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I've acquired a diff from a 71 with 3.36 gears to replace my current diff with 3.08s in my 73. The stub axles had some end play and I've followed many of the differential threads and papers on the subject, Gary's threads in particular. I've watch Tom Eaton's video on posi tuning. The ring gear looks good, no evidence of pinion seal leak and the pattern is good so I don't think I'm pulling the pinion. Before I get started I want to be sure the stub axles are decent and I've noted the spec from snap ring to axle end should be .187. I pulled the axles and they measur .180 and .175. Can I make these work? Also, one of the axle dust caps is in rough shape. Is there a trick to getting this off or just dremel it off and press a new one on? Lastly, there's many comments on the hardening of the axles; losing the hardened surface as the axle end gets worn and I'm wondering how re-hardening an axle is done and the kind of shop that does it if needed. As always thanks for the help.
Assuming your 73 differential is mostly original, consider swapping out the posi carrier from the 73 with the 71. The 73 likely has the 3rd generation posi case and is stronger than the 2nd gen that is likely in the 71. Easy to check with the cover removed. Irrespective of which case you use, be sure to check it carefully for cracks.
Depending on whether you “tune” the posi, the axle measuring 0.180 may be ok, the 0.175 likely not. With the axles installed, measure the gap between the axle tip and the cross shaft and report back to the forum.
Find a friendly local machine shop and get the axles hardness tested to see where you are at. I’m sure you can find a company that does heat treating near you if needed. Nothing exotic about the thermal cycle needed to achieve the desired hardness.
I've acquired a diff from a 71 with 3.36 gears to replace my current diff with 3.08s in my 73. The stub axles had some end play and I've followed many of the differential threads and papers on the subject, Gary's threads in particular. I've watch Tom Eaton's video on posi tuning. The ring gear looks good, no evidence of pinion seal leak and the pattern is good so I don't think I'm pulling the pinion. Before I get started I want to be sure the stub axles are decent and I've noted the spec from snap ring to axle end should be .187. I pulled the axles and they measur .180 and .175. Can I make these work? Also, one of the axle dust caps is in rough shape. Is there a trick to getting this off or just dremel it off and press a new one on? Lastly, there's many comments on the hardening of the axles; losing the hardened surface as the axle end gets worn and I'm wondering how re-hardening an axle is done and the kind of shop that does it if needed. As always thanks for the help.
The 187 spec is one I posted years ago, it is not a GM reference from a print. It is meant to be a reference check only, as in if you had 160" or so it would be worn out. The spec was more than 187" in fact.
If you tune the posi it must be balanced, have no endplay, have no lash, have no binding and run-in about 800 times. The type of clutch used is important, you can't for example tune fiber clutches.
The last revision of the Eaton used in the vette actually started in late 71. The pictures of the posi cases shows the difference in the OD. They were stronger but still cracked. Your 71 posi may be the same as the 73 but the 73 will have better spiders and weaker clutches. The 69-mid71's had weaker spiders and better clutches. How it is built will make the most difference.
Axles can be through hardened, I do that all the time. I just finished this diff and it has original GM axles- not rebuilt, not cored, not Chinese. They are hardened and cryo treated. That is what I use now. The dust caps tap off with chisel and press on. Use Loctite primer and 271 on them. They have to be press fit on the axle, some of the junk sold today is not.
The .170 axle is probably going to give you a far amount of slop, which will affect the handling, since this part is basically a suspension mounting point. Ideal is almost no in/out movement of the axle stub.
Another alternative is a good machine shop should be able to machine the tip of your axle and insert a hardened valve shim, from a motorcycle or such. It would trade the hardening service fee for a machine shop fee. If yours is too short you may have to take this approach. Should work fine as long as the clearance came out OK. IIRC Gary prefers something like .006 clearance.
BTW tuning the posi section drastically tightens up the axle clearance. Mine actually became too tight. I had to have Gary cut my axles down a few thou.
Last edited by leigh1322; Jun 24, 2023 at 12:10 AM.
Thanks for the responses. Out of the two differentials I have three "decent" axles measuring .168; .175 and .180 from snap ring to axle end (the fourth one mushroomed badly almost to the snap ring groove). The measurement from axle tip to center pin is .035 for the .168 axle; .028 for the .175 axle and .025 for the .180 axle. I'm going to try tuning the posi for the 3.08 as a trial run before doing the 3.36.
You can buy a set of "Hardness test files" online that are not that expensive and work pretty well. At least you will know if you have worn thru the hardened section or not.
Differential Stub axle question
