P6 thrust bearing dodged a bullet…

Ronmar · Sunday at 6:04 PM

I am very glad I did this! Looked great inside, popped off the outer clutch pack snap ring and lifted out the clutch pack and carrier, discs and spacers look great. Center tower bushing, springs and piston look great, roller bearing looks OK until I try and lift it out.. The front/outer race and needle carrier lifted right off with no resistance leaving the inner race behind on the center tower…

It was just a matter of time before the outer race and needle carrier popped forward off the inner race and down to rest on the carrier. Then the lip on the inner race and crimps that are supposed to hold it all together, would have methodically chipped away at the composite roller bearing carrier/spacer every time the clutch carrier slid back against it, until it released the rollers to thrash around in the clutch-pack and on into the rest of the transmission…

Whew!

Inner race is still on the center tower.

after I lifted out the thrust bearing inner race…

MugOfPaul · Sunday at 7:18 PM

Are you going to replace with the composite washer “upgrade”?

Ronmar · Sunday at 7:36 PM

MugOfPaul said:
Are you going to replace with the composite washer “upgrade”?

Sort of

Plasa · Monday at 4:34 AM

Do you have a transmission that is affected by the Allison letter to change the bearing if transmission is before a certain serial number?

Ronmar · Monday at 11:31 AM

All transmissions prior to ~Feb 2011(need to check s/n on the bulletin) are potentially affected by this...

Here is the basic chain of events. In the 90’s they produced with a particular p/n bearing, lets call it A. I think they knew about issues then, because even the military A0 LMTV manuals produced in the 90’s say to replace this bearing WHENEVER you are in the transmission/transfer case for any type of work.

At some point Allison officially acknowledged the failure with a tech bulletin stating that the type A roller thrust bearing should be replaced with a improved roller thrust bearing with a new P/N, lets call this one B… they also listed a s/n break point where production included bearing B.

In Feb 2011, I assume because bearing B was also failing or they suspected a possibility of failure, they issued a second tech bulletin directing that you replace bearing B with a composite thrust washer 3 times as thick as the A or B roller thrust bearings… They again listed a S/N breakpoint where this new thrust washer was included in production…

I have no idea if a bearing B has ever failed, but Allison said replace it with a thrust washer for some reason, and they shifted production to the new thrust washer…

Here is a link to the most recent failure discussion. It has links to previous discussions and the tech bulletins…

1998 LMTV Hot transmission - No drive

Hi everyone, I am in the process of un hack jobbing my 1998 LMTV, previous owner did some questionable things, I've already replace the motor due to a cracked block. I had the driveshafts balanced and then did a test drive, transmission felt great - smooth shifts and no issues over 50 or so...

www.steelsoldiers.com

NDT · Monday at 12:41 PM

If you cut the bed away to allow cover removal, how many hours are we looking at to do this repair? Do we have to drain the fluid?

Ronmar · Monday at 3:15 PM

it depends on your vehicle...
I believe you must drain the transfer, but that's the easiest part

. You don't need to drain every last drop, just till the flow slackens near the end and put the plug back in, maybe 3.5 gal. (clean 5gal bucket, 3/8 drive ratchet or breaker bar, about 5 minutes)...

A 1078 you can trim the bed folded sheet metal crossmember. you can also unbolt the bed and lift the front 3-4" over the upper frame rail. Depending on your available tools and skillset, either will take perhaps an hour or longer. A plasma cutter would take perhaps 10 minutes to setup and cut out the interfering material... you need to be able to move the P6 clutch assembly~6" to the rear to slide the clutch center carrier off of the protruding center splined shaft...

There is a lower bolt-in crossmember in the main frame but it is far enough back to not be much of a factor except to reach around to access hardware or manipulate the assembly while lowering it out... not much of an issue...

If you have a 1079, get ready for the pain. I own a 1079 chassis... There is a heavy welded crossmember(same channel material the upper frame rails are made out of) in the upper frame, approximately 3" behind the P6... Besides having a m1079 box on top of it, it is huck bolted to the main chassis frame with 18-20 fasteners. you need to remove those fasteners and lift that upper frame rail ~8", or slide it 6" to the rear, to clear the P6 enough to remove it. The other option is to cut it out and turn it into a bolt-in crossmember... It is 1/4" 2" X 6" channel, so no easy task with the 1079 box in place, then prepping and drilling the remaining ends for a bolt plate or welding it back in...

Since I am converting that upper frame rail into my habitat floor, I did not want to cut that crossmember. but since I am making it a hydraulic captured spring habitat to chassis connection, I had already cut all the huck bolts out. I simply had to unbolt the front and lift the habitat floor and cargo boxes 8" to clear the P6.

