Looking for a posi rear end for my 66 skylark. I ended up swapping out the 2 speed for a 200 r4 last year. Now Looking for a posi rear end. Preferably a bolt and go application. I'd like 64-67 or 71-72. I hear the gears on the 68-70 are hard to come by. Thanks
I would start by removing the rear end cover and identifying the ratio you have. I could be wrong however I think you can still buy gears for your rear end it’s the 70 version that’s obsolete
That posi unit is for a big car with the 9 3/8” 12-bolt. I would suggest two options: 1: Rebuild your existing 8.2” with the gear ratio of your choice and a posi. 2: Swap to a 71-72 8.5” 10-bolt. Plenty of parts and ratios available and one of the best rears ever made. I can build an 8.5” for you,if you can’t find one local. Is this more of a cruiser,or is this a street/strip application? If you do swap to the 8.5”,you will need to shorten your driveshaft 3/4”. I’m assuming you shortened it once when you swapped the 2004R.
First things first I'm clearly not a mechanic. Now that we cleared that up. I wasn't aware that I could keep my rear end and repurpose it. I found a few gm rear ends locally for $200-$500. I just was hoping for unbolt and bolt up application. Now that I found out about about adding posi carrier with desired gear it kind of gives me a price point. I found a local guy with a 65 8.2 posi with 3.23 gears for $600. Waiting to see how this pans out. Drive shaft was not shortened these are bolt up applications for 66.
When swapping from a 2-speed or a TH350,to a 2004R,the driveshaft needs to be shortened 1/2” to prevent bottoming out or binding in the transmission. It doesn’t sound like a lot,but it is critical. Can you rebuild your existing rear with a posi and gears? Yes. However,you will need to find a shop that is competent in doing the installation and setting up the gears correctly. The 65 8.2” 3:23 posi is a possibility. If it is all original,and never been serviced or rebuilt,now would be the time to do that,since it is out of the car. The 64-65 rears are also 5/8” narrower on each side,so it will move your wheels slightly inboard. If you have a very early 66,it could possibly have a leftover narrow rear under it already. Just something to check and consider.
From all my research and discussions on here both st300 and 200r4 are 27 3/4", no shaft modifications were necessary. I believe modification was necessary if if had gone with with 350 trans? Correct me if I'm wrong, but I discussed this with guys on here at length before installation. I thought the narrow shafts 1/2" were 64-66. 67-72 were wider with 70-72 being 8.5?
I'm still trying to figure all this out so any and all knowledge is helpful. I usually do a ton of research, mainly on here, about mods I'm planning.
I think the issue comes in not the overall measured length........but in the amount of travel the slip yoke physically has. Its possible the trans measure the same length but the new trans doesn't have the same internal amount of travel.......or the yoke for the swapped in trans could be a different length with the same internal spline.........so if the yoke on your old shaft was longer it would fit but have less travel...... Typically if pinion angle geometry is correct when the rear axle drops the yoke slides out, and when it goes up it slides in..........
Mounted without issue, they've been doing this for 40 years now. My main question was about rear gears and got sidetracked here.
BOP cars had the narrow rears for 64-65,and some of the very early 66’s had the leftover narrower rears. 66-72 BOP cars had the wider rear. The difference is 5/8” on each side. There are plenty of people and “professionals” that have swapped a 2004R transmission in without modifying the driveshaft,but that doesn’t exactly mean it’s correct. If the shaft is too long,it will bottom-out inside the transmission when the suspension travels,which can and will cause damage. There is a simple check for this: with the suspension under load,NOT hanging,unbolt the rear u-joint from the rear yoke. You should be able to push the shaft forward, 1.25” into the transmission,before it bottoms-out. If you don’t have that much movement,the shaft is too long.
Great information. I have a vintage mechanic that does all of my installation of more involved mods. I'm seeing him on Tuesday. I'll bring this up to him. Also I'll measure my rear end to see how wide it is. I believe I decoded my rear end a few years back when I was trying to figure out what gears I had. Might do this again to see if it tells me what year/length rear end I have.
The 17-2 posi unit is a EATON freeway posi unit 2.28 2.41 2.56 ratio unit. ( I have to double check the series in my paperwork...) They have 71-1, 17-3, 17-4 posi unit also. It can be 28 or 31 spline...Likely the 28 since thoise were a later unit. it will be a derivative of the OLD 65-70 Pontiac 8.875 rear. NOW called the 8.75 because the pinion shaft is 1.625 like a 8.5 etc. It will have 12 bolt holes for the ring and be 7/16" left hand threaded bolts to hold the ring on...AGAIN a rear that no one makes parts for. They were used all the way to 1987 wagon, deville, full size cars. A36 bearing just like the 12 bolt chevy ...we see tons at swap meets people trying to sell them off as 12 bolt chevy posi unit. terrible trick to play on someone... Jim JD Race
Do not forget to check what ratio you already have in the car...(this was mention already...but making sure you do it.) So you know how far to go with the next rear. YOU might want to leave it alone till the trans is in and drive it to see what it feels like to know how much more gear you need. Over drive will typically drop the rpm down 1000 from 3rd to 4th non lock up.
I have factory 2.78. The transmission has been in for 2 years now. Just looking for a little more get up out of a stance. Leaning towards the 3.42 or 3.55. Haven't got much feedback from guys with similar set ups.
These might or might not help some https://m.facebook.com/marketplace/...\":503,\"value\":0}","ftmd_400706":"111112l"} https://m.facebook.com/marketplace/...\":503,\"value\":0}","ftmd_400706":"111112l"} https://m.facebook.com/marketplace/...\":503,\"value\":0}","ftmd_400706":"111112l"} https://m.facebook.com/marketplace/...\":503,\"value\":0}","ftmd_400706":"111112l"}
First one on that face book is NOT 8.2 Pontiac it is the FULL SIZE car 8.875 the other guy was looking for in another thread...Someone should send that ad to him. Jim
