USAT Diesel Motor Block Tips


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. This makes good electrical pickup, but these need to be kept clean and lubricated. The lube will attract dirt and grit, so periodically pull the sideframes off the axle tips and clean them out. 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). He recommends vegetable oil, which will clean the mess up, but not attack the plastic.


Below is a list of common issues and how to address them.

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.

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

"Skate" problems


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

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:


 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

Often in older units a short has melted this 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.

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

Axles bushing improper reassembly / damaged gearboxes

Show bushing alignment for all versions.

show picture of right and rwong.

Emphasize gearbox destruction resulting from improper assembly

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.

Strange shorts or poor running


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


Notice the small metal posts just to the right of the square housing that holds the motor?

Notice how the lower wiper wire (that is attached under the metal tab of the lower skate) is very close to the post?

Apparently someone had put metal sleeves on the end of the gear casting, and in the process got the wiper wire to touch that metal post, thereby making a connection between the motor and a track pickup, big problem. You can nip the posts shorter so the wiper cannot touch it.



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