Once the interferences are out of the way, Besides the 5 minutes to drain some oil, with a powered ratchet and a breaker bar, it is under an hour to remove all the hardware and pull off the p6 assembly(there is a notch on top to insert a screwdriver to pry/loosen the seal on the p6 cover, providing all the hardware spins out OK(15MM bolt heads, in 3 different lengths). It weighs ~55# but it is supported by the center shaft and a case guide ring, so you can get all the hardware out easily before you try and move it(nothing will freefall

). not horrifically heavy and not until you slide it off the center shaft.

On the bench, 2 minute disassembly. Remove the upper/outer snap-ring with a screwdriver, and lift the center carrier and all the rotor and stator discs(perhaps 25#?) straight up and out of the housing and set aside to inspect later. That will leave you staring at the P6 thrust bearing on top of the tower like in the above pics...

Good luck if you should decide to accept this mission... This tape will self destruct in 5... 4... 3...

MatthewWBailey · Monday at 8:50 PM

Ronmar said:
it depends on your vehicle...
I believe you must drain the transfer, but that's the easiest part . You don't need to drain every last drop, just till the flow slackens near the end and put the plug back in, maybe 3.5 gal. (clean 5gal bucket, 3/8 drive ratchet or breaker bar, about 5 minutes)...

A 1078 you can trim the bed folded sheet metal crossmember. you can also unbolt the bed and lift the front 3-4" over the upper frame rail. Depending on your available tools and skillset, either will take perhaps an hour or longer. A plasma cutter would take perhaps 10 minutes to setup and cut out the interfering material... you need to be able to move the P6 clutch assembly~6" to the rear to slide the clutch center carrier off of the protruding center splined shaft...

There is a lower bolt-in crossmember in the main frame but it is far enough back to not be much of a factor except to reach around to access hardware or manipulate the assembly while lowering it out... not much of an issue...

If you have a 1079, get ready for the pain. I own a 1079 chassis... There is a heavy welded crossmember(same channel material the upper frame rails are made out of) in the upper frame, approximately 3" behind the P6... Besides having a m1079 box on top of it, it is huck bolted to the main chassis frame with 18-20 fasteners. you need to remove those fasteners and lift that upper frame rail ~8", or slide it 6" to the rear, to clear the P6 enough to remove it. The other option is to cut it out and turn it into a bolt-in crossmember... It is 1/4" 2" X 6" channel, so no easy task with the 1079 box in place, then prepping and drilling the remaining ends for a bolt plate or welding it back in...

Since I am converting that upper frame rail into my habitat floor, I did not want to cut that crossmember. but since I am making it a hydraulic captured spring habitat to chassis connection, I had already cut all the huck bolts out. I simply had to unbolt the front and lift the habitat floor and cargo boxes 8" to clear the P6.

Once the interferences are out of the way, Besides the 5 minutes to drain some oil, with a powered ratchet and a breaker bar, it is under an hour to remove all the hardware and pull off the p6 assembly(there is a notch on top to insert a screwdriver to pry/loosen the seal on the p6 cover, providing all the hardware spins out OK(15MM bolt heads, in 3 different lengths). It weighs ~55# but it is supported by the center shaft and a case guide ring, so you can get all the hardware out easily before you try and move it(nothing will freefall). not horrifically heavy and not until you slide it off the center shaft.

On the bench, 2 minute disassembly. Remove the upper/outer snap-ring with a screwdriver, and lift the center carrier and all the rotor and stator discs(perhaps 25#?) straight up and out of the housing and set aside to inspect later. That will leave you staring at the P6 thrust bearing on top of the tower like in the above pics...

Good luck if you should decide to accept this mission... This tape will self destruct in 5... 4... 3...

Your theory in a previous thread was that the moving drive gear is heavy enough to slam into this thrust bearing when engaging 1. Is that perhaps the reason for the thick composite bearing?

Ronmar · Monday at 11:13 PM

Now that I have had a chance to look at the setup with my own 2 eyes, there is nothing I can see that would push the carrier race back against the thrust bearing. The P6 piston pushes the disks forward and away from the thrust bearing, so the only possible effect would be to hold the carrier where it is at, as the discs bind together and apply load to the splines between case and carrier with torque applied… or, since the rotating discs ride on the carrier I believe it would tend to push the carrier race forward, away from the thrust bearing Before the disks, carrier and shaft bind together completely. I certainly don’t see it applying any bearing load…

Now it is possible that when the piston releases the discs, the whole assembly(carrier, rotors and stator discs, ~25#) could possibly rebound, or even be pulled slightly back toward the thrust bearing, by oil suction/surface tension on the piston face to the discs, as the piston retracts. As soon as there is not enough disc compression/friction to hold the discs, they and the carrier on the shaft will start to spin, since the carrier probably cannot move under tension/while clutch is locked, I think you would probably lose any oils suction before it could move. The discs and carrier starting to spin again could possibly create a push of the carrier to the rear as the discs spread out...

