanation of how the system works, but also a real story of how I figured out what problems I had, and how to correct them. You will see that this page is written mostly in a narrative form.

Since that first exposure, I have expanded this section with experiences from other Aristo-Craft locos, like my E8's.

The basic problem was that I found in this specific case is that there is often no power pickup from several of the drivers in a loco. (Please remember my Northern is built from 4 modular gearboxes with a motor block created from two Aristo Pacific locos, i.e. a 4-8-4.)

Also, as you read this, you wll see that there are a number of issues that may need to be corrected or at least addressed even on new locos.

There are TWO main sections, power pickup and tracking, be sure to read both if you want a full understanding.

Quick links in this page:

Basic Design - the basic parts of how this works

Power Conduction - the complex path from the rail to the motor

2 axle motor blocks with the prime mover differences

Wheels used on the Prime Mover

Warning: red Loctite causes disassembly problems:

Before working on ANY Aristo-Craft motor block, be aware that quite often Aristo-Craft slathered the drivers, axles, and fixing screws with red Loctite, the stuff that is intended for permanent assemblies and NOT to be disassembled ever. The application of Loctite came about due to the ongoing problem of slipping wheels on axles. Irritating on diesels, and often fatal to steamer rod hardware and gearboxes.

Depending on a number of variables, the amount of Loctite and the difficulty to remove varies widely among Aristo locos with the prime mover design.

The worst problem is with the screw that holds the driver on. Some people have been successful just pulling the screws out, and it works sometimes, BUT when it does not work, often you snap the head of the screw off in the metal half axle and you are "screwed". Also, these screws are cheap soft steel, so stripping the Philips slots into an unusable conical depression is also a real possibility.

A good tip from Paul Burch: get new stainless steel screws, they are ordinary metric screws with round heads


First, of course, you need the best-fitting screwdriver you can find, one that locks into the Philips screw. I have about FIVE Philips in the same "size" so when I remove something critical, I can pick the one that fits best.

First, assess the situation, if you see red Loctite all over the screw and star washer, my advice is you don't want to try it without heating it.

No red stuff: put in the screwdriver, press firmly and give it a grunt. You should get it the first time.

But if you have red stuff all over, the best way to remove the screw is to use a micro torch, with a small pointed flame, and just concentrated on the head of the screw. 30 seconds on, 10 seconds off, 30 seconds on again will do the trick, be sure to remove the screw or bolt right afterwards, before it cools down again, once it starts moving, complete the action.

To repeat: that red loctite will re-harden right away after the removal of heat, work quickly.

Several people have reported heating a Philips driver and then inserting it into the screw. I tried this several times, even heating the tip of the screwdriver to red hot, and my findings were that the driver did not conduct the heat very well or quickly to the screw, and I tried several that fit the screw very closely. (besides destroying the temper of the screwdriver).

Below is a picture of the Bernz-O-Matic ST500 3 in 1 micro torch, which makes the fine point flame to heat the screw head.

Bernzomatic ST500 micro torch

More on disassembly BEFORE YOU START:

The details of the design and construction are below, but there are a few more tricks to disassembly:

The main issue will be, (after removal of the motor block cover), the removal of the motors: you have slots on each side of the motor block where the motor brush tabs are soldered to "forks" in the housing, 2 slots each side since 2 motors (except GP-40).

The slots to get to the forks to desolder them are behind the middle wheel.

The best way is to try to remove the blob of solder with solder-wick, and then pull on the motor as you alternately touch the forks with the iron. If you are really successful with the solder wick, you might remove enough to break them free without applying the iron again.

I've used Ted's pictures below to illustrate the fork-tab soldering point, look at the second and third pictures below.

AristoMRL SD45locoRearTruckCenterAxleResolderedWasColdFactoryForkSolderJointComposite


Here is a picture of the motor block with the motors removed, you can see the 4 "forks" in the motor block in the center.

new top2

A picture at an angle to show the "forks":



Finally, you will see various rubber/silicone pads under the motors and gearboxes: Until you become an expert, leave them alone!


OK, on to the basic Design:


The motors have a hex drive which engages the gearboxes. Shown below is the newer (production 2001 and later) metal hex drive. Earlier versions had a plastic hex drive, which usually disintegrated due to wear.



The key component in all Aristo-Craft "Prime Mover" drive trains is a modular gearbox. The picture below shows one gearbox "module". You are looking at the hex drive "sockets" on one end, which actually goes completely through the gearbox. This is how power from the motor is transmitted.

To use multiple driving axles, you just "string" them together with a metal rod with a hex drive on each end. Often the connections between gearboxes have a flywheel as part of the hex drive shaft. The motor likewise has a hex drive on it.

In steam locos, there is a motor at one end, then a gearbox, then a flywheel, then another gearbox, etc.

In diesel locos, there is usually a motor between each gearbox module. One motor for a 2 axle truck, two motors for a 3 axle truck.

