8 3/4 Thrust Block

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Logan

Logan

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I've been reading about the 8 3/4. Specifically the thrust block used in conjunction with Mopar tapered roller axle bearings.

I don't understand how it is expected for the bearing rollers to be properly engaged in the bearing cup simultaneous with the tip of the axle pressing with the correct force on the thrust block. It seems that if the axle was only a few thousandths short the thrust block wouldn't do any good. Conversely, if the axle was a little long, the bearing wouldn't seat properly.

I'm plan on having my axle housing narrowed and from what I've seen about how it's done I worry about the necassary accuracy for this to work.

Am I missing something?
 
I had similar questions and needed axles and my honest opinion is that Dr. Diff is thee best guy out there , out there meaning right here in the USA.

check out www.doctordiff.com and call Cass for any tech questions
 
Ack. I had a browser meltdown. Yes, Cass is an excellent source for information.

The thrust block is two pieces that push in from each side, and meet in the middle of the carrier. They can slide back and forth in the center. As you tighten the bearing on one side, it pushes against the block, which slides against the other axle shaft, which will tighten the bearing on the other side.
 
I haven't contacted Dr Diff yet but planned to since I notice on the web site they will add spring perches. I had a rear once that has warped from too much welding on the housing and it would eat axle bearings for lunch! I want mine checked for straightness after all the welding is done.

On my initial inquiry I'm still stumped though. I realize the thrust block shifts to press against both axles but suppose the combined axle length was too great [or the axle tubes were a bit too short] preventing the bearings from seating fully. I just can't see proper seating of the thrust block and axles at the same time. Maybe the machining is just more precise than I imagined?
 
There is at least an 1/8" of adjustment on the stock setup. Open diffs have large contact surfaces and are one piece. The funny part is when you see the thrust block piece for a sure grip. It's a lightweight little thing that looks like a stretched out thread spool held together with a roll pin. On top of that, it floats around in the two cross side gear holders shafts (whatever they are supposed to be called). It's one of those things that seems weird but works great.
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If you are worried about proper bearing endplay then use green bearings. It elimantes adjusting the bearings all together. But just so that you know there good for strait line driving and not road racing.
 
Quote: I realize the thrust block shifts to press against both axles but suppose the combined axle length was too great [or the axle tubes were a bit too short] preventing the bearings from seating fully. I just can't see proper seating of the thrust block and axles at the same time. Maybe the machining is just more precise than I imagined?

That's why the axle bearing retainer on the passenger side is threaded. Install the axles both sides then screw the retainer in until both axle end bearing are seated in their cups (axle pushes against the thrust block which pushes against the other axle which seats it's bearing in the cup). Once the screwed in bearing retainer is snug and you've pulled back and forth on both axles and they don't move then back the retainer off until you've got .010" passenger side axle end play. Reinstall retainer lock.
 
68 superstocker said:
If you are worried about proper bearing endplay then use green bearings. It elimantes adjusting the bearings all together. But just so that you know there good for strait line driving and not road racing.
Click to expand...

I agree. Just use the greenies and forget about it. As far as them not being suitable for road racing, this may be true, but they are more than adequate for spirited street driving. So far, the rear end guru mentioned above, has sold me two sets knowing that they were going into street/strip applications.
 
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