T138 sprag replacement and rebuild

[JasonS]

I have a bobbed M818 which I purchased two years ago. I found that the front axle did not "pull" when the transfer case was cold but did work when the transfer case was hot and the oil was thin (reverse works fine in all conditions). Since I had experience with deuce sprag clutch, I embarked on swapping the T138 sprag clutch (which, unfortunately, turned into a full rebuild; more details and explanation to follow).

For now, I just wanted to show what a worn sprag unit looked like. The 5 ton sprag is the exact same design as the deuce sprag; just different size and actuation method. The sprag clutch relies on some friction between the sprag segments, inner sprag race, and outer sprag race. This means that in forward motion, when the front axle is overdriving the transfer case, the forward sprags and races are wearing.

The wear mark towards the top third is from the forward sprag; I can just catch the grooves with my fingernail. The lower portion, where the reverse sprags ride is like new.



You can see the wear in the forward sprag as a shiny area across the top of the sprag segment.



This is the reverse sprag, note that there is very little wear.



This is a picture of a new sprag clutch (inner race and one clutch).

 

[JasonS]

The front drive section can be slide forward and out once the driveshafts are removed.


The sprag clutch can then be removed and replaced.


The front output shaft is piloted by the rear output shaft; these two didn't want to part ways due to wear. Note that this is steel on steel; no bushing.

 

[Wreckclues]

I found that the front axle did not "pull" when the transfer case was cold but did work when the transfer case was hot and the oil was thin (reverse works fine in all conditions).

Wow, how prophetic! Experienced the exact same symptoms with my M813 yesterday trying to climb an icy incline over a stone wall in my backyard. Although the rear wheels were spinning the front wheels wouldn't engage. Temps were in the teens and my gearbox oil was like Carmel syrup.
Looking into magnetic preheaters to start. Works fine in warm weather. Had to get a running start, didn't help that the rear NDT's are bald, weird feeling sliding backwards in a 5 ton

 

[JasonS]

So, the transfer case came out for repair. I used a transmission jack and engine hoist to get the transfer case on the ground; then put it on an engine stand.



When I initially bought the truck, it wanted to pop out of gear on occasion. I found that the input shaft was moving axially due to a missing spacer between the double roll ball bearing and the low range gear (#7 below). I fabricated a spacer, installed it, and all was good.


However, a lot of metal was sheared off and caused damage. You can see the three washers that I used as a replacement spacer below.



So, what happened to the missing spacer which caused all of this mess? It was placed between the front and rear output shafts. And, it appears that this is what bent the shift fork and didn't allow the front axle to be entirely in neutral.


I replaced the low range gear and the front drive driven gear, input and output shafts, both shift forks, and all bearings. The gears looked OK and the bearings really weren't too bad but I decided to replace anything which was suspect and I was able to save a LOT of money by ordering all of the bearings from eBay (NOS Timken).

 

[GopherHill]

Interesting and informative.
Thanks

 

[JasonS]

Wow, how prophetic! Experienced the exact same symptoms with my M813 yesterday trying to climb an icy incline over a stone wall in my backyard. Although the rear wheels were spinning the front wheels wouldn't engage. Temps were in the teens and my gearbox oil was like Carmel syrup.
Looking into magnetic preheaters to start. Works fine in warm weather. Had to get a running start, didn't help that the rear NDT's are bald, weird feeling sliding backwards in a 5 ton

New sprag clutch assemblies are available for $200 + shipping.

 

[silverstate55]

Great info and helpful pics. Thanks! Gotta subscribe to this one for sure

 

[US6x4]

So it was the transfer case that was popping out of gear? Like popping out of high and going to neutral?

 

[JasonS]

So it was the transfer case that was popping out of gear? Like popping out of high and going to neutral?

Yes, there was just enough axial movement for the synchronizer assembly to pop out of gear and into neutral. It only did it on rare occasion. After installing the homemade spacer, it worked as it should. While I was picking up quite a lot of metal on the drain plug magnet, the transfer case ran cool and I made several 125 mile trips without issue. I would have probably just continued to monitor and flush the oil had the sprag not been an issue.

One of the issues which is frustrating with "non-standard" vehicles (we have several) is the difficulty in getting parts or the correct parts. Things such as wrong exhaust manifolds, defective brake boosters, defective turbocharger, wrong transfer case shaft, eBay sellers not shipping at all let alone timely, etc, are par for the course. Owning a vehicle like this and properly maintaining it really has to be a labor of love, takes patience, and diligence. Here is another example... I need one copper washer for the air actuator as one was missing. Ten years ago, it was easy to get copper washers at NAPA; no longer the case. Found a pair on eBay; received yesterday and, of course, it was not the size listed. Back to searching Amazon which is, generally, the best place to find them.

 

[JasonS]

The only tricky part of the reassembly is setting the tapered roller bearings. The TM has two methods: the first requires a fixture which I don't have, the second involved removing all shims, torquing the bearing retainer, measuring the resulting gap between retainer and case with a feeler gauge, then adding 0.003" shims to that thickness. When I did this, the drag seemed high; and more than before I disassembled the transfer case. I could add 0.010" shims and dramatically decrease the drag without the bearings being obviously loose. I was not comfortable with this and contacted gringeltaube for his thoughts. He recommended using a dial indicator to set the axial movement to zero which is what I did. I was able to spin the assembled transfer case in low range with a cordless drill.

I used a harbor freight transmission jack to get the transfer case in place and a cherry picker for safety just in case. It actually went back in pretty easy.

The transfer case is in and I have made several shorter trips. The bearing retainer locations warm up to a temperature equivalent with the rear pinion bearing. I did measure with a temp gun, but since it is cold here, the absolute temperature of 140°F doesn't meet much.

I have a strong magnet on both the fill and drain plugs. The fill plug magnet has caught a very tiny amount of fine fuzz which I believe to be residual cast iron dust. I plan on monitoring for awhile for anything abnormal.