In the picture below, the gearbox has been opened. On top you see the nylon worm. It is supported by ball bearings at each end. It engages the nylon worm gear. The metal "half axles" fasten to the plastic worm gear. In the picture below the metal half axle is pointing directly at you. You can just barely see the 3 small screws that affix the axle to the worm gear. There is no play or slop in the bearings, so the alignment of the worm gear with the worm stays constant.

Note, this is a worn gearbox, the worm and worm gear are damaged.

(A bit of terminology: the long skinny gear is called a "worm" not a "worm gear". Why? Because the larger round gear on the axle is called the "worm gear".

Another feature of the Aristo gearboxes is the ability of the worm gear to move side to side in the gearbox, thus allowing side to side motion in the drivers while still transmitting power.

In the picture below, you can see the width of the worm gear. It is allowed to slide side to side in the gearbox, to help the loco around tight curves. The extra width of the worm gear ensures that the worm and worm gear stay meshed. Again this is a damaged gear.


The half axles are screwed to a central plastic drive gear, thus the metal axles are insulated from each other. The axles have tapered ends, where the drive wheels are attached. Quartering of steam loco drivers by simple alignment, there are no keyways on the shafts. The gearboxes are assembled in the motor block, and then the drivers added.

Again, this is damaged, the end of this axle is bent. They will bend if you drop the loco on the ground. This is a dead givaway.

The 3 small screws are a weak point in the design, the screws on each side use the same holes, so they cannot be very long, and with the small size and fine threads in plastic, they can be pulled out from as little pressure as running through tight flangeways.

Most drive wheels have a recess in the center, "outboard" of the taper.This is to hold a "star" type lockwasher. A screw goes through this washer and goes into the axle. The screw bears on the washer which bears on the recess in the wheel. The threads pull the tapered end of the axle into the taper in the wheel. The integrity of this connection relies on the taper fit, kept tight by the screw.

By inspection, it appears the tapers on the ends of the half axles are cast, not machined. More on this later.

Overall the construction of the gearbox is very robust, there are 4 screws holding the gearbox halves together. The fit of the halves is excellent, I detected no seepage of grease or oil at the seams on any gearbox. The gears are likewise large and appear very "tough". From conversations and observation of many posts on several forums, it's clear they can handle very heavy loads with no failures.

There is a patent number on the motor blocks, but the motor block is not what is patented, patent D437,334S, but the patent is a "decorative patent" on the appearance of the gearbox only. I guess you cannot patent the gears or the bearings. I actually think there are some clever bits to the design that would be patentable, but basically you cannot make a gearbox that looks similar.

Power conduction details:

The conduction of power from the wheel treads to the main circuit board is clever, but involved and has many places that often fail.

Basically, power is picked up from the wheel treads, goes to the axles, then to a spring loaded ball bearing that rides on the axle, then to a springy metal clip bolted to the gearbox, and then to straps inside the motor block, then to some wires and then to a connector on the motor block, and from there to the internals of the locomotive.

My explanation will proceed in the way I investigated the power pickup failure in my Northern, it was quite interesting and educational!

So I will start from the "outside" in, since to get to the answer I had to completely tear the motor block apart!

Power connections to the motor block - Steam locomotives:

There is a 4 pin socket at the end of the steam loco motor blocks. The picture below shows this, it is the white object on the extreme left end:

2 of the pins go to the motor brushes. The other 2 pins go to two wires that run down the long grooves of the motor block.

 The picture below is a closer shot, you can see the two wires, and near the 2 screw holes, a small piece of metal just touching wire:

I thought that this small contact area could have been my problem, but a test with the ohmmeter said the connection was OK, so I looked further>

 These small fingers are actually part of metal straps inside the motor block, the ones in the shallow u-shaped notches:

In the picture above, you can see two straps on each side of the motor block interior, one on the near edge an one on the far side. The picture above is a steam locomotive, but this arrangement is the same for diesels.

 Power connections to the motor block - 6 Axle Diesel locomotives:

In the latest versions, power from the locomotive comes to a circuit board in the underside of the truck:

 The lands on the circuit board engage (or should engage!) the contacts on the top center of the motor block:

3 axle prime mover

3 axle prime mover 2

In the picture above, there are two small rectangular pieces of metal in the center, with two "fingers" each, those are to the motors.

Outboard of these two small rectangular pieces, running almost the length of the motor block on te edges, are two long strips of metal, and each has two small square "tabs". These are connected to the wheels, i.e. track power pickup. These connect to the straps inside the motor block, just like the steam locos. Again, those internal straps connect to the spring power clips on each gearbox assembly.

Power connections to the motor block - 4 Axle Diesel locomotives with the "prime mover":

The GP40 is the only 2 axle motor block that uses the prime mover. There is a circuit board that has contacts that go to the power pickups, but the motor is soldered to the board, and the board has a 4 pin connector. The picture below shows he circuit board and the 4 pin connector that goes into the loco.

