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confused about differential preload

May 9 2008 at 3:57 PM
kevin cozzo 

woo hoo, I finally understand how synchros work after a lot of studying, and reading, and now have the gear clusters all reassembled and a gasket kit on the way. I decided to strip the whole case down, and deep clean, and paint it, upon doing so I discovered a crack in the case, right next to where the axle stubs bolt to the axles, crack was all the way through. I had another 5 speed, and took it apart, to use the other case half, funny thing is it ALSO had a dent in the same place, looks like the axle may have come loose at one time and the CV joint slammed it
Anyways, my question is how to set preload for the differential, or if I even should, the races do have some very light pitting and I'm thinking of replacing the bearings, I see from the manual how they say to do it, but isn't the race gonna be tight. I'm guessing that I need to reassemble the case halves with the new races/bearings in place, and the race retainer removed, and bolting the case halves together will push the race out, then use the old shim to measure the difference, and get a new shim,is this correct?
BTW, I found another way to remove the axle stubs without a slide hammer, just use long 13 mm bolts to push the shaft out against some wood wedges
Thanks in advance for any advice, Kevin Cozzo

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if there is any doubt,

May 9 2008, 4:12 PM 

replace the diff bearings.

Assemble with no shims, measure the gap - say .010". If you need .005 preload, add the gap plus the preload (.010 + .005) and shim by that amount.

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By the book

May 9 2008, 4:42 PM 

By the book worked beautiful for me. I obtained a whole stack of different thickness shims and ended up using the very same thickness upon reassembly. Two new TIMKEN type bearings installed.

As you have probably found....the measuring/determining task is really pretty easy. The jump to the next size available shim will seem WAY OUT of tolerance.

lezesig '79 X 1/9

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kevin cozzo

ok, I see now,but..........

May 9 2008, 7:24 PM 

I spotted Bernice's description in "best of" Now I'm concerned about the mix and match on the case halves, and Isn't the reverse shaft a different dia, from 79 to 86, don't know if my guts will fit, guess I better go test fit some parts

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Mixing Transaxle Cases

May 11 2008, 8:36 AM 

As mentioned by others, I don't recommend this since they are line bored during production. If you cannot get the case welded (make sure the case is well baked prior to welding due to porosity and oil absorption), try getting another case in better condition, they are not that difficult to locate.

Check to see if the corners on the diff carrier tapered roller bearing cover are not warped inwards as this is common and if bearing pre-load measurement is made at the warped corners, it won't be accurate. The amount of "corner bending" could equal or exceed the gap pre-load spec in some cases.

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Andrew Patterson

Aren't you not supposed to mix and match case halves?

May 9 2008, 4:46 PM 

Thought I had read that somewhere; the reason being that they are bored together as one unit. So another half may not match up as exactly as it should.

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kevin cozzo

um, didn't think bout that.........

May 9 2008, 5:15 PM 

still got the other side if I need it

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Andrew Patterson

Might be OK

May 9 2008, 8:03 PM 

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kevin cozzo

well that's cool, but.....

May 9 2008, 10:02 PM 

the extra trans parts are off a 79, the one I'm trying to save is from an 86. The reverse shaft on the 79 is smaller in diameter than the one in the 86, long story short, the reverse shaft and gear have to stay with it's own case, and reverse on the 79 is trashed. I was thinking about taking the whole mess down to my machine shop, and having them cut down the diameter on the end of the shaft that would fit into the 79 case halve ugh, what to do, Kevin Cozzo

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I'd machine the other way

May 10 2008, 5:44 AM 

Don't cut down the reverse shaft, instead bolt the mismatched case together and have the 1979 half measured for alignment, and rebored if necessary.

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kevin cozzo

that would be a no-no

May 10 2008, 8:41 AM 

Was thinking about that, but the 86 case has some extra metal around the hole for the reverse shaft, kinda reinforced, because the larger hole is a lot closer to the input shaft bearing. I guess the cheapest thing to do is to get a new reverse idler (shaft and gear) for the79 case, but I would really like to use the thicker shaft from the 86, they beefed it up for a reason

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The idler shaft was beefed up because...

May 10 2008, 9:06 AM 

The original reverse idler shaft would bend if too much torque was put through when in reverse gear. Gears tend to want to push apart when transmitting torque, more so with more torque applied. If one tried to push start the sohc Fiat in reverse for instance, this could overstress the reverse idler shaft and bend it such that the idler gear moved away from the other two meshing gears.

The beefier idler shaft can safely be turned/ground down at the end to fit the early gearbox, but thin down only enough length to fit in the bush, and be generous with the radius where thin becomes thick. The extra meat to resist bending is only needed around the middle third of the shaft's length - the extra thickness is redundant at the ends.

Cheers R

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weld up the boss and rebore.

May 10 2008, 10:24 AM 

If the case half alignment isn't correct, that would be the procedure for the mainshaft and layshaft bearings.

Trying to refit the 79 reverse idler is a fool's errand (IMHO)

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Mixed case halves worked for me when I had to do it

May 10 2008, 5:19 AM 

It shimmed out just like Lezesigs, the same LOL

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kevin cozzo

answer to my own question

May 10 2008, 8:59 AM 

called Robert out at shadetree enginetrics, he said he could weld up the crack, for about 20-30 bucks, so I can still use old case

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Steve Hoelscher

I was going to suggest this...

May 10 2008, 2:53 PM 

This is a common place to crack the case and is easily welded.

As for using mismatched case halves, it does work but because the cases are bored while bolted together, the line bores for the shift rails, reverse shaft, main and lay shafts, these may not line up properly. On some miss-matched assemblies, I found the shift rails didn't move smoothly and at least one which had excessive gear and bearing whine no matter what gearset or diff was installed. I determined that inaccurate alignment must have been the cause.

Shimming the diff for preload is a fairly simple process. Remove the left side bearing/seal retainer. Its held on with four bolt. Leaving the rest of the internals out, install the diff and bolt the two case halfes together.

Install the left side outter diff bearing race. Tap it into place using a brass drift or flat nosed chisel. Make sure it is fully seated against the diff bearing. Now install the factory bearing shim and the bearing/seal retainer and its bolts, torquing the bolts to 18 ft/lbs.

Fit the two axle stub shaft in the diff only far enough to engage the splines and turn the diff a couple of complete revolutions. Unbolt the bearing retainer and use a feeler gauge to measure the clearance between the back side of the bearing retainer and the mating surface of the case.

The clearance should be between .003 and .005 inches. Shim accordingly. The thing that makes the process difficult is sourcing shims of the appropriate thickness. .010 is way too much preload and will overheat the bearings. Zero preload with wear the bearings excessively along with the pinion gear. Less than zero preload will result in the axle seals leaking excessively along with the other problems.

Hope this helps.


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Dave Evans

bearing preload after rebuild

May 10 2008, 7:49 PM 

Steve, while you are on the subject of diff bearing pre load, I have another question. Do you need to check pre load after splitting the case to perform other work?

I did some work on my trans a little while ago and put in 3 new bearings and a neutral spring. Used a new gasket set and bolted it back together. Should I have checked the pre load? I find now that I have a bit more neutral gear rattle than before and am a bit concerned because I can hear what sounds like gear rattle while the car is decelerating mostly in first and second gears. This noise goes away if I apply power while in these gears. Is this anything to be concerned about? Can I check preload with the trans in the car?

I have MTL in the trans now and was wondering if going to MT90 which is thicker might help.

Dave Evans
'79 X

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