Gear question
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Joined: Jan 2014
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From: Spring Hill Florida
Gear question
Currently I have the standard 3.70 in my L76 Roadster but would like to go to a 4.56. I understand there are different carriers and center sections, but don't know what one I would have or possibly need if I switched. Are the gears themselves relatable to a 10 or 12 bolt GM rear or special for the Corvette?
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Joined: Aug 1999
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From: Connecticut, USA
Currently I have the standard 3.70 in my L76 Roadster but would like to go to a 4.56. I understand there are different carriers and center sections, but don't know what one I would have or possibly need if I switched. Are the gears themselves relatable to a 10 or 12 bolt GM rear or special for the Corvette?
I don't know if any new vette 10 bolt 456's are left or made today. You can call Tom's differentials and see if they have any.
Depending on the power level of your car, trans, traction and usage, the 456 may turn your stock diff into "instant junk" unless you properly build it with good parts and procedures, and still nothing is bulletproof.
You want the best vette IRS diff with 456's, Tom's still has some 12 bolt 456 conversion gears. If you have never built a 12 bolt vette conversion, good luck. I have done a lot of them. They take a lot of time hand and machine fitting the parts- but they are the best. NOTE - 12 bolt conversions are no good with 63-66 housings in case that is what you have. Also, you will need a lathe, Bridgeport, and surface grinder and lastly the knowledge on how they are built.
Tech Contributor
Joined: Aug 1999
Posts: 15,176
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From: Connecticut, USA
65 Differentials were not strong. This was a transition year for the diff, moving away from the junk used in 63-4 but still carrying over some of the same issues. The posi case was changed to the first design Eaton but it was very prone to cracking. Once a crack forms and starts to travel it is a time bomb. It can not be drilled and welded it has to be replaced before the crack reaches the cross shaft bore and the case implodes.
If you find NOS or good used GM 456 gears you will need a 4 series posi case. The 4 series gears of that era were two designs, one bad and one better.
If you plan on racing the car you will need to make a lot of mods to keep it on the track, especially if it's original.
If you find NOS or good used GM 456 gears you will need a 4 series posi case. The 4 series gears of that era were two designs, one bad and one better.
If you plan on racing the car you will need to make a lot of mods to keep it on the track, especially if it's original.
Racer
Joined: Feb 2014
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GTR1999 is the resident expert on the C2-C3 IRS. His advice is gold. What are your intended use for this car? If driving is involved, a transmission swap makes sense. With your 3.70 gears, a close ratio TKX will give you 4.56 launching with its 2.87 first gear and 2.56 cruising with it .68 OD.
My money would be on a case swap in the ‘65 housing and a TKX. And that’s exactly what I did on my own ‘64.
My money would be on a case swap in the ‘65 housing and a TKX. And that’s exactly what I did on my own ‘64.
Drifting
Joined: Jul 2014
Posts: 1,555
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From: Massachusetts
GTR1999 is the resident expert on the C2-C3 IRS. His advice is gold. What are your intended use for this car? If driving is involved, a transmission swap makes sense. With your 3.70 gears, a close ratio TKX will give you 4.56 launching with its 2.87 first gear and 2.56 cruising with it .68 OD.
My money would be on a case swap in the ‘65 housing and a TKX. And that’s exactly what I did on my own ‘64.
My money would be on a case swap in the ‘65 housing and a TKX. And that’s exactly what I did on my own ‘64.