The picture below shows the circuit board loosened. You can see the 2 wires that the circuit board rests upon. Not an impressive connection here, no spring contacts or anything. You can make out the ends of the straps that go down into the motor block to contact the gearboxes.

Another view, where you can see the parts of the circuit board that contact the wires.

Power conduction from the motor block to the gearboxes:

OK, so far so good. We have power from whatever touches these straps inside the motor block to the locomotive electronics, and have shown the methods for the steamers, the 3 axle diesels and the unique GP40 2 axle block.

So, now how does the path work from wheels to these straps?

Well, those straps inside must be the key to review:


So there must be something on the gearboxes, and there is, a clip with a springy bit that touches the straps inside the motor block:


In the picture above you see a metal "clip" on the side of the gearbox. Look at the part below the axle, and you will see a rounded "spring" area. That is the part that contacts the strap in the motor block.

OK, as I was tracing this in my Northern, still nothing looked wrong. I saw several areas were problems could occur, but the conduction (tested with an ohmmeter) was fine.

At this point, I was scratching my head. I knew power was not being picked up from several drivers, but nothing was obvious yet.

At this point, I figured the power went from the wheel tread, to the axle to the bearing to this clip and we are done. Nothing makes sense yet, all of this looks pretty solid.

So, further disassembly was necessary.  Let's remove the clip:


Well, this is what I expected to see. A metal ball bearing on the axle shaft. So why the heck wasn't power flowing? The power went from the wheel to the axle, through the bearing, then the clip and to the loco. How the heck was this not working?

Wait! What is that little spring I see behind the ball bearing? What is that doing there? The ball bearing surely holds the axle in place.

 So I removed the ball bearing:

What in the heck is going on? There is no way this is there for kicks. It cannot be locating the axle, the ball bearing does this. It definitely looks like a power pickup of some type. But while it's clear that the ball rolling on the axle will be conducting, how the the power flow from that spring? And why do this at all?

Hmm, let's take another look at that clip I removed.

Wait! I missed that little "finger" up at top. Could that be contacting the spring? After looking at it, that's what is intended. But I was still wondering why this "extra" mechanism when you had the ball bearing conducting already. Or were you?

Quick, get out the ohmmeter and measure the resistance between the inner and outer races of the ball bearing on the axle.

OPEN CIRCUIT! The ball bearings on the axle does not conduct electricity!

So all the power pickup goes through that single ball on a spring and the finger on the clip MUST touch the spring. And this is where my unit failed. The little finger on the clip did NOT touch the spring, and no power pickup.

By the way, running heavy loads (current) through ball bearings is NEVER recommended by ball bearing manufacturers, the very thing that makes them low rolling resistance is what makes them liable to pit when moving current, the extremely small contact patch between the balls and the race.

In older designs, apparently Aristo-Craft did pick up power via ball bearings. Actually, Aristo-Craft "did the right thing" in using "hybrid"  bearings here, i.e. the races are metal, but the balls are made of ceramic. Ceramic is non-conductive, so no pitting from electricity, and also, they have even lower friction, because ceramic is harder and deforms less, thus lower rolling resistance. Therefore the ball bearings on the axles play NO part in power pickup. All the power from the rails is conducted through that single ball bearing riding on the axle, and the hit or miss touching of that spring with that little finger.

So, now I understood, and found where the power pickup failure was, but had to completely disassemble the loco to do it.

Final analysis and the answer to my power pickup problem:

So the conductivity path is:

  • wheel tread > wheel center
  • wheel center > axle
  • axle > single ball bearing riding on axle
  • single ball bearing to > spring
  • spring > "finger" on clip  (most common point of failure)
  • clip > strap in frame of motor block
  • strap > "bus" wire
    bus wire > circuit board
  • circuit board > 4 pin connector and thence on to the main circuit board

There are several weak points in the power conduction path, and any or all of them can contribute to a poor running loco.

So, in my case, the problem was the little "finger" not getting power from the spring. It is just angled inwards, in the HOPE it will touch a part of the spring. I checked the rest of the wheels, yup, the wheels that pick up power fine have that little finger touching the spring.

On the wheels where there is no power pickup, the little "finger" was not touching the spring. Push on the little finger a bit, whammo, conductivity. Note: BE GENTLE, a little at a time.

The problem is that the finger" comes in at an angle to the spring and depending on the spring's position, it may or may not touch. The contact area is very small.

So finally the mystery of the smoke coming from my siderod to crankpin was solved. The driver that was picking up power had no clip to pick up power in the inside of the motor block, because my Northern was bashed from a Pacific, and when Rex extended the motor block, he did not extend the power pickup bus wires. So the power went from the wheel to the gearbox, but could not be collected, so the current went to the crankpin, through the siderod that had oil and a very loose fit on the crankpin. The concentration of current through that very small and intermittent contact made enough heat to vaporize the oil!

