Posi series 3 versus series 4

[Joel T.]

Hi;

Can anyone fill me in as to what the differences are between Posi "series 3" versus "series 4" units?? Are there particular years/engines particular to each??

Also, would anyone have a procedure for properly shimming a replacement pinion and ring gear set?

Thanks,

Joel

[Chuck G.]

Re: Posi series 3 versus series 4

3 series carriers are used with rear ratios of 3.08 to 3.70.

4 series carriers are used with 4.11 and 4.56 ratios.

You can use a 4.11 gear with a 3 series carrier but you need a ring gear spacer. They're about 1/4 inch thick as I recall. All the speed shops used to sell them.

4 series ring gears are thinner than 3 series ring gears.

You cannot used a 3 series ring gear on a 4 series carrier.

Confused?

Chuck

[Joe L.]

Re: Posi series 3 versus series 4

Hi;

Can anyone fill me in as to what the differences are between Posi "series 3" versus "series 4" units?? Are there particular years/engines particular to each??

Also, would anyone have a procedure for properly shimming a replacement pinion and ring gear set?

Thanks,

Joel

Joel-----


The difference is the thickness of the flange on the posi case where the ring gear mounts. If I recall correctly, the "4" series case uses a thicker flange to "compensate" for the fact that 3.90:1 and numerically higher ring gears are thinner than those used for lower numerical gear ratios. The "3" series case is used for differentials with ratios of less than 3.90:1; the "4" series case is used for differentials with a ratio of 3.90:1 or numerically higher. It has nothing to do with engines or year model.

Complete procedure for differential set-up is included in the Chevrolet Service or Overhaul Manual. However, special tools are required and I do not recommend this work be performed by the inexperienced.

[Joel T.]

Re: Posi series 3 versus series 4

Hi Chuck...

Yes!!!! Would you happen to have anything on proper shimming?? I pulled the carrier out of my 63... it was originally a 3.70 (stamped code) but "Bubba" swapped gears for 3.36's which hurt drive ability (after I installed a close ratio T10). I replaced the whole carrier but want to put the original back in with 3.70's. Everything I read talks about proper gear mesh and shimming.

Thanks!!

Joel

[Duke W.]

Re: Posi series 3 versus series 4

3 series carriers are used with rear ratios of 3.08 to 3.70.

4 series carriers are used with 4.11 and 4.56 ratios.

You can use a 4.11 gear with a 3 series carrier but you need a ring gear spacer. They're about 1/4 inch thick as I recall. All the speed shops used to sell them.

4 series ring gears are thinner than 3 series ring gears.

You cannot used a 3 series ring gear on a 4 series carrier.

Confused?

Chuck

The proper term for the machined casting that contains the differential gears/positraction clutches and ring gear is differential case.

The differential carrier is the external housing that contains the case and drive pinion.

See any COM. All '63 to '79 Corvettes use essentially the same carrier, but there are two cases as stated.

Duke

[Chuck G.]

Re: Posi series 3 versus series 4

Hi Chuck...

Yes!!!! Would you happen to have anything on proper shimming?? I pulled the carrier out of my 63... it was originally a 3.70 (stamped code) but "Bubba" swapped gears for 3.36's which hurt drive ability (after I installed a close ratio T10). I replaced the whole carrier but want to put the original back in with 3.70's. Everything I read talks about proper gear mesh and shimming.

Thanks!!

Joel

Joel:

I'm no expert on setting up rear ends. As a matter of fact, in times I've done this, I'm the "general contractor".

I'll pull out the rear and then bring it to one of several local places who have VAST experience setting them up. When I get it back, I reinstall it.

I personally wouldn't mess with trying to set up a rear. Too many specialized tools, too many tricks.

Tony Avedisian at Tony's Corvette Shop in Gaithersburg, MD has done a few rears for me. George Gallant at Gallant's Corvettes in Bel Air, MD has done a few for me too.

In your situation, with a 3 series "differential case", you'll be OK with the 3.70 gears replacing the 3.36's already there. You will not need a spacer.

Chuck