Hello, I have what I believe to be a 1971 8.5 rearend that codes out to originally be a Buick 2.56 open rear. The axle tube stamping is LK B270 (2?) This rearend was supposedly upgraded with posi and 3.90 gears. Can the ratio be verified with the following ring gear stamping? Now, the part I am not quite understanding about these rearends! These are supposed to be the GM corporate rearends, but did BOP rears still differ from Chevy due to bolt in axles vs c-clips?? I am looking to have the rearend narrowed in order to run the widest wheel/tire I can in the stock wheel wells. And, this is going in a 67 GS so the extra width of this rear is a problem with the 235's on it now. Are getting axles for this going to be a problem? Above is the current setup of the rear with upgraded disc brakes, coil overs, and upper/lower trailing arms. Any help or references to figure this out would be greatly appreciated. Jarrod
Nice setup! Ratio is 3.23:1, found via the stamped numbers 13 and 42. 42 ÷ 13 = 3.23. Hopefully others will jump in to answer your other questions. Devon
Thanks so much. That is the ratio I was thinking based on that but wanted to be sure since I was told otherwise!
Jcamp, I have done all of this numerous times for customers and guys on this site. I recently did an 8.5” for a member on here with a 67 that wanted to run an oldschool wheel that didn’t have enough backspacing,so a narrowed it to the desired width and had axles made for it. Just so you know,that 71-72 8.5” is the same width as your existing 67 rearend. The BOP cars only had the narrower rears in 64-65. The Chevy cars had them for 64-67. When swapping from your existing 8.2” to the 8.5”,you will need to shorten your driveshaft 3/4”.
I agree Charlie, I am glad to hear that they were in fact 3.23’s! Thank you for the info Brian. The rear was already being used in the car so I assume the driveshaft was already shortened. Funny thing is I bought the exact size tire that was already on it, but went with some new reproduction 15X7 Buick rallyes. I guess the backspacing was just different enough so now it is rubbing! Who would be a good source to have the axles made for it? Also, is there a way to identify the type of posi (cone/clutch)or brand (Eaton/GM/etc) simply with the cover off?
Those are all GM parts in that rear. That unit was the better unit that came in the 8.5’s back then. Very good unit and rebuildable. Now,if you are going to narrow it and have axles made,I would suggest upgrading to 30-spline and be done. The cost for the shafts is the same,whether they are 28 or 30-spline. However,you would need to replace the posi unit to a 30-spline unit. I wonder,by chance,if those Buick rally’s were made with less backspace,like 4.0”,instead of 4.5”.
Wheels / rims GM Most all GM factory rims except for corvette were 4.25" backspacing on a 7" wide rim. Measure the back of the rim down to the wheel mount flat surface that would mate to the drum. That should be the same on both rims ...THEN see if the RIM ITSELF is wider or the same. Jim Narrowing the Buick rears or any bolt in axle with a OILED bearing is pretty trick as you will need to cut into the lower control arm bracket some and weld in there not to screw up the bearing land inside of the axle tube. Just not a lot of room to narrow them bolt in axle a body housings. AND if you do not get that end back on DEAD center you will have LEAKING for the rest of time with those rears...they are bad enough with trying to get the correct bearing and seal not to leak on a factory stock straight rear. Jim JD
they got the gears from a 1973 rear... likely the posi too and likely a OLDSMOBILE as 3.23 was a normal 442 ratio in the 1971 - 1976 era olds cutlass performance cars.
When I narrow those rears,I make a vertical slice up the inner side/corner of the shock mount and remove the shock mount. The control arm bracket itself does not get disturbed. You can comfortably remove 1.25” off each side and still be able to get the welder around it without interfering with the control arm bracket. I do all of this with the bar and pucks. Some look rather questionable,but they are now straight. Once the ends are welded back on,the shock mounts get welded back on. They now get mounted inboard,behind the lower control arm. The left side goes on the right side and the right side goes on the left side. This is why I make a nice vertical cut up the corner. The benefits from this are to maintain clearance from a 14” or 15” rim that will now sit inboard deeper,and to take some of the angle off of the shock and allow it to work better,much like a G—body design. Here are two examples. The unfinished housing is a 65 8.2” that I narrowed an additional inch on each side to accommodate an oldschool wheel. The finished one is the 8.5” that I mentioned earlier. I narrowed that 3/4” on each side for a 67 GS that also had an aftermarket wheel that didn’t have enough backspacing.
Wow guys, what awesome information you have shared. I greatly appreciate it. Where to source the shortened bolt in axles? Also if the posi unit is possibly a 73 olds along with the gear set, would it be smart while all this is happening to update the posi? Thanks again,
I have Moser make the majority of my shafts. Yes,I would upgrade to a 30-spline posi unit and have 30-spline shafts made.
Yah but you are picturing a 66 of older 8.2, that axle bearing is a seal axle bearing. The 8.5 already has a headache with the tapered bearing and seal system then cutting that tube end off...IT better be perfect...or again more headache with sealing. And like you said you can see the deviation on the tube end from the main tube... being a 50+ year old rear end they never go back on square from years of punishment. (I should say they will be on true from the saddle to the bearing end...BUT the end will have a step and not LOOK true even thought the bar is.) So if that is the case then the lower control arm geometry will be off too...not a ton BUT yes stiff off. At 55 years old I am done fighting these old rears...I will leave that to the next youngster to do that needs to make a name for themselves...lol. Sounds mean...but I have worked hard for 30+ years...I am getting tired, these rears are getting worse and worse from years and years of over abuse. Jim JD Race
I also like to use cores from a “low-stress” application. As you know,they are much more likely to be straight,or have less issues. I’ve thrown out 4 housings in the last few months for that reason. They came from something like a big block 4-speed or something that has history of racing and abuse. They made a slinky look straight. I will not mess with those. Unless it’s the original rearend to that car,and the owner must have it at any cost. Otherwise it’s scrap. I don’t care what codes are in it.
Are Chevy 12 bolt housings less likely to be bent than the 8.5? I’ve seen a lot about bent housings lately.
No. Anything can bend. All it takes is power. Welding the tubes to the pumpkin will not prevent that. Yes,I weld them any time I have a housing in the jig for other welding,but the tube will just bend wherever. This is also why you see a back brace on a 9”. With a rear-bolted cover housing,you have to get a little more creative with bracing. I’ve done a few where I have welded to the lower rib of the pumpkin and went out to the control arm bracket with a brace.