Specific problems and how to mitigate

 The sections below will go over specific problems and how to solve or mitigate them

 Procedure: power pickup testing / troubleshooting

The areas of potential loss of power pickup have been extensively described above. Be sure to "wiggle" the axles around the axis of the motor block, and also slide the axle in and out (side to side) while checking. The best way is to use a precision ohmeter to test, i.e. be able to read the difference between .1 ohms and 8 ohms, for example. You should get darn close to zero ohms between any wheel and the corresponding "bus" wire (outer 2 pins in the power connector).

I read 15-50 ohms on half of the drivers on a new from the box Mikado provided by Aristo themselves brought to a show to "show me" that everything was fine.

If you do not have access to this type of meter, you can remove the side rods when testing.

The most common problems are the little finger on the metal retaining clip not touching the spring inside, and the metal straps not soldered to the bus wires as outlined above.


Problem: Poor power conduction to diesel motor block: 

There are 8 contacts on top of the diesel motor blocks, which engage a circuit board on the body.

See the picture below:

The first and most obvious problem is that the metal used for these "spring contacts" is NOT springy at all, but just cheap chrome plated steel, which means these contacts will eventually flatten back and stop making contacts, as well as the fact that chrome is not the greatest conductor. It does resist oxidation though, and that's why it is used.

The next problem is the 4 outer contacts, which are the track pickups. They were completely flat on my loco, and from conversations with others, were on theirs. By not being bent up, they often do not make good contact and affect the power pickup.

The last problem is the inner 4 contacts for the motors, on the 2 small rectangular pieces of metal. This is a bad problem, because these smaller "fingers" flatten more easily, and when you lose contact here, then you can have the problem of no power to a motor block, while the other motor block pushes the loco. This can destroy gearboxes. Intermittent operation will be first, and this might be the reason some people just destroy gearboxes... many of the original E8 run had this problem, and bending the contacts up cured it.

See the picture below, which is a closeup, 2 of the 4 fingers on my E8 block were not bent up at all, it took a bit of fiddling with an X-acto blade to get them back up. The contact on the left foreground was not touching, obviously. Look at the very bottom of the picture. 

prime mover contacts 2

See that the left hand contact is almost flat? No contact! No conduction!


The only solution that is easy is to remove the blocks and bend up the tabs in hopes they touch the springs. Be careful! I use an x-acto blade inserted parallel to the motor block. This will work for a while and then eventually the tension will relax and contact will go away.

A little grease on the tabs might help oxidation issues, but I have not seen this, and the small amount of motion between the motor block and the "A frame" probably helps keep the contact good, although that same motion helps eventually bend the tab back down and interrupt power flow.

The only permanent solution would be some kind of phosphor-bronze spring material soldered in place that would not lose tension.

Problem: poor contact between"pickup" straps and "bus bars" on steam locos:

Recap, the power pickup from the wheels to the gearboxes goes to straps inside the motor block.

These straps come up a touch the 2 wires that act as "bus bars" back to the power socket. In the picture below, you can just see 2 of them, one on top and one on the bottom "bus bar", just in line with the 2 screw holes. Clearly the contact area is not large!

Unfortunately, this connection is just from touching, and there is no adjustment to keep good tension, or keep it clean. The contact area is very small. Thus there can be problems in power conduction here.  The finger only contacts 25% of the outer circumference of the "bus wire".

I cannot comment on how often this is or is not the problem, but, Aristo has obviously seen this problem in the past and changed the assembly process to solder them. This is one of the first places to look if you have a pickup problem.


The solution to this problem is to solder the little "finger" to the "bus bar" wire. Be very careful, you can melt the plastic nearby easily. You cannot avoid all damage, but a little melting of the plastic won't hurt anything. Of course disassembling later will be a nightmare, but you should not ever have to do this. I have heard that the repair people in the US will solder these when a loco is returned for problems. If you have an Aristo steamer, you would be wise to solder these. Get a bit of flux on the joint first, since you are soldering to chrome.

Further explanation:

These 2 "bus bars" are simply bare wire that come out and solder to the circuit board that the motor solders to, and also the white 4 pin connector that goes into the loco. To remove the board you need a high wattage iron, or a way to remove the solder. My recommendation is if you need to remove the board, is to unsolder the 2 "bus" wires, and pull them out IF the straps are not soldered to the bus wires. Then you can heat the motor tabs and rock the board off. If the clips are soldered to the bus wires, you need to get all the solder off the motor terminals and the bus wires before you can remove the board with solder wick or a vacuum system. . Clean all the solder away from these connections when re-attaching. You should never have to pull this apart unless you need to replace the motor or the bus wires. (There is enough flexibility in the board to allow the removal of the gearboxes without disturbing the soldered connections.

