Stainless Steel Wheels for Aristo-Craft Prime Mover Diesels

Stainless Steel Wheels For Aristo's "Prime Mover" Diesels - A viable alternative
Ted Doskaris
April 22, 2010
Rev. GE-B
October 3, 2011
Rev. GE-C  Added GP40 excessive axle displacement info.
October 11, 2017
Rev. GE-C1 Repaired broken links

October 6, 2018
Rev. GE-D Added reference to custom made metal coupler box


Since track powered locos are typical for standard product offerings, the quantities involved make wheels increasingly important for the manufactures to provide them with lasting durability for good power pickup.

Aristo-Craft brand diesel locos with the so called "Prime Mover" ball bearing type gear boxes use the same wheel design. To date, in the order of introduction, these include the SD45, GE Dash-9, E8 / E9, and GP40.

Though wheels on Aristo's predecessor locos (FA1 / FB1, RS3, U25B, RDC1, Eggliner, etc., also having ball bearing axles) look the same, their hub taper that engages the axle is different, and they don't include a ridge on the back side of the wheel hub area.

Shown below is the back side of an Aristo prime mover motor block diesel wheel with a ridge protruding from the hub area.

Shown below is an example prime mover gearbox with stub axle and bright metal retaining clip. The heads of the screws that hold the clip are protected from wheel rubbing wear by the ridge on the back side of the wheel in the event the wheel were to be pushed against them. At least that appears to be the intent - though in some cases there still seems to be evidence of a wear ring on the back side of the wheel like the one in the prior picture.

An Aristo wheel problem:

From my experience, Aristo's first production run GP40 loco wheels suffer from quality issues in that their plating would quickly wear through or peal off the tread surface.
The picture below shows one of the wheels on a Aristo Union Pacific GP40 with minimal run time that appears to have shed its plating!


This is discussed in another article, Aristo GP40 problems - Smoking motors & Debilitated wheels! .

Examining several other Aristo diesel locos, including SD45s that I have, also showed evidence of poor plating on some of the wheels in the way of blisters on the tread surface.
For example, I have a Santa Fe "Kodachrome" SD45 with such a condition.
As to the GP40, the picture below depicts an example Cotton Belt GP40 wheel with blistered plating.

When using track power, degraded wheel plating is of significance as it results in excessive electrical arcing and black carbon and / or copper oxide deposits on both the rail head and wheel tread surfaces.

It is to be appreciated that degraded wheel plating is only one of several areas that effect power pickup. For example, there is a latent problem with the design of the gear box retaining clip that relies on a "finger" to make electrical contact against the side of a coil spring within the gear box. The reliability of such contact can be one of happenstance. Greg Elmassian has done excellent research on this issue which is covered in  Aristo Prime Mover Basics on his web site.

As to the wheel, the combination of carbon and copper oxide (I have brass track) - resulting from electrical arcing - along with whatever dirt, etc. constitutes debris picked up from the rail heads. Retention of this collected debris on the surface of the wheel tread is exacerbated by blistered or vacated plating resulting in uneven surfaces or crevasses.

Progressive degradation results as the wheel surface worsens from all these factors - serving to further increase the tendency for more electrical arcing as less and less electrical contact area between wheel and rail is available; thus, resulting in more debris on both the wheel and rail head - repeating the cycle of events until the loco runs in a jerky fashion or completely stalls for lack receiving power. I found that cleaning such wheels helps only for a short time as debris is quickly re-collected.

One interesting side effect is that the debris can act like a traction tire to increase the pulling ability of a loco, but this is of little consolation when power is interrupted to the loco.

For track power users this has become a significant dilemma to confront.

Though Aristo may be able to supply replacement wheels (at a quoted $10 dollars each when I called to ask), it seems using them would likely be risky with like kind quality of what is on the GP40s as previously described!
Moreover, at the present time aftermarket wheel suppliers such as North West Short Line (NWSL) and Gary Raymond no longer make / choose not to make / or don't make a diesel type wheel common to Aristo's SD45, GE Dash-9, E8 / E9 and GP 40.

A Solution:

Whilst discussing the wheel issues with an automotive fabricator / mechanic friend of mine, the suggestion was made to consult with a highly qualified CNC machining business - namely, Datum Precision, Inc. located in Grass Valley, CA.  This is the same company that later made the metal coupler boxes that I had designed described in vignette, "Custom Made Coupler Boxes & Kadee Centerset Couplers for Selected Aristo-Craft Locos"
I followed up and spoke with president / owner John Jans as to the feasibility for a replacement wheel.

