I'm faced with some unhappy and likely costly decisions regarding a solution to my desire to replace my non-original 3.70 open diffy with a 3.70 posi. I had purchased what was supposed to be a 3.70 posi from a forum member in a housing that was too far rusted to salvage so I also sourced a good used housing with the idea of rebuilding everything and selling my open 3.70 which was rebuilt with new bearings and seals approximately 10,000 miles ago.

Unfortunately the supposed 3.70 was actually a 3.36! This was discovered after opening it up and at that point the seller refused to take it back nor would offer any credit toward purchasing new 3.70 gears. My open 3.70 is a 64 model unit, the good used housing is also a 64 unit but I have no idea what year model year is the 3.36.

Zip offers completely rebuilt posi-traction units for a grand but they also require a good 1965 or later core or add a $500 core charge. I know there are other sources for rebuilt units other than Zip including some forum members with great reputations for quality and for units built to withstand considerably more horsepower than our cars came with originally.

Any thoughts about the Zip units? Recommendations for another supplier, perhaps one that will accept an earlier non-posi core?

Presently I'm leaning toward rebuilding the 3.36 in the 64 housing and selling both it and my open 3.70 to offset the cost of purchasing a commercially available rebuilt unit but I'm not optimistic about being able to sell either in today's market.

 

You may want to PM Gary GTR1999 and get his opinion. He will give you an unvarnished and professional advise. Good luck going forward. Jerry

 

Thanks, that was my first action.

 

A little off topic but the seller advertised a 3:70 and gave you a 3:36 without a refund or credit? Please list who they are in seller feedback section so we know who to stay away from please.

 

You might want to find out who does the rebuilds for Zip then talk to Gary. He has some stories about others company's

 

If I were to speak openly about what I have seen I would probably be banned.

If the gears are GM they have a date code on them, see if they were mixed up from different differentials- seen it happen.

63-64 original diff's were NOT good. They were DANA posi's and they were weak. Parts are not available for them and not worth it if you did find them on someoe's shelf. The 63-early65 pinions were what I call square shoulder and some broke in half.

65-68 were Eaton first design, prone to cracking and imploding used 10/18's ok but not the best.

69-late 71 were Eaton and a little better than the previous ones.

Late 71-79 were Eaton 2nd design, much better than the previous ones. 10/17's but had junk Snowflake clutches. some places still sell and use Snowflake clutches.

80-82 back to DANA and I think, just my opinion, they are also junk.

You will find places to sell you whatever you want to hear. Money over quality is the game today, bait and switch methods are alive and well.

The way I see it, you can go cheaper and maybe be fine. You can go better and while it may cost more upfront if you know what parts were used and procedures followed will be done with it. It is common to think diff's, boxes, and TA are all the same and price shopping it the best option, be careful.

 

I didn't know this existed......where exactly is it?

 

https://www.corvetteforum.com/forums...-feedback-153/

 

Thanks Robert I didn't know this existed for members. Good to know. Very interesting reads on that forum.

 

I recently faced a similar decision. Mine was easier because I'm rebuilding for reliability and I chose Gary.
Your decision is harder because you have a strong L79. How you plan to drive it? To me that will make the difference.

 

Gary spelled it out fairly clearly.

I wouldn't waste anymore money on the rears you have, just try to sell them as is. Even if you can find the parts for the 64 3:36 Posi, it's really not worth the money to try to rebuild it, you'll never get your money back. There's just not much market for 63-64 rears.

 

Actually I have 500+hp 406sbc but I've been driving it fairly aggressively with the open 3.70 for close to 10,000 miles without an issue. But, being an open diffy, it's got a built in fuse with the tendency of lighting up one tire quite easily.

 

The open differential is stronger than the Dana posi

 

Very sad to hear you were treated poorly by another member. I know we can’t do it, but it would be great to publicly shame the bum. God is watching him and justice will be served.

65air_coupe;1607016749]I'm faced with some unhappy and likely costly decisions regarding a solution to my desire to replace my non-original 3.70 open diffy with a 3.70 posi. I had purchased what was supposed to be a 3.70 posi from a forum member in a housing that was too far rusted to salvage so I also sourced a good used housing with the idea of rebuilding everything and selling my open 3.70 which was rebuilt with new bearings and seals approximately 10,000 miles ago.

Unfortunately the supposed 3.70 was actually a 3.36! This was discovered after opening it up and at that point the seller refused to take it back nor would offer any credit toward purchasing new 3.70 gears. My open 3.70 is a 64 model unit, the good used housing is also a 64 unit but I have no idea what year model year is the 3.36.

Zip offers completely rebuilt posi-traction units for a grand but they also require a good 1965 or later core or add a $500 core charge. I know there are other sources for rebuilt units other than Zip including some forum members with great reputations for quality and for units built to withstand considerably more horsepower than our cars came with originally.

Any thoughts about the Zip units? Recommendations for another supplier, perhaps one that will accept an earlier non-posi core?

Presently I'm leaning toward rebuilding the 3.36 in the 64 housing and selling both it and my open 3.70 to offset the cost of purchasing a commercially available rebuilt unit but I'm not optimistic about being able to sell either in today's market.[/QUOTE]

 

Oh that's interesting. Didn't consider there might be a difference. After finding myself in this situation, I've considered just putting it back in and saying the hell with it, I don't need no stinkin' posi!

 

Purchased a rebuilt diff off ebay with a good seller my car 64 coupe and the diff was from a 67 bb-looked great and new-installed it and when i drove it it made so much noise and would not go away-seller refunded my money great except all the labor of laying on the floor twice so point is pay the extra money and get a very very good rear end and please post where you will be getting your next one from.

 

Yeah, I've been burned once and not going to open myself up to that possibility again. I thought I'd be safer buying from a forum member but I guess not.