Problem: growling from motor alignment, or poor power feed to motors in diesels

In a diesel, the motor brush terminals connect to metal forks, and the original design just had the motor brush leads "sliding" in these forks, letting the motor "float" a bit.

This was a good design to allow the motor to self-align with the gearboxes, but the movement of the motor would loosen the "forks" and power conduction became an issue.

Later, Aristo began to solder the motor leads to the "forks", but introduced a new problem, misalignment of the motor with respect to the gearboxes. Since the motor could no longer self-align, the motor needed to be well aligned before soldering.


From many comments about noisy motors, and the repair method discovered by Raymond Manley, it's clear they are just soldered as they lie in the block. Raymond discovered that if you press the motor firmly up into the motor block and resolder the connections, this almost always cures the misalignment and noise problems.

I've done this several times with success, do one motor at a time. Raymond takes the bottom cover off and dives down inside. I remove the wheel, and there is a slot that you can see where the soldering has been done.

I find my method easier, remove the bottom cover and wheels, and then pinch the motor tightly (fingers on motor, thumb on motor block other side. Now you have a nice force on the motor. Touch the iron on one side and you can feel the motor move.. wait a bit, then do the other side. Be sure to press in the center of the motor.



Problem: all locos - poor conduction and spring tension problem between gearboxes and motor block:

OK, this is a more severe problem, and takes more time to fix.

Lets look at the gearbox again and view the metal retaining clip on the each side, below the axle in this picture. When the gearbox is put in the frame, these gearbox clips press against straps on the frame. (These are the curved bits below the axle in this shot).

The amount of flexing (rotationally along the axis of the motor block) concerns me that this can become a problem area with extended use, or poor repairs, or defective "clips". These "clips" are not spring steel, or phosphor bronze, nor any other material normally known for retaining spring tension over time. They are easily deformed when handling.


So my recommendation is to be sure these curved ends have good shape and are springy enough to have good contact to the straps on the motor block. A bit of grease here will keep oxidation and corrosion at bay. Any good quality grease will be fine. If you have disassembled a loco to this stage, make sure they have a nice snug fit into the motor block housing.

Since the contact area is large, I am not worried about normal dirt or corrosion, but losing spring tension from motion over time. If your power pickup is intermittent, this is a possibility. You can check it by twisting the gearbox side to side and see if continuity is broken.

Another problem" on the diesel motor blocks, the "straps" inside the motor block are only fixed at one end. If you are not careful, instead of the spring contacts on the gearboxes going on top of them, they get "caught" between the plastic housing and the UNDERSIDE of the spring clip, making a mess. A worse problem, sometimes they get so far away from the side of the motor block housing, they bend completely UNDER the gearbox and make NO contact. I have not seen this assembly problem from the factory, but I have seen this in blocks that have been apart. Use an x-acto knife to reach in and move the strap tightly to the motor block housing when reassembling. Look carefully as you put a motor block back together. 

As always, test power pickup on EVERY wheel after reassembly. 

Problem: all locos, power pickup caused by no power conduction from "spring" to "clip finger"

This is the really bad one, and when Lewis Polk told me I could not put my web site in my forum signature because of my comments on the "weakness" of the prime mover design, I agreed to pull those statements, PROVIDED he meet me at the Big Train Show in California with a NEW Mikado motor block so I could show him and Navin the problems.

Well, fully HALF of the drivers on NEW Mikado motor block they brought to the show had this problem. Showed it to the head Aristo-Craft repair tech right at the Big Train Show. This happens ALL the time, and is the #1 reason for power pickup problems.

Basically the problem is that the little finger has to touch the spring enough to make contact, but not so much it distorts the spring. There is also a "random" element here, how the spring is positioned affects if a loop of the spring is in the "right place" for the finger to touch.

All you can do is check the conductivity, and if it is an open circuit, get in there with a little screwdriver and bend the finger inwards a bit.

Be prepared for the finger to break off if you bend it too much, and if unsuccessful, you have to take the clip off and mess with it.

Now that finally makes sense. It is also where my problems were. Unfortunately these clips are inconsistently formed, they vary all over the place in contour, in the way they fit to the module and the angle of the "finger". The critical part is that the "finger" MUST touch the spring to make conduction. The way the finger contacts the spring is a poor design in my opinion.


The contact area is small, at different angles, and there is no way to "line up" a coil of the spring with the fingers contact surface. Pressing the finger in too far can interfere with the spring tension, not enough (as my case) and you don't touch the spring at all. I am considering a way to improve this, but all you can do right now is try bending the finger to see if you get it working.

(As an aside, be CAREFUL, many people have reported losing this spring and ball bearing. Being forewarned I did not, although seeing it caused me to order 2 spares! If you remove the axle ball bearing, take this spring and single ball bearing out immediately and put them aside. Put them back in AFTER you replace the axle ball bearing)

Also reported by Bob Grosh, some of the early RS-3 gearboxes were of a similar design, and the tab was bent up at right angles, so that the spring seated flat on it. But you can see that trying to keep the spring compressed while installing the clip is virtually impossible. I might try doing this myself, but an assembly nightmare.