John indicated he could make stainless steel wheels in quantities estimated to be 240 wheels from type 303 stainless steel bar stock using a minimum order size of 1 and 5/8 inch diameter by 12 foot long. The price quoted to do so was notably less than Aristo's $10 per wheel and would further lessen with increased quantities.

Since Datum Precision typically makes parts for other business, a price markup from a distributor or reseller is avoided, thereby keeping the per piece cost reasonable, but with the tradeoff of having to order in large quantities by one individual.

Considering that Aristo's wheel is plated steel, pursuing a superior stainless steel wheel is a comparative bargain! Of course, the minimum order requirement would have to met, so based on my needs of retrofitting many Aristo "prime mover" diesel locos needing several hundred wheels, I decided to seriously entertain the prospect of a 2 bar stock order.

In comparison to the Aristo wheel, the way the new wheel was to be made would not affect the price, so the logical next step was to look into improvements on the wheel design.
In this regard Aristo "prime mover" diesel loco wheels are consistently spaced too narrow and have an overly pronounced flange depth and tread taper.  

So I decided to check with the NMRA standards, namely S4.3. At this point I decided to consult with Greg Elmassian as he had been involved with the NMRA standard (and familiar with G1MRA standard) and is acquainted with knowledgeable individuals having familiarity with wheels vs. standards.

Together, Greg and I came up with a set of specifications and submitted them to Datum Precision, Inc.

As the business name suggests they are accustom to manufacturing tight tolerance, high-end precision parts, including for military applications, with the numerous CNC machines they have.

I then placed an order for the Stainless Steel wheels that were made using a Mazak QTN200MY CNC machine.

The picture below is out of focus, but Jason at Datum Precision had been running the CNC machine making the SS wheels at the time I visited.

The picture of the Mazak machine below is similar to the one being used by Jason and shows its "turret" arrangement. A 12 foot long stainless steel bar stock of 1 and 5/8 inch diameter used for making the wheels was cut into 3 foot lengths for loading into the turret heads.

Shown below is the first sampling of the wheels that Jason had made prior to the hub taper machining operation.

The order consisted of two groups of wheels as follows:

Group 1 - Generally applicable to Aristo diesel GP40 4 axle locos and the 6 axle locos dated 2004 and newer.
Group 2 - Generally applicable to Aristo diesel 6 axle locos dated prior to year 2004 (typically the SD45 introduction date 2001 and the follow on in 2002.)

The collective actual yield quantity of two groups of wheels made from two bar stocks was 464 wheels (16 wheels short of the estimate) mostly owing to a first of this type of part for Datum Precision. Shown below are some of the wheels as I received them, including a purpose build wheel puller.

There is no outwardly visible difference to the two groups of wheels.
Shown below are a couple of pictures of the new stainless steel wheels

The reason for the two groups has to do with the back to back wheel spacing differences.  The wheels are the same except that the wheel's axle hub taper depth is made slightly different - being machined less for the earlier than dated year 2004 loco applications.

By different measurement methods, the back to back spacing for the earlier 6 axle diesels was determined to be mostly about 0.035 inch narrower than the newer production runs.
So the group 2 wheels meant for the earlier locos were machined with slightly less hub taper depth so they would not seat as far on to the stub axles, thereby the back to back spacing would be compensated for by an increase - specifically +0.035 inch - when a PAIR of wheels is installed on an axle.

Shown below is a picture of the back of the SS wheel. The hub taper is 8 degrees in conformance with the prime mover axles. The spirals as brought out by the camera shot that you can see within the tapered hub area are in the order of 1/10,000 inch according to Datum Precision owner, John Jans. This can be advantageous to "lock-in" the wheel on the axle, or should one choose to use lapping compound, provide for a tooling effect.

The wheels were purposely specified to favor a slightly wider back to back spacing than the NMRA S4.3 Draft v1.6 nominal spec of 1.575 inch when installed on an axle.
This was purposely done as it is easier to remove material than to add material such as using shims.

Accordingly, the SS wheels I have installed on about 10 locos thus far trend to measure about +0.010 wider than the NMRA S4.3 back to back nominal spec. However, this is within the asymmetric allowed tolerance of +0.019, -0.015 inch.

For 36 wheels supplied to Greg Elmassian for his 3 Aristo E8 locos, he has successfully optimized the installed wheels to the exact back to back nominal spec. on two locos by using automotive valve lapping compound in order to do so. He has a simple procedure to do this with minimal effort.
Since most of the material removed will be from the axles (the SS wheel is harder) keep in mind that if you were to re-install the original Aristo wheels they would become even closer together than they were before - being too close at that. This only would be of concern if one were to part with the loco and elect to keep the SS wheels.

