USAT Diesel Motor Block Tips


In general these are rugged and long lived, even though no ball bearings in the drivetrain anywhere. They come in a 2 axle variety, and a 3 axle variety, which is a 2 axle block with a pivoting 3rd axle attached.

The designs are simple, rugged and pretty foolproof. Also parts are normally readily available, and Mike in USAT parts is exceptional which is a boon to people who have struggled to get the right part for any other loco.

USAT diesels use a sturdy motor, and this motor can draw a lot of current, specifically it's stall current is high. This is a factor when using decoders, make sure you are running a true 4 amp or greater continuous decoder. Soundtraxx decoders are a poor choice here.

The design uses a plastic axle with the gear cast into it, then 2 half axles with the wheels pressed on. These plastic axles ALWAYS crack after a while, but there are parts available and a simple fix that fixes them permanently. On the good side of this design you can adjust the gauge easily.

Most of the locos will have one axle in a motor block with traction tires. This can really help traction, but it has other downsides, you can convert to all "solid" wheels easily.

At the end of the motor block there are 4 pins, the outer 2 pins are the motor, and the inner 2 pins are the track pickups.

Power pickup comes from several places. First, there are the 2 skates on the motor block. Next, there are brass journals in the sideframes, with wires attached. The power from the wheel goes to the axle tip, into these journals. In addition, in the 2 axle blocks, often there are 2 "whiskers" that rest on the axles to pick up power.

2 axle motor block:

In the 2 axle blocks, the axles are held rigidly on the block, running in brass bushings.(The NW2 and S4 do not have these bushings, the axles ride directly in the plastic motor block casting !!)

In this picture you see:

  • power pickup skates
  • power pickup "whiskers", note how they rest on the axles, clear of the brass bushings
  • the motor, and a bit of the worm on the motor shaft
  • the black plastic worm gear, with the half axles pressed into it, and the brass bushings which are wheel bearings


 The motor blocks for the F units, the GP9, are the same, there is a smaller version for the NW2 and for the Mighty Mo.

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.

It appears that the same motor block, with the same axle spacing (2.835") is used on all the 3 axle diesels, but the gearing and the wheel diameters vary.

Note that the 3 axle blocks have the extra idler gear shaft. No room for the power pickup whiskers apparently,.

USAT SD40 axleMotorBlockLidOffSkates P7020014


One thing not shown is that the axle tips run in brass bushings in the side frames, not ball bearings. The weight of the loco is through these bearings.

The brass bushings between the wheels inside the motor blocks do NOT support the loco weight, just the thrust of the gears. All the weight of the loco is supported by the brass inserts in the side frames that the axle tips ride in.

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.

When these bushings get worn, you can rotate them 180 degrees and get basically a new bushing, since the weight of the loco is always pressing down. Alternatively, drilling out the side frame a bit you can fit a 3mm x 6mm x 2.5mm ball bearing. You may have to sand down the axle tip a bit. You can find these ball bearings on eBay.


Basically you remove  the bottom covers to access the axles and gears. The design of these motor blocks suspends the weight of the loco through the axle tips in the side frames.

Read carefully: the motor block itself is NOT connected to the loco... it "floats" around the axles.

You remove the motor block by either spreading the side frames to release the axle tips, or unscrew one side frame (the 3 axle blocks need the 3rd floppy axle released first)

BEFORE removing the bottom cover screws, use rubber bands from the axle tips around the backside of the motor block to hold the axles in place. The #1 cause of issues with people working on these motor blocks is getting the axles in wrong, i.e. the axles moving our of place when re-assembling.

(photo courtesy Ted Doskaris)


The most important thing is the alignment of the brass bearing/bushing in the motor block and it is DIFFERENT between 2 axle blocks and 3 axle blocks.

In the 2 axle blocks, the "flat" of the bushing is parallel to the bottom cover

In the 3 axle blocks, the "point" / "corner" of the bushing is pointing outwards.

(Who in the hell designed this???)


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 axle 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

Below is a video of a loco with split axles and also internal damage from improper reassembly

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.

The video above had this problem, and in the case of 2 axle blocks, put in wrong will put the gears too far apart and destroy the gears.

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 design!).

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.



Below is a picture of a 44 Tonner that was clearly improperly assembled. Look closely and you can see all kinds of things:

44tonner destroyed

First, when put back together NONE of the brass bushings were installed properly. The "points" are up as opposed oriented to be "flat" where the lid goes into place. The bushings are not only improperly oriented, but look at the lower right one, it's not even in the "slot" for the bushing. So the gear on the right axle has a half moon cut out of it by the worm (on the motor shaft).

Another assembly mistake is that the 2 wires that pick up power from the axles (under the 2 metal clips that have the black and red wires) are not touching the axles, mainly due the the brass bushings being too far inboard.

You can also see what must have prompted the disassembly in the first place, look at the upper half of the left axle, it is cracked from the gear up.

So, read and study and be careful, or your "foray" into your motor block will wind up the same way.

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 was a REALLY DUMB decision USAT!) In any case, if you assemble these wrong, you will be "crushing" the gears together, and will destroy the gears in short 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 for repairs/replacement:

  1. Remove the motor block from the side frames first:
    1. For 3 axle, you need to disengage the "floppy" axle from the plate it slides in, and attaches to the chassis
    2. Next you can either unscrew one side frame, or bend the side frame out to clear the axle tips from the side frame journals
  2. Now 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.
  3. Finally you can unscrew (carefully and gently) the gearbox cover.

Do this right and you won't disturb anything. It's almost impossible to reassemble the gearbox without something to hold the axles in place. Improperly assembled, the gearbox can destroy itself in just a few minutes.

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 locos 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.



If you need to replace the slide shoes please follow these instructions carefully:
1. Unscrew the screws on the truck bottom plate and carefully take off the bottom plate cover.
2. Lift out the slide shoes. Remove the springs and install them on the replacement shoes. Reinstall the slide shoes.
3. The slide shoes will have to be raised slightly as the bottom plate cover is being seated on the truck. There are tabs on either side of the bottom plate to hold the slide shoes in position. The bottom plate cover must sit flat on motor block before securing with screws. If bottom plate cover is not sitting flush the axle bearings are not seated properly.

NOTE: Gears will strip if axle bearings are not seated properly. There should be no need to interfere with the axle and gears but in the event of accidental movement the axle bearings might become displaced from their sockets. The large axle bearings must be seated at 45° angle and smaller axle bearings are seated flat.

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 journal bearings

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.

Weather Underground PWS KCACARLS78