I believe this was the original intent, but it caused a lot more time in assembly, so the current shortcut was assumed. Since all wheels pick up, and the individual wheels are not tested for continuity, it happens all the time. The poor pickup is because of non-functioning contacts, not dirty track or wheels.

Problem: Bad / non-adjustable wheel gauge or back to back dimension

A disadvantage of this design is that you have no way to adjust the wheel gauge. There is no way since the axles are tapered, so the wheel goes where it goes, it stops when it seats on the axle. In fact over time, there were 2 fairly distinct "dimensions" on axles and wheels, and if you swapped wheels, the gauge was WAY off.

As a side note, just before Aristo ceased operation, they came out with "D cut" axles, where there was basically a non-tapered axle with a flat on one side of the axle, and the recess in the wheel was the mirror image, like a "D". You could adjust the gauge with a washer, or grinding a bit off the end of the axle.

Solution: gauge too wide:

You are lucky, you can "open up" the taper in the wheel to have it sit further on the axle. This could be done easily with some valve lapping compound.


If you do this make sure you do not cause the axle to protrude past the recess in the wheel, where the screw seats. This is where the screw seats on the axle tip before pressing on the wheel recess. The wheel will not be tightened. from the wheel, since the lock washer sits on the wheel and the screw pulls the wheel to the axle.

If the axle tip protrudes into the wheel, then the washer will sit on the axle and not "clamp" the wheel. If this happens, you need to grind a bit off the end of the axle until it is just a bit recessed from the wheel when assembled.

If the gauge is too tight, you might be able to use some very thin brass shim stock between the wheel and the axle, but this often makes the wheel not run true. Fiddle enough and you can probably get it right. Also you might have slippage from the extra shim. Use Loctite for sure.

I purchased some custom-made shims from Grant Kerr of the old Outback Turnouts in Australia. They are precision made, and about .009 thick. You may use between 1 and three of them per axle, split them between sides. Thanks Grant!

One problem is that the Aristo flanges are often WAY too thick. This makes it problematic to set the back to back without exceeding the gauge width. To compound matters, the Aristo-Craft SS track I have is right at the NMRA minimum. So I shimmed the wheelsets to make the wheel gauge meet the same number, and that helped the back to back.

Unfortunately this made the gauge of the wheelset too wide in some cases. I think my only resort is to turn down the flanges. (Remember I am using a synthesis of the NMRA and G1MRA standards on my track and switches)

The picture below illustrates what happens when your back to back gauge is too narrow. Continued running this way will break something, usually ripping the axle half from the drive gear.


Problem: Axle Slippage - caused by poor friction fit, exacerbated by Loctite

The most common and frustrating problem is a steam loco driver coming loose on the axle. When this happens, it almost always involves damage, gears stripped, axles ripped from the main gear, bent or broken connecting rods.

As stated earlier, the friction fit relies on the taper on the axle and drive wheel, and the screw clamping them together.

Historically, Aristo tried various applications of Loctite red to secure this, often glopping it all over the screw, the washer and the recess in the drive wheel.


One common problem is when the factory apparently has applied the Loctite and it has set before the screw has been tightened correctly. In this case the wheel WILL come loose eventually. Loctite is not epoxy glue, and has little strength unless in a very thin coat. Solve this problem by removing the screw, washer and wheel, clean everything up and reassemble.

I went a step further, and used a 3.00 - 0.5 mm tap and chased the threads in the axles, cleaned out all the loctite red in there. What a mess. You normally destroy or damage the screw head (cheap metal) removing them, so getting some spare screws and lock washers is a good idea.

Properly done, no loctite, or a dab of loctite blue (designed for vibration) is what you want. Loctite red is for stuff never to be disassembled. It is unreasonable to use it in this application. Do not put it on the axle ends or the lock washer.

Problem - Axle Slippage - caused by axle protruding too far into wheel

This one also happens fairly often and is harder to solve. The problem is that the axle enters the wheel so deeply that the screw and washer bears on the axle end, not the wheel. So tightening the screw only fixes it to the axle, no pressure is applied to the wheel.


The cure is normally to grind off the end of the axle so it is recessed properly. Be aware that this might be related to another problem, that the gauge of the wheelset might be wrong. It might be worth considering purchasing another wheel. It's not clear what the problem is here, the wheel recess too deep, or the axle taper too skinny.

Problem - Axle Slippage - caused by poor fit between wheel and axle

This is really, in my experience, the primary cause of the slippage, along with loctite red that cured before the tapered fit was snugged.