Speaking of valve lapping compound, it can be used to help remove the little M3 screws that retain the wheels by putting some on the screw driver tip. This trick was suggested and demonstrated by John Jans of Datum Precision as he removed a wheel from one of sample SD45 motor blocks I provided him with.
Whilst sometimes you may not need to do this, it is best to do this on all the screws so you won't risk rounding out the screw heads. Also, I found an Aristo Dash-9 had red Loctite factory applied to the screw!

If you do round out a screw head (as I did before knowing about this trick) I used a small reverse twist drill bit and drilled out the screw. It turns out the screw's metal is soft enough to drill through - so this method worked OK.

Wheel Puller:

WARNING! When removing the wheel/s do not attempt to pry them away from the motor block as you will certainly damage the gear box half shaft retaining mechanism. Use a wheel puller.

When I first started to remove the wheels, I used a Craftsman battery terminal puller as shown below. This worked very well - though it requires the wheel retaining screw be left in place but backed out some for the puller to work against.

However, when John Jans saw the sample GP40 and first generation SD45 motor blocks I had provided to him, he was inspired to design and made a purpose built wheel puller that fits exactly over an original Aristo diesel wheel.

Like the Craftsman puller, the purpose built puller can be used on the Aristo 2 axle and 3 axle motor blocks without removing them form the loco.
You can just remove the loco side frames, take out the wheels' retaining screws, slide the puller over the wheel and use an Allen wrench to tightened down in order to "snap off" the wheel.

Though the 3 axle blocks come out easy, the GP40 2 axle blocks do not as you have to remove loco's shell and unscrew the motor blocks from within the chassis. So being able to pull the wheels off with the motor blocks left in place makes for a easier and faster effort.

Comparisons - New Stainless Steel Wheel and Aristo Original Prime Mover diesel wheel

The pictures below show the new Stainless Steel (SS) wheel on the left with an original Aristo GP40 wheel on the right.

The SS wheel tread taper was chosen to be 3 degrees rather than the 5 degrees of the Aristo wheel. Also, the SS wheel's flange taper was chosen to be reduced compared to the Aristo wheel.

The wheel tread width is similar, though the overall wheel thickness is marginally less than the Aristo wheel.

The flange of the new SS wheel is about 0.080 inch deep with respect to the base at the tread fillet.

The NMRA S4.3 spec. indicates 0.090 inch max.for the flange depth, whereas G1MRA spec. indicates 0.079 inch max. The original Aristo wheel flange measures about 0.120 deep. The NMRA Draft v1.6 / 4.3 appears to be a concession to legacy wheels like Aristo's with a spec of 0.118 inch max.

The picture below shows flange area against the side of rail head with a loco having new SS wheels placed on Aristo stainless steel track.

Shown below is an example SS wheel mounted on a GP40 stub axle. Note there is a desirable recess to the axle hub so that when its retaining screw (Metric size M3 by 6mm) and external tooth lock washer are fastened, the wheel is allowed to be snugged down.

When first placing the wheel on the axle some may seem to have a very slight wobble, but once it is twisted a bit and snugged down with a tightened screw, the wheel is firmly seated when the screw is backed off.

The pictures below show the SS wheels installed on the GP40, sans side frames.

Correcting the GP40's Excessive Lateral Axle Movement

Lateral axle displacement is the amount of distance the axles are allowed to move from side to side in the motor block.

Whilst the Aristo 3 axle prime mover motor blocks by necessity require a notable amount of lateral axle displacement in order to negotiate curve tracks (so as to avoid wheel binding on the rails), the 2 axle motor blocks, like used in the GP40, do not.

Thus, during operation, the GP40 motor blocks having excessive axle displacement can result in walking of the axles such that the gears are in a less than optimal mesh contact within the gear box - thereby, risking failure under load.  Moreover, the displacement is pronounced enough such that coupling to cars may be problematic should the loco twist about its axis -  particularly when using Kadee couplers.

Placing a 0.032 inch thick Nylon washer having a quarter inch hole over each axle stub will limit the axle movement to a reasonable amount.

Note: To avoid having to remove wheels to install a Nylon washer on an axle, the washer can be half split with a pair of dikes, allowing it to slide over the axle; however, once it's on the axle, it must be flattened out.

The following video serves to demonstrate the GP40 axle displacement issue:


Performance Issues to date:

Aristo Turnout Latencies:

I can't speak to brands of turnouts other than Aristo as I don't yet have those, but in the case of Aristo's production run years (generations) of Wide Radius and No. 6 turnouts that I have, it seems these turnouts were compromised to accommodate Aristo's own typical under spec., overly narrow, back to back wheel spacing of locos and cars - sort of two wrongs making a right!

