Joe Inama

Little help. My memory is fading. What is the gear ratio split on axle ratios. when do you need a 4 series posi unit, after what ratio??? if I remove 4:11 ring and pinion do I need to change posi unit going to a 3:55. Thanks in advance

DZAUTO

You MUST, repeat, MUST use a 3-series carrier (posi unit) with 3-series gears (3.08-3.73). PERIOD. You CANNOT, repeat, CANNOT use 3-series gears on a 4-series posi unit. The reason is because the flange on the 4-series posi unit (where the ring gear is bolted to) is CLOSER to the center line and there is not enough room for side-to-side adjustment.

BUT, you can install a 4-series ring gear on a 3-series posi unit simply by installing a spacer between the flange and the ring gear--------------NOT THE BEST CONFIGURATION!!!!!!!! WHY??? Because to use a 4-series ring gear on a 3-series carrier (posi) with the spacer in between, it is necessary to use longer bolts----------------WHICH GREATLY INCREASES THE SHEAR FORCES ON THE LONGER BOLTS!!!!!! Translate: potential broken rearend.
Thus, bottom line: if you plan to change from a 4.11 to a 3-series (such as a 3.55)-----------BUY A NEW 3-SERIES POSI!

Here is a 3-series posi with a 4.11 ring gear bolted to it, with a spacer in between.

Joe Inama

Thanks that what I now remember. I would not use the spacer. Thanks a bunch.

street62

Some gear manufacturers make thicker ring gears so you won't need a spacer

409/409

But you still need longer bolts!!!

vettsplit 63

Unless you are going to ron slicks and launch it really hard, the longer bolts arent gonna give you any trouble with the thicker gear. I drove the snot out of a 55 Chevy rhat had a spacer and 4.56 gears, and tore up a lot of stuff, but never had a problem with the ring gear bolts

wmf62

with a thicker gear, longer bolts don't have any effect; they could be a mile long and it wouldn't matter... shear is shear and is only present at the mating surface...

with a spacer you MIGHT have differential movement at the carrier's flange mating surface, but the frictional forces of the mating surfaces, properly torqued, should negate that.


Bill

409/409

I'm not talking about shear. I simply said "you still need longer bolts".
The bolts should be as much longer as the thickness of the spacer to make sure you have enough threads.

SW Vette

Maybe a stupid question, but is it ok to swap a 3-series "pumpkin" into a 4-series diff case?

wmf62

sure, go for it...

Bill

DZAUTO

The outside case is the same for all rearends-------------------it's the guts that is different.

And of course, for every rule, there is always an exception. On the INSIDE of a NON-POSI 57-62 case (-64 for pass cars), there is a gusset which has to be trimmed is a posi unit is to be installed. But otherwise, NOTHING is different.

This is the gusset inside a 57-62 NON-posi case.



This is the gusset after it has been trimmed to accept a posi unit.



This is inside a 57-62 posi case. As can be seen, the gusset does not exist.

C2 Hot Cam Guy

Do you rebuild these since you have such knowledge. Can a non posi mid year be coverted to posi ? Thanks

DZAUTO

Been rebuilding them (62-older style) for over 50yrs.
For 63-later rears, the outer case is the same (for the year of concern). It's only the guts that are different. Thus, a NON-posi rear can be converted to posi simply by installing a posi unit. But I avoid 63-later Corvette rears ('cause I just don't like 'um) like the plague. Although I do rebuild GM 10 and 12bolt style rears.

Chevelle 10bolt



Chevelle 12bolt

Kerrmudgeon

Same here....went from a 3.08 to 4.10 with a spacer and beat the snot out it for over 15 years with no troubles.
I thought the cutoff was the 3.90s for case change? Memory is fleeting though.....

My addiction

Memory may be fleeting but your brain working fine! You got new eyes so that you can find the things you forgot where you put them! Makes perfect sense