The axles are chrome plated. If you look at the taper on the ends of the axles, it is not smooth or even. Now, I cannot tell if this is from the plating or that the axle is cast and the taper not machined.

The bottom line is that this design relies on these 2 surfaces mating well, and in my opinion, they do not.

I would have to say that Aristo agreed, otherwise why is loctite red (made for permanent assembly) used all over the place?

(after improving my "fit" I have put NO locking compound on my axles or screws and have not had anything loosen)


After the chrome plating, the bottom line is that it's just not good enough, in my opinion. Ted Doskaris also had some custom wheels made, and supplied motor blocks to the manufacturer. It was reported back that the tapers on the wheels did not exactly match the axles!

I took some fine valve grinding compound (available from auto parts stores) and lapped the 2 parts together. You take some small dabs of the compound and put it on the axle tip. Spin the wheel on it with moderate pressure. Hold the gearbox so that the axle you are working with protrudes as far as it can from the gearbox, helps keep the stuff away from the bearing. You can tell by the drag between the axle and wheel when you've lapped it enough. You will wear off most of the chrome plating. Keep the wheels with their mated axles from now on. You will see when you put the screw back on this will never slip. I clean the axles and the insides of the wheels thoroughly.

Tracking problems

Now we shift from power issues to getting the truck to run over track.

A lot of these issues are related to the variations in assembly over the years, and inconsistent "theories" on how this affects tracking.

Basic design and revelations:

First, understand the design. These motor blocks are NOT sprung, in any possible stretch of the imagination.

Most people do not understand this, but put a loco on it's back and you will quickly see. All the gearboxes pivot around the long axis. Push down on a wheel on an axle, and the other wheel moves up. The gearbox pivots. This is actually pretty nuts when you think about it. The only thing this helps is track that goes in and out of cross level. If your track is level (cross level, across the rails), this is an UNSPRUNG motor block with absolutely no accomodation for track grades starting and stopping.

Pads under the gearboxes, why and where?

There is an additional issue. Uncontrolled, a locomotive can wind up with a permanent "list" to one side or another.

Therefore, at least one of the gearbox assemblies in a truck/motor block needs to be held rigidly in place, i.e. not pivoting. This will force the locomotive to stay upright.

Over the years, Aristo has added different foam/rubber pads of various materials, thicknesses and positions to limit the motion of one or more gearboxes. These pads go between the geabox, and the "top" of the motor block, thus they cannot be changed or seen without removing the motors and gearboxes. This is one of the reasons so many "experts" don't understand this, because they have not fully disassembled the motor block, or just did not notice the pads.

In it's final years, Aristo "experimentation" stopped, but they they originally came with the center axle locked (3 axle diesel), or last axle locked (steam), all steam locos typically come with a thick black rubber pads under the leading gearbox, and all diesels typically come with thick translucent white nylon pads under the outermost axles on the loco.

So for diesels, the first axle into a switch is one that is locked, and the first axle into a switch in the forward direction for steamers.

Problem: not all axles even with track, high centering

Recently I have been experimenting with wheels with smaller flanges and a less "toylike" wheel tread contour.

This brought an immediate derailing problem on one particular Aristo wide radius switch. My friends who were also testing these new wheels also were experiencing derailments.

These wheels correct the back to back spacing to NMRA and G1MRA standards, have a tread taper of 3 degrees, and G1MRA and NMRA compliant flange thickness and depth.

I realized that my wide radius switches had no shims on the flangeways. Stock Aristo Wide Radius switches have a guardrail flangeway width of 0.123" or so, way over specification. I aim for 0.106", a common standard. So I took some 0.020" brass and wrapped it around the guardrail. That did not solve the problem, but I did not expect it to, because I was not "picking the frog point", but derailing at the switch point.

So I was really befuddled. Finally laid down on the ground and watched. The first wheel of the motor block went straight through the switch! I thought maybe it was a problem with the switch points, but no, they fit the stock rail perfectly, and everything looked very good.

More low angle inspection, now sighting down the outer rail with the switch set to the diverging route.

The wheel was not even contacting the rail !!! What the heck? It was about 1 mm off the rail. No wonder it "ignored" the switch point and the diverging route!

How was this happening?

I took the loco off the rails and put it on a completely flat surface. The center axle was 2mm LOWER than the two other axles!! This meant the truck could rock end to end, with only 2 axles in contact at a time.

Well, I should have remembered George Schreyer's page on this.

Investigating this, it was apparent that the "saddle" in the motor block was too high for the center axle. 

No problem, tear apart the motor block and grind a bit out of the saddle. Oh yeah, the motors are soldered in place now. They used to just slip in, but the "slip contacts" for the motors would get bad from the motors torquing, so Aristo now solders them in place.

OK, break out the solder wick, suck out all the solder on the 4 contacts and pull the whole assembly out. Mark which pieces go where and the "up side" of the motors, or you may get a motor block with motors running the wrong way or in opposite directions.