Shown below is an example Aristo first generation D&RGW SD45 motor block axle with a way under gauged back to back wheel spacing!

Shown below is the same axle with the new Datum Precision group 2 made SS wheels installed.

That said, it's been my experience that Aristo's turnouts still need modifications (like example narrower guard rail to stock rail clearance for conformance to NMRA specs.) to make them work even with some of Aristo's factory supplied locos!

See articles hosted for me by Greg E.: Aristo Wide Radius Turnout Improvements and Aristo Craft #6 Turnout issues .

The Aristo locos typically have a wider truck wheel base than the cars, so axle skew (angle of attack) becomes a more significant issue with the intentional wider back to back wheel spacing when the new SS wheels are installed.

The pictures below illustrate a pronounced wheel angle of attack with an example 3 axle truck placed on an 8 foot diameter curve track.

Note the pronounced angle of attack of the outer axle's wheel.

Note how only the center axle wheel is parallel to the rail.

Note the pronounced angle of attack of the other outer axle's wheel.

Therefore, given the new SS wheel, it is understandable and anticipated folks will more likely have problems with unmodified turnouts with the new SS wheels if only because of their intentional wider back to back spacing - albeit in conformance to NMRA specs.

In this regard, when I built my under house layout I hand selected some turnouts and later modified other turnouts (Aristo Wide Radius & #6) for Aristo locos and rolling stock to pass through them without derailing. But I had not done all of them because the remaining ones seemed at the time to work as is -  owing to happenstance but favorable factory tolerances.
Now that I have proper spec. wheels, I discovered when operating most of the locos I have retrofitted thus far, more turnouts now need to be upgraded / modified. - No surprise here. Interestingly, I do still have a few turnouts that still seem to work with the new loco wheels.

The fundamental problem issue concerns operation of a loco going into the diverging path of a turnout so that it faces the point rails and frog point. Riding up on the frog point has been previously addressed even with respect to locos having stock factory wheels by shimming the guard rails or completely replacing the guard rails with ones having proper proximity to the stock rail as I had done and described in the referenced articles. Another problem concerns the point rails.
I captured some pictures of my example Aristo problem turnout that is one of three Wide Radius turnouts in the 10 foot diameter curve of my 270 degree loop back.
In this test case, all 3 WR turnouts are thrown in the curved path for operation of the loco in a complete 10 foot diameter circle. These turnouts are factory and had not modified with corrected guard rails or frogs until after I did the tests and took some pictures as will be shown.

The D&RGW SD45 loco with Datum Precision wheels is shown going clockwise with long hood first approaching into the point rails of the example problem WR turnout.
The pictures will show the wheel on the inboard side of the curved path passes between the right point rail OK, whilst the outboard wheel flange rides on the left point rail before dropping down.

Shown below is an example problematic Aristo Wide Radius 2nd generation turnout.
In the picture/s the loco is running long hood forward (clockwise), but the same thing happens with short hood forward (again, clockwise direction).

The edges of the point rails can be seen in the below picture.

The picture below shows the leading wheel in the foreground properly passing beside the open point rail and frog.

In the background of the prior picture, you can see the opposite wheel riding up on the point rail that is against the outer curve.
In the below picture, that side of the loco is shown with the wheel up on the point rail.

Shown below is a close up of the problem point rail that is thrown against the outer curve.

In comparison, shown below is another WR turnout point rail. But the loco with SS wheels has no problem passing through this turnout. The difference is the subtle relief made at the top of this point rail that allows the point to better seat in the undercut of the stock rail.

So it appears the problem turnout is to be "tweaked" in like kind fashion to make it work like this one.

I have since upgraded the problem turnout (along with many other turnouts) with respect to the guard rails and "tweaked" the point rail so that the locos with the new SS wheels successfully go through them without incident.


The first thing I tried was to pull a 66 car freight train around my under house layout.
This train required about a 4.6 lb draw bar force to break its stiction - dropping back to about 4 lbs when underway.

The force to pull the train is high since my layout is fairly circuitous consisting of mostly curves. Three locos pulled the train around the layout's main line whilst going through several turnouts without problems.

Whilst my under house layout is fairly stable and level, I decided to run 3 locos with the new SS wheels outdoors on what little track is built thus far (approx. 45 feet).  Since I had not had time to continue building the outdoor extension of my layout, this section suffered from a couple of years of disuse and seasonal weather conditions. There was a minor mud slide on part of its track work. So I swept out the debris from the tracks and wiped off the stainless steel rails with a rag.

