BOP vs. Chevelle rearend

I posted this over on Team Chevelle, but didn't get much response, so I'm asking you guys...

My step-father has a 71 LeMans Sport convertible and he's pretty sure something broke inside the rearend today. He's was driving along @ 35 mph and he said something went "bang" and the back end "jumped up" and it locked up the rear tires. It wouldn't move forward under it own power so I had to tow him home. Now it is leaking gear oil - we haven't got it up on his lift yet to find out where it's coming from.

I have a couple of questions:

1) Will a 68-72 Chevelle 10 or 12 bolt in or are there differences in the brackets? I know there are some physical differences in the center section and the shape of the diff inspection cover.

2) His BOP 10 bolt is a posi - will the posi unit fit in a Chevelle 10 bolt? I may give him my 10 bolt and find a 12 bolt for my 70 Chevelle.

3) If the Chevelle rearend is a direct bolt-in is there any benefit over a BOP? Or should we just rebuild what he has? Never rebuilt one yet, but we have pretty much everything we need tool-wise, plus he's a 30-year
machine repair-man.

4) Any other ideas/suggestions I might not have thought of...?

Thanks in advance for your help.

Eric

 

I believe that the spring perch on the Chevelle axles is directly over the axletube, whereas the BOP style has the bracket spring perch. I might try the swap if I had a free axle assembly hanging around, but with the difference in the spring perches I would be hesitant to spend on anything but a BOP assembly. I have heard that they can be swapped, but I have not tried it myself.

for the 8.5" rear end 10 bolt, the posi units will interchange with no problem.

rebuilding the axle assembly really isnt all that difficult. You just need to get pinion depth exact, and then work on the pattern. Get the special tool to measure pinion depth or at least a generic one like Summit sells.

Use an old pinion bearing with the ID ground out a little for a slip fit to determine the correct shim pack for your depth. Beware aftermarket shims that catch on the little radius between the pinion shaft and head. OEM shims are much better, or you can modify the aftermarket ones. Once you find the right shim pack for the depth, then you press on the new bearing.

Another handy tool is the "magic stick" used to hold the pinion yoke stationary while torquing the pinion nut onto the crush collar. I have also used a large pipe wrench for this.

I dont think there is an advantage or disadvantage to a BOP 8.5 10 bolt versus a Chevy 8.5 10 bolt. I think theyre the same thing, excepting the spring perches.

scratch that - The one thing I did not like about the BOPs that I have rebuilt is that the wheel bearings, or pressed-on axle bearings, were difficult to locate, and expensive when I found them. The BOP uses different bearings than the Chevy at least in the 8.5" 10 bolt configuration.

good luck

 

Seventy chevy, where are you located? local yard has 71 pontiac lemans rear, still under car. don't know gear ratio, not posi. Old man is very reasonable in price with me, as I help him out from time to time. This 71 pontiac lemans still had 8.2 rear; buick olds and chevy stepped up to 8.5, based on my findings in local yard. pm/email me if want.

 

I bolted a 12 bolt right in my 71 GS, too many times actually! I can't make up my mind what gears I like. :spank: Used the same control arms and springs. There are two different u-joint sizes. I have two 12 bolts and each uses a different one. So when I switch the rears around (4.10 vs 3.55) I switch drive shafts as well. I think someone makes a u-joint that fits the Buick GS driveshaft and the Chevy pinion.

The 66-67 12-bolt is about an inch narrower between the lower control arms bracket and the backing plate for the brakes. They both bolt in the same, just one tucks he tires ina little more. With the 67 I can run a 30X10.5 slick on 4 7/8" back space wheels.

 

Thanks, guys, for the input. Dad got the car on the lift last night and found out his posi unit failed and something caught under the ring gear and exploded the bottom out of the diff case. He said the hole was big enough to stick his fist in. So he'll be looking for a new rear.

He just bought this one a few years ago - not sure what happened with the posi as he doesn't drive it hard at all. As a matter of fact he was just cruising at 35 mph when it let go.

 

Right; doesn't drive it hard. I wasn't driving my "mildly modified" turbo limited hard when I spit the bottom half of the crank out either. ERR, at least not at that minute...
Nor when I spun the driveshaft out from under our 70 impala 454 back in the early 70's. I was taking some younger kids (ok they were all my friends and I was the oldest of the bunch, and I had just gotten my unresticted liscense a few weeks earlier) home from church, and ...and I hit a wet spot in the road. Those log black marks back behind the car dad never saw? no idea; somebody must have skidded on that side on the fresh pavement.o No:
Actually, it could have had a stress fracture that finally gave way from damage months or years earlier. That is how the ring gear on 1 of my turbo cars broke; I honestly NEVER abused that car; it was too low miles and too high optioned to risk damage. Too bad a tree fell on it during a hurricane.
See my email to you regard rear. UMM. its a lemans, not a tempest. actually both are there,but tempest rear was robbed of axle, and it is posi. bummer can't be both in same rear end.Ya never know.

 

Eric,
Tell you dad to scrap the 8.2 in the car now and any plans to reinstall one. Get the 71-2 Cutlass/Skylark 8.5" rear and have it built. It is best rear for the money. You have a wide selection of posi units and gears. ONLY 71-2 Cutlass and Skylark had the corporate 8.5" rear......no other models from the factory. This has been discussed here many times and I have a couple articles on this rear on our site below.

When going to the 8.5" rear, you will need to shorten your driveshaft 3/4-7/8 of an inch and also a good time to have the d-shaft rebalanced.

 

the 8.2 chevelle will bolt into the bop mounts. a guy just put my old bop 8.2 under his chevelle. they use the same control arms and springs. the bop is stronger though.