USAT Diesel Motor Block Tips

Common issues of the 2 basic types:

This page is for common issues on USAT diesel motor blocks.

There are basically 2 axle motor blocks and 3 axle motor blocks.

In the 2 axle blocks, the axles are held rigidly on the block, running in brass bushings.


The 3 axle motor blocks are basically a 2 axle block with a third "floppy" axle in a pivoting and sprung housing, much like a leading or trailing truck in a steam loco.



The axle tips run in brass inserts in the side frames, not ball bearings. The axles also ride in brass bushings in the motor block, but note well: these nice looking bushings do not support the load of the loco, just the thrust of the gears. All the weight of the loco is supported by the brass inserts in the side frames.

These side frame bushings need to be kept lubricated, and as clean as possible, but they are out where they pick up dirt and moisture. The lube will attract dirt and grit, so periodically pull the sideframes off the axle tips and clean them out. Grease is best, not oil. A tip from Paul Norton: use inexpensive "Q" tips to clean inside the journal bearings. (The inexpensive ones are smaller in diameter and fit inside). For cleaning, Paul recommends vegetable oil, which will clean the mess up, but not attack the plastic. Use good grease, since these bearings will wear quickly otherwise.

Common issues and how to address them:


#1: Loss of traction, wheels turning without the motor, the famous "cracked axle" issue.

This is very common, and NO, it has not been fixed. USAT did modify the design a bit, but did NOT fix it. Many people do not realize they have this issue, as the wheels will turn even though they are slipping and the loco just does not pull as well, or you get abnormal gear wear (one truck does all the pulling).

90% of ALL USAT locos have this issue one time or another, even BRAND NEW locos, unused.

The handling of "cracked axles" has it's own separate page: Split / Cracked Axles

Axle bushing improper reassembly resulting in damaged gearboxes

This is the number one cause of damaged gearboxes, and worn gears. All you have to do is open the gearbox, and the axles will normally pop out a bit and the brass bushings will become misaligned and when this happens the gear mesh is wrong.

In the two pictures below, the proper alignment is shown and it is DIFFERENT between the 2 axle and the 3 axle motor blocks!!! (Talk about dumb!).

Again, if you improperly reassemble a motor block, you WILL destroy the gears! You have been forewarned.

Below is a picture of a 2 axle motor block: Look at the brass bushings next to the wheels, note that they are "flat" or flush with the surface of the motor block. If you put these in wrong, you MIGHT notice it's hard to screw the bottom cover down, and if you persist in screwing the cover down, you have effectively moved the gears apart, so the engagement of the gear teeth is much less and you tear up the gears, because just the tips of the gear teeth are taking all the load.



NOW, below is a picture of a 3 axle motor block, notice the same bushings are in a different, orientation, 45 degrees, so instead of having a flat side presented, you see a "point" sticking up. These are actually the same bushings, but in a different orientation. (This is REALLY DUMB) In any case, if you assemble these wrong, you will be "crushing" the gears together, and will destroy the gears inshort order. Normally the extra pressure makes the loco run slow and noisy, but not always.

USAT SD40 axleMotorBlockLidOffSkates P7020014


I cannot emphasize enough that you pay careful attention. My first advice is DON'T open these blocks unless you NEED to. If you do open them, remove them from the side frames, and then when reassembling, put a large rubber band on each axle, going around the "top" of the motor block. This will hold that axle down in place so it won't pop out of alignment when you replace the bottom cover. I have seen dozens of people mess this up, take my advice.

Pickup Shoes / "skates" / sliders

I have read that LGB pickup shoes can be substituted for the USAT ones. This could be helpful if your USAT loco's pickup shoes catch on turnouts, like the Aristo wide radius. The LGB ones are larger. This could also backfire, by being larger, can short to adjacent rail in turnouts past frog. Check carefully, several MTH customers have melted wheels by having sliders too wide.

In my opinion, keep the track clean (or use stainless steel), swap out any traction tired wheels with solid ones, and remove the sliders. The sliders can be troublesome by shorting out at some turnouts, or they physically "catch" in turnout gaps or poor trackwork.

Often there are issues with the "skates", the metal sliders that give additional power pickup.

Below is a bottom view of a typical 2 axle USAT block (F3 in this case)


Remove the 6 larger screws and more is revealed:


At this point, the skates will just lift out, and the springs will stay in the skates:


I personally do not like skates, and prefer to get good power pickup other ways, but people who have oxidized track, or LGB foamers swear by them.

I swear AT them, since they often get hung up on switches, or can cause shorts on switch frogs, or the biggest thing for me, is that the pickup current seems to go through the wimpy spring, and one derailment with a short circuit overheats the spring and takes the temper out of it and now you don't have the spring tension and the skate does not pick up power.

Also, sometimes they get real hot and melt into the motor block. 

So, if you keep them, you need to check them periodically, and I strongly recommend you wire a PolySwitch in series with each one, so if something shorts through the skate, you do't melt and damage things.

Internal power pickup explained

Some of the diesel blocks have an internal "whisker" that rides on the axle for power pickup. I believe these are only on the 2 axle blocks on the smaller locos. The picture below shows an F3 unit:


Note well that the "whisker" rides on the AXLE, NOT the brass bushing.

Often in older units a short has melted this "whisker" and rendered it ineffective. This is a good thing to check when opening the motor blocks. Also check that they have not overheated and lost their spring temper.

You can see the 2 whiskers in the picture of the motor block above. Note where the whisker "rides", when reassembling, be sure not to get them in the wrong position, often people miss this and cause problems.


 The 3 axle blocks have an extra reduction gear in them, and it appears that there was no room for the "whisker"

USAT SD40 axleMotorBlockLidOffSkates P7020014

Power is also picked up from the journal bushings, in the sideframes. Be sure to check the wires here.

Wear of the axle journals

The tips of the axles ride in plain bushings in the sideframes.

The motor block is literally suspended in the sideframes, which are affixed to the chassis, therefore, all the force of the weight of the loco, is at these bushings.

It is critical to keep these well lubed, and the grease seems to get "dirty" very quickly.

This may be because power pickup is also partially done through the axle tips also.

In any case, it is critical to keep them lubed, but they WILL wear just because they are not ball bearings and seem to accumulate dirt quickly.

Normally the wear is at the 12 o'clock position in the bushing in the sideframe, which will make perfect sense to you, that is the force of the truck pushing up against the weight of the loco.

Be CAREFUL not to crush the bearing when you turn it. I recommend using a crimping tool that has smooth curved jaws.

This wear does not seem to affect operation much, but you can carefully turn the bushing in the sideframe, try 180 or 90 degrees. Done before the wear gets really bad, you can get 4 times the life of these bushings by rotating 90 degrees. Wait too long and the wear won't allow you 90 degrees, 180 will be your only option.

Strange shorts or poor running


 Recently reported, excessive current draw and different running in one direction.


Notice the small metal posts to the right of the square housing that holds the motor? Right hand axle, there are 4 of these posts (actually the ends of heavy wire). Two of these are just to the right of the end of the motor, and two are just to the left the the gear casting.

Notice how the lower wiper wire (that is attached under the metal tab of the lower skate) is very close to one of the the posts? Potential for a short. You should inspect and clip the post if it is too close to the "whisker".

Also notice posts near the axle gear casting? No big deal unless you have the "cracked axle" problem and have put metal sleeves on the end of the gear casting to repair.

Now if the metal post touches the metal sleeve and the whisker, big problem. You can nip the posts near the motor shorter, and move the posts near the axle so stuff cannot short. Weird, but this has happened.



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