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I'm finally going through everything in the back end, and have run into some surprises. My differential appears to be a 78/79 based on the stamped code "OH." No issues whatsoever getting it (or the trailing arms) out of the car, so that was cool. However, I've always had an issue with the left side yoke slipping out of the diff housing by a noticeable amount. Well upon inspection, and from what I gathered based on reading other posts here, the C clip came off at some point, then the yoke was hammered away until all that's left is what you see here.
I also saw that the diff cover had helicoils installed in the spring mount holes.
I have a local shop that can work on the diff that is reputable. But at the same time, knowing these issues may just be the tip of the iceberg, what's my best approach?
My goal is to have a rebuilt 3.55:1 differential to put back in when all is said and done.
Right now I'm looking at Zip Corvette, assuming I might get nothing for my core, and feeling like ~$1500 isn't a terrible proposition. WWYD?
Aren’t the rear differentials the same from like 65 to 79?
Call David Crane in SD and see if he has a good conditioned rear diff case.
You can order hardened yokes from Lonestar which will keep that C clip from coming off again.
A rebuild kit from Paragon is $180 and comes with Timkin bearings.
Order correct gears and find someone local or send it to Gary Ramadai (sp) and have him rebuild it. He’s supposed to be the best in the business and does a lot of the NCRS guys stuff.
edit* Gary’s handle on this forum is GTR1999
Id pm him before I do anything.
Common problem - GM did not properly heat treated the axles and the tips wear down. You need to be very careful about sourcing a replacement axle as there are horror stories abound about suppliers selling junk.
You might get by with a simple replacement of the axles but properly rebuilding the differential takes a lot of knowledge to do it right. If yours is original, it’s best to rebuild it as opposed to the crapshoot with a replacement. Your posi case, if original, is the strongest of the 3 versions GM produced and your gears, if original, are desireable (vs some of the repro sets available). Contact Gary Ramadei (gtr1999) and get his advice. He is the best source of knowledge and experience with these differentials.
Common problem - GM did not properly heat treated the axles and the tips wear down. You need to be very careful about sourcing a replacement axle as there are horror stories abound about suppliers selling junk.
You might get by with a simple replacement of the axles but properly rebuilding the differential takes a lot of knowledge to do it right. If yours is original, it’s best to rebuild it as opposed to the crapshoot with a replacement. Your posi case, if original, is the strongest of the 3 versions GM produced and your gears, if original, are desireable (vs some of the repro sets available). Contact Gary Ramadei (gtr1999) and get his advice. He is the best source of knowledge and experience with these differentials.
Finding good quality inner axle yokes is going to be your biggest issue. All 73+ ones are softer and wear badly. Yours are worn over .200" if the c-clip groove is gone. Good ones measure .185" or more above the c-clip. Earlier years were hardened differently and may not have much wear even after 100k miles. Your best bet may be to find some respectable looking originals. With meat left above the c-clip groove. If not worn too badly, the yokes can be properly heat treated. If some wear needs to be corrected, hardened inserts can be added. It is a lot of effort compared to buying some rebuilt or new chinese ones, but the track record on those is just awful.
PM GTR1999 he will be glad to help with your diff, he is the singular diff expert in our hobby. He is a master machinist that trained other machinists.
Common problem - GM did not properly heat treated the axles and the tips wear down. You need to be very careful about sourcing a replacement axle as there are horror stories abound about suppliers selling junk.
You might get by with a simple replacement of the axles but properly rebuilding the differential takes a lot of knowledge to do it right. If yours is original, it’s best to rebuild it as opposed to the crapshoot with a replacement. Your posi case, if original, is the strongest of the 3 versions GM produced and your gears, if original, are desireable (vs some of the repro sets available). Contact Gary Ramadei (gtr1999) and get his advice. He is the best source of knowledge and experience with these differentials.
crapshoot as opposed the the improperly heat treated Gm original junk that lasted 40+ years?
I looked for my 3.55 rear end for about 6 months before finding a freshly rebuilt unit for $500 on the facebook classifieds. I did have to drive almost 3 hours to get it but it was worth it. mine came with goodies like the heavy duty diff cover and new yet to be installed gaskets so I could examine the internals when I bought it.. If you look you can still find good deals out there.. I thought I saw zip offering rebuilt rear ends for under a grand recently?
Based on my experiences trying to save a few bucks, your best bet for the money is to have someone who's intimately familiar with how to properly setup a corvette diff do the work. These are not your run of the mill 10 or 12 bolt chevy rear. Vendor replacements are crap shoots at best. They're built to a profit margin. @GTR1999 is one of the best there is and he's very reasonable. I ended shipping mine to him and I have zero regrets and have had zero issues. I wouldn't do anything diff related until speaking with him. Good luck!
Differential Advice Needed
and tune it! 
