I am trying to swap the stronger ? 8.5 10 bolt for the small rear in my 86z. I don't want to put any expensive gears or axles in the factory 2.73 single track since i already have a 8.5 3.73 with a locker. Does any one have any info on how to mount the trailing arm from the tranny to the front of a big pinion 10 bolt? I have access to a machine shop and thinking of a rear cover mounted bracket to run foward to attach trail arm to. Any suggestions or a better idea or proven mounts would be great.

 

Is the 8.5" a leaf sprung rear? If so, you'll hafta to fab LCA, shock mounts, and coil spring mounts.

I think I've seen 9 inches use a torque arm mount, that attaches to the pinion bolts, around the front of the center section.

 

DO a search, a few months ago two members showed how they did up an 8.5" 10 bolt. The spring, LCA, shock and panhard mounts were simply moved from the 7.5", or freshly made, and welded onto the tubes (easy).
The Torque arm (the hard part, since you already know welding to cast iron is tricky) was done with a few different ways i've seen:

-One guy used a HUGE block of aluminum, 2 piece, and ground it until it fit nicely onto the pinion area. It used two 1/2 bolts to clamp onto the pinion snout, then had the holes to mount the TA. It was large enough it should have held.

-The other guy made the bracket out of 1/8" or 3/16" steel, and just mig'ed it to the center section. He said it hadn't cracked yet.

Personally i'd use 1/4" steel and brace it well. I'd also pre and post heat the center section, and use a high nickel weld-all type of rod. Perhaps peen the welds as well.

 

The 8.5 is a big 10 bolt from a 76-78? i think monte carlo. It appears before any measurements that the lca,stabilizer and shock mounts look very similar and mount in the same general area.I have started on a mount that will bolt on to the back cover (between the housing and cover) that will run under and foward to mount the torque arm to. It kinda looks like a mini skid plate. I plan on using gr 8 or 10 studs instead of the gr5 bolts.I still have to fab a bracket to clamp the front (pinion snout) to the rest of the plate it looks like a wide c-clamp with 2 clamp bolts to prevent any deflection of the bracket. Hope it works cause bud,s want one too. I plan to torture test it in 3 weeks at the local drag strip to see if it leaks or breaks off. Wish me luck!!!

 

I will try to post some pics as this nears completion.

 

Right on man.
The 8.5" 10 bolt is pretty common, Blazers, etc got it as well. AFAIK it is a leaf spring setup, so you'll have to remove those.

You're taking it from a monte carlo? How's the width? I've heard the 2nd gen camaro ones are preferred since they are the exact right length for our cars, as well they have the right wheel bolt pattern.

 

I was also going to question the width.

By the time you're done fabricating and modifying to install a junkyard 8.5" 10 bolt, you could probably buy an aftermarket direct bolt in 12 bolt or 9" for the same price.

The torque arm is the biggest obstacle to installing some other diff under a third gen. To fabricate a mount, it needs to be strong enough to handle the stress put upon it. The aftermarket torque arm bracket to mount the torque arm onto a 9" housing uses three 5/8" bolts. The bolt mounts are welded to the steel housing of the 9". Trying to do the same thing to a 8.5" 10 bolt or a 12 bolt means having to weld to cast which requires a lot of heat to get proper penetration. Too much heat and you'll distort the housing and the alignment of the bearings inside.