Force = Mass X Acceleration… i can pull the center carrier approximately 1/2” forward from where it was resting on the roller thrust bearing, until it brings all the discs into contact/tension. If the acceleration is inertia related(vehicle movement/clutch rebound) the shorter the distance it has to accelerate, the less energy it can deliver, so a taller bushing(.335”) would restrict it’s movement significantly(nearly cut it in half?)… Even a gentle movement with a 25# sledge is quite a bit of energy...

87cr250r · 2025-08-19T00:25:50-0400

I don't know anything about this transmission but...

Roller bearings don't like to run unloaded. If there isn't a constant force pushing on them that can cause trouble. A plain bearing is just as happy loaded as it is unloaded.

Ronmar · 2025-08-19T00:28:56-0400

87cr250r said:
I don't know anything about this transmission but...

Roller bearings don't like to run unloaded. If there isn't a constant force pushing on them that can cause trouble. A plain bearing is just as happy loaded as it is unloaded.

Yep, I also think roller bearings probably don’t like being repeatedly accelerated by random contact and then released… this doesn’t bother plain bearings either

87cr250r · 2025-08-19T00:50:18-0400

The gentle tap with a big hammer is an interesting phenomenon. Some of my tugboats have a ~6" diameter 20' long solid steel shaft connecting the engine to the drive. The shaft uses split cylindrical roller bearings which use 10-32 screws to clamp the inner race to the shaft. They have a very limited axial load rating of around 5000lbs. Every once in a while one of these shafts slips forwards. Other companies have blamed some random phenomenon like incorrect warm-up of the engines. The reality is that the captains were hitting the ships too hard. With only a knot or so of speed that 2000lb hammer could generate a lot of force.

MrGoldman · 2025-08-19T17:58:22-0400

Ronmar said:
Sort of

Do tell.... I just spent almost $500 on a damn thrust washer today....

NDT · 2025-08-19T18:44:14-0400

MrGoldman said:
Do tell.... I just spent almost $500 on a damn thrust washer today....

Omg the Allison part costs that much???

Ronmar · 2025-08-19T19:14:24-0400

Yes it costs that much… it took me a half hour to cut a blank out of aluminum plate with a pair of hole saws and about a half hour to turn it into a bushing on the lathe…

MugOfPaul · 2025-08-19T20:30:52-0400

Oh I just remembered I took photos when I inspected my 2007 A1R a few months ago.

87cr250r · 2025-08-19T20:33:53-0400

An aluminum bushing?!?

NDT · 2025-08-19T20:45:33-0400

So your aluminum bushing you made is to duplicate:
"composite thrust washer 3 times as thick as the A or B roller thrust bearings"

Extra points if you share the dims of the bushing you made .... and which alloy

Skyhawk13205 · 2025-08-19T20:54:43-0400

87cr250r said:
An aluminum bushing?!?

It just needs to take up the space, and be softer than the contact metals. Most thrust washers I have seen are usually brass, but I don’t see why aluminum would not work. It is prob comparable to Allison’s plastic washer. It also has the ability to not deteriorate with high temps or breakdown from organic solvents.

Ronmar · 2025-08-19T23:40:16-0400

Yep, Aluminum… Whats it going to do, void the warranty

the Allison part looks like a fiber re-enforced thermoplastic, probably popped out of a injection mold machine run by Chinese slave labor for .50 each…

There is nothing I can find in this design that forces the clutch carrier back against the bushing. I think it only experiences casual intermittent contact…

Since the pressure lubricant flows in from the rear, around the end of the center shaft(past its bushing) and out to the top lip of the center tower that the bushing slides over, the bushing is basically under a waterfall of lube, in fact when the center carrier race slides back to contact the bushing, the bushing will basically be pressure lubed as that space where front race contacts the bushing is the only real way for the pressurized oil to leave the center tower except perhaps along the shaft splines…

pretty certain it will last longer than I do

As for dimensions, well Allison was kind enough to provide the new bushing thickness, 8.5mm or .335”, in the second bulletin. Also that it is 4.9mm thicker than the original which was ~3.5m…. The roller bearing is 1.992ID and 2.862 OD, but I am machining the bush to a slightly larger OD as the races it sets upon appear to be 2.920.

only partial extra points as I can find no alloy stamps on the piece of 1/2” plate I have available… it drills and machines easy enough so it is not one of the harder alloys, but not as soft as plastic… I might be more concerned if there wasn’t so much lube available. I do not expect much wear…

P6 thrust bearing dodged a bullet…

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