Is there a list somewhere of what posi rear will fit our cars? Am wanting to put on in my 70 Skylark conv. Will eventually have a mild 455. Street use no racing. Auto tranny. What is a good gear ratio?
This is an age old question and the answer is depends on so many things like intended: highway use; tire size; trans to be used; who you ask, etc. If I planned some highway use a 3.23 or 3.08 ratio are good compromises. Avoid the high number ratios if you are not heavy into racing. These Buick engines have enough low end torque on a street build to burn the tires with a 3.08 or a 3.23.
My opinion, go with a 8.5" 10 bolt unit. Parts seem more readily available for them, and the water gets muddy when you start talking about 8.2s and parts.
Here is the biggest problem. We only have one aftermarket choice for gears on that rear. 68-70 are 8.2 BUT they are all on their own TOTALLY! Brad at Fabcraft can set you up with a 3.42 gearset and that will be a great all around ratio for street or strip. I do not know how many are left cause the gear run was for 30 gear sets THAT is all for now. May never be run again if they do not sell well. You can get all the ordering info in the parts section sticky right at the top of the aprts for sale section. Or ask me I will help you get it done too. You can ask me any questions on these diffs and I will explain everything about them to you. Ok? Jim
My main question is. What model cars can I get a posi rear from that will fit the 70 Buick. Does it have to come from an A-body car, have to be GM? I am looking for my option as to what will fit.
71'-'72 A body for the 8.5" plenty strong and reasonable $$ Chevy 12 bolt from I believe 70'-72 will be stronger but higher $$ JKaz
The 12 bolt is not that much stronger, and the 12 bolt is a C clip rear. I didn't even consider a 12 bolt when I was looking to replace my 8.2. The 8.5 is a good stout rear, has bolt in axles, and excellent aftermarket support.
Yep! You gotta love the factory c-clip eliminator with the 8.5 10 bolt. I seem to remember reading somewhere on here awhile back that the 8.5 actually has MORE pinion support than the 12 bolt. May have been Roger (OPH) or maybe Jim (Monzaz) who pointed this out.
The 8.5 has the same pinion support as the 12 bolt, same pinion shaft diameter also, I believe. I built my 8.5 with 30 spline Moser axles and 30 spine Eaton Posi. It's a great rear IMHO.
The weak point on the 8.5 is the axle shaft diameter where the press on bearing goes. 1.382 compared to stock 12 bolt 1.40 Not hugh but steel is steel and going from 28 -30 spline will be aqbout the same size in difference at the spline end...So if we think increasing the spline size is as be a deal then lets not for gwet the bearing side.... The 8.5 has a better yoke then the 12 bolt chevy. The metal surrounding the pinion spline is thicker diameter so that is a plus...BUT you can take the 8.5 yoke and use them on the 12 bolt rears also. They make a conversion seal for this. 12 bolt tubes I think are also a bit thicker roll then the 10 bolt 8.5 . The 12 bolt trucks are thicker roll tubing even still. But that is understandable since the truck has to carry more weight in the bed. you have 2 more bolts giving stability to the ring gear support. But the bolts on the 8.5 are larger so that might be a wash...lol. Crush sleeve on a 8.5 if inferior to the 12 bolt version.... means you could have trouble keeping pinion bearing preloads correct under hard driving...DRAG racing. Solid crush sleeve would solve that. So all in all it still comes down to YOUR choice... Either is a good one. Jim
I found a 8.5 10 bolt from a 76 Trans Am for $200.00. Is this a direct bolt in? Drum spacing the same? Decent price? John
Now that you mention it the pic kind of looks that way. I got this from the Buick performance website- The two best choices are the 1972-81 Trans Ams/Z28s and full size station wagons such as the Caprice, Cutlass, Electra and Catalina. Most of the Trans Ams/Z28s have the units. Most of the time you will see a small metal tag attached to one of the cover screws that states "USE LIMITED SLIP DIFF. LUBE ONLY." Also found these two posi rear end out of my 1970 trans am,,,gears i believe to be 323,,,never had the cover off 10 bolt rear end out of a 66 chevelle posi 2.73 gears Is 2.73 to low for a mild street 455? John
John, You asked what rear. That means entire rear end to me. The quote from the article you mentioned talks about how to find posi units (carriers) in a bone yard, what cars to look for.
Sorry, I did mean the entire rear end. I am not sure what other entire rear ends are a good fit. As far as bolt in, and length.
You need an A body rear, 64-72. They have coil springs in the back. Chevelle, Cutlass, Lemans, Skylark. The 71-72 Skylark/GS rears were the 8.5 10 bolts. Those are the best ones to look for. That rear has wide aftermarket support, so new parts are readily available. The 8.5 posi units came in the cars mentioned in the article you quoted. The GN's use the same posi unit, but the G body rears are a little different, so look for a 64-72 A body rear.