There happens to be tilt in part of an uphill straight track section just prior to a curve. So this could serve as a test for the smaller than Aristo sized wheel flanges to see if the wheels would lift up high enough to expose the flanges as it would go along the tilted track and transition to / from the rest of the track that was not tilted.

The  3 Aristo locos (all equipped with SS wheels) used for the test are a E9 with 3 axle trucks employing the current Aristo practice of rigid mounted outboard axles, a first generation SD45 with 3 axle trucks having the rigid mounted center axles and a GP40 with two axle trucks.
This would serve to test the effects of short and long wheel base locos and motor blocks of various types.

Whilst closely watching the 3 locos traverse up and down the outdoor track, the wheels' treads did not noticeably rise above the rails.

Other people who participated in testing the new SS wheels:

I thank all who participated in testing the wheels as the feedback afforded is how improvements can be made.

First, it is to be appreciated that Aristo's turnouts must be upgraded before judgments can be addressed when operating with the new SS wheels.

Greg Elmassian, of course, received 36 new Datum Precision made SS wheels for use on his 3 Aristo E8 locos. He had to address a guard rail issue on a turnout and do some track leveling after which the locos with the new wheels performed well. In the process of observing what was happening, Greg discovered a constraining design issue with Aristo's 3 axle motor blocks that I will let him speak to.
RJ Deberg received 16 wheels for one of his Aristo first generation SD45 locos, and 8 wheels for Joe Mascitti's GP40.
I learned that RJ had original Aristo wheels re machined so Aristo locos would work on Aristo's turnouts, and not surprisingly, there were derailments with the new SS wheels with emphasis on one Aristo Wide Radius turnout that has its diverging path in a curved section of track.

Joe Mascitti reported he used Aristo's wheels with big flanges on the lead axle of his GP40 and the new SS wheels on all the other axles so he would not have derailments on his layout.

Nick Savatgy received 24 new SS wheels for testing that he placed on a two Dash-9s that he used at the York train show. He indicated a modular layout was to be setup for the train show. Nick expressed concern about the wheel flanges not being sufficiently deep, but he reported that the locos ran OK on this layout. However, the modular layout has properly tweaked turnouts. Nick reported different results on an outdoor layout where derailments resulted, including in areas other than at the turnouts.

Plans for future wheel versions:

At the present time Aristo and other suppliers of replacement wheels for the "prime mover" diesels appears to have vacated the market place with respect to a quality stainless steel wheel offering.

Since I have a large investment in Aristo diesels, pioneering an effort for a SS replacement wheel became a logical necessity to pursue. Finding a highly qualified company (Datum Precision, Inc.) willing to make the wheels at an affordable price made it viable. And, interestingly, this did not require going "off shore" to another country to do so.

With turnouts upgraded, the new SS wheels worked well when installed on different type diesel locos operated on my under house layout and would work for outdoor layouts that are well constructed and maintained. Since I have yet to build most of my extended outdoor layout, this affords an opportunity for me as to what to do for optimal performance during the construction.
However, to expand the application of the SS wheel that satisfied my needs to others having less than ideally constructed or maintained outdoor layouts, Greg and I are working on a follow on wheel version that will provide for more forgiving outdoor train operation should future group orders be made.

Another possible Aristo replacement wheel in consideration is the pre prime mover types having ball bearing axles like those used on the later versions of Aristo FA/B1, RS3, U25B, RDC, Eggliner, etc. 2 axle locos.  At the present time I don't have sufficient quantities of these to justify a minimum order requirement.

In any case, the turnouts used on a layout still must be upgraded for working with any of these new SS wheels that principally borrow from the NMRA and / or G1MRA specifications.

The mechanism to procure train wheels from Datum Precision is sort of an open source one, and it requires a minimum quantity order conforming to one bar stock of stainless steel material. Hence, individuals who can group together to meet this requirement becomes necessary whilst keeping the per wheel unit cost reasonable as there is no distributor or reseller to stock inventories of wheels that understandably would add a layer of added cost, including a profit.

It is to be appreciated, too, that Datum Precision makes high precision parts for various customers and may interleave train wheel orders with other work loads, and there is a lead time of obtaining material (about 2 weeks in my case) - albeit lightening speed compared with going off shore. Moreover, Datum Precision cannot be expected to enter into a dialog with customers having questions with respect to model train issues until such time they become familiar as their business is to make parts to information provided to them in the way of specifications and drawings.


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