I tested it in the motor block without the cover, and it seemed to have all the axles in line now. Screwed the cover back on, and now it was bad again! What the heck?

Take the cover off, look at the motor block carefully. The motor block casting is bowed in the center! Screwing the cover on straightens it out a little. OK, so pull the dang thing apart again and take some more off. I know I have it this time!

Wrong! What the heck again. OK, this means that something is not letting the center axle tuck up into the motor block. It could only be the motors. Sure enough, there are rubber pads on the top and bottom of the motor recesses. I took the pads out of the upper motor block, put it back together, and success! There was just enough rubber there to push the motors closer to the rails, drive the gearbox down towards the rails and not let me enjoy the modifications I made.

So, the first solution is to be sure your wheels are all in the same "plane". Put your motor block on a flat surface, and if it's way off, the above procedure can help.

Wait, only help? Not solve? Read on!

Fundamental issue with unsprung 3 axle trucks

OK, there is a fundamental issue here. You have a long, unsprung truck. This can cause derailment issues on vertical transitions on your track. There is NO way around this.

The following shows this: (thanks once again to Ted Doskaris for the excellent pictures)


AristoSD45 3axleMotorBlockTestExamplesOnOutdoorLayoutCrownedGradeChangeComposite

AristoSD45 ATSF MidProductionRunWithStandardDeepFlangeDepthWheelsGradeTestComposite

Notice this is a small, minor grade transition, but the wheels do indeed leave the rail head. What keeps these locos from derailing? The deep flanges!

There's no other solution to this problem, and your wheels WILL leave the rail head.

I had a mysterious derailment problem, and the locomotive always derailed at a switch, and I could not figure it out. Actually the wheels had left the railhead 6 feet earlier, and one wheelset was no longer on the rails, but the loco continued with the 2 other axles guiding it, UNTIL it hit a switch and all hell broke loose.

This is so common that it really bears repeating, you really have to look where the derailment actually occurred, not where it got too bad to continue.

Misc. Issues


Problem: brake shoes rubbing and interfering with the axles motion, warped sideframes

I had read about the brake shoes rubbing on the 3 axle diesels, so decided to take a look as I buttoned up the E8. I also remember that they were delayed for "warped sideframes".

Good think I looked. Some of the sideframes were warped. bowed in the middle, away from the motor block.

What this does is "bend" the brake shoes on the center axle even closer to the wheels. In fact there was an additional problem, some were bowed enough to interfere with the wheel's lateral motion, i.e. the edge of the wheel hit the brake shoe.

I used a Dremel tool with a drum sander "bit" and thinned the brake shoes, they need it most closest to the rail head. After that, I put a 45 degree chamfer on the inside of the brake shoes, from the inside. Done correctly, this does not show from the outside.

Problem: strange wear patterns on axles, poor power pickup, bad tracking.

The single ball bearing pickup can cause excessive wear on axles and side to side binding. This ball bearing is rarely lubricated. As it rolls on the axle, if there is a lack of lubrication, it will "carve" one or multiple grooves in the axle:


I have never experienced this on my locos, because I keep the axles well lubed, but this appears to happen when the axle will sieze in one ball bearing race.

Now the single ball keeps riding in the same spot, and due to no lubrication, will wear down a groove into the axle.

Notice the narrow groove the ball wil wear. On the more broad wear spots, that is apparently a siezed ball bearing.


Keep the axles themselves lubed - I use the "heavy gear oil", and periodically inspect the lateral movement of all drive axles.

Some people never have this problem, but there is a club in Florida that had it all the time, and apparently never figured out they needed to lube this part.



"D cut" wheels and axles, the last gasp from Aristo

Wow, finally a solution to the wheels turning on the axles, only found in the very last production run of the SD45's, just before Aristo folded.


d cut1


d cut


A flat on the axle, and a "D" shaped hole in the wheel.

From RJ DeBerg:

    • Wheel dia  1.35
    • Wheel overall diameter (including flange) 1.57
    • Flange thickness  0.064
    • Back to back  1.59
    • gauge  1.72

My SD45 high hood:

    • wheel diameter 1.374 (very near the flange)
    • wheel overall diameter 1.576 (including flange)
    • back to back 1.559 to 1.565
    • gauge 1.68 to 1.69

Also the wheels appear to be sintered iron/steel, magnetic, and have the typical black stuff on the surface, just like the AML sintered wheels on their rolling stock.

I wish we could have seen where Aristo was going with this. On diesels, slipping wheels was not a huge problem, since they just slipped. But on the steamers, slipping wheels destroyed the rod gear and/or gearboxes.

Clearly an issue to be solved is "phasing" the drivers after the axles are already assembled to the gearboxes and drivetrain.

Oh well, perhaps someone will come up with a good solution to retrofit the huge number of Aristo prime movers out there.




Weather Underground PWS KCACARLS78