I’m planning on long trip in my 71 455 car this summer so I’m wanting to find a replacement differential . The car has a 12 bolt differential in it which I believe is the original piece for this car. At some point it looks like someone put 3.73 gears where it should have 3.42 in it . The 3.73 ‘s are great around town but they are a little deep for extended highway travel . It seems like a 2.73 would be good fit for the tire size I have but I if I re-gear the 12 bolt to this gear I think I would need a different series carrier for that so.. can any one point me to another diff that would be a pretty painless swap? This car has a 1330 series conversion joint on the rear of the driveline which I believe is stock so if there is something that would swap/ interchange I guess I would like to know what to be looking out for. Thanks
https://www.differentials.com/technical-help-2/carrier-breaks/ 12 bolt carrier breaks are 2.76 and down, and 3.07-3.73, and 3.90 and up. Might be easier to go with the 3.08. Using a 28" tire would get you to 2.91. https://www.summitracing.com/parts/rmg-4900941
Well re-gearing the 12 bolt would certainly help me avoid drive line issues. So the 3.08 would be 3000 ish at 80 with my 275/60’s that are on currently on the back. That might be the best compromise . 2500 at 80 would be pretty nice too , I ll have to keep saving my penny’s for the GV.
Your car would have originally came with the 8.5” 10-bolt,so someone swapped rears. You could re-gear it to a 3:07 or 3:31. Same posi carrier.
If it is a 455 wagon, the rear is probably a 9 3/8”. If it is a factory 350 wagon it is likely an 8.5.
Those wagon 8.5 are a I retty cool part to find. They use larger axle bearing...set10 instead of set 9. So someone could fit 33 spline axles through a set 10 if needed instead on just 30 like set 9. I dont recall if there is a drive shaft length difference between 10 and 12 bolts. Something to weigh out in your mind also. If your having to have the rear put together not doing it yourself you could be spending as much as 50% of the price of an over drive unit. Slowing down to 65 from 80 will save you almost 700. I know no one wants to slow down. But that would put you at the same rpm as 3.08 gears......now you just saved 50% of the money needed to buy your od unit! In fact you will be within 150ush rpm of what you would be turning with a gear vendors unit
Some Canadian built Skylarks came with a 12 bolt. My first Buick, a 1972 Skylark, came with a 12 bolt 2.56 peg leg rear. The OP doesn't really tell us what car he is talking about, just a 71 455 car. For all we know, it could be a big car 9 3/8 rear.
No 9.3 or 9 3/8" were ever installed in a A body chassis. We are talking about a Skylark 1971 here correct?
Trouble with a 12 bolt to 8.5 swap is the drive shaft length and u joint. Do you have pictures of your u-joint area? The 1968-1970 Canadian built skylarks and other BOP with 12 bolt Chevys did have the Mechanic 3r yoke BUT you still have a LENGTH issue from a 8.5 to a 12 bolt Chevy. Soooo Be careful there too. It would be easier if your looking for a driver rear to get across the country find a Pontiac/ Buick Olds 12 bolt as your driveshaft length will be the same as the 12 bolt Chevy. Jim JD Race
Jim, we don't know what car. The OP doesn't specify that. His description is 71 455 car. His avatar is a 67 GS400. His previous threads confirm that. So, we are talking about a 71 455 transplant. In any case, we are talking about a 1967 A body. That would have come with a BOP 8.2 10 bolt, not a 12 bolt.
The title says 71 GS,so I assumed and A-body. It would have originally had an 8.5” 10-bolt. Sound like someone swapped a 12-bolt Chevy with 3:73’s in there at some point. Yes,the 8.5” takes a shorter shaft than the 12-bolt Chevy. Not sure if they just did an adapter u-joint and “made it work”,or changed to a longer driveshaft. If it is the original shaft,it doesn’t have as much trans/yoke engagement as it should.
Look at his avatar, then look at all of his threads. https://www.v8buick.com/index.php?search/53588907/ My guess is a 1967 GS400 with a 1971 455 transplant. That means the car originally came with a 1967 BOP 8.2 10 bolt. At some point, the 12 bolt was swapped in.
