Wednesday, March 12, 2008

Rebuilding a 9 Bolt, 7.75” Borg Warner Cone Style Posi

This is basically a cleaned up version of a post that I made on thirdgen.org, the title links to the original thread. Like in other cases, I got a number of requests to put it back together with pictures, so here it is.

As always, the pictures are all clickable for a larger version.




Rebuilding the posi: There are pretty good, detailed instructions in the FSM and in the tech section on this site ( thirdgen.org ). Look them up, they’re a good start.

I’ll add a quick nutshell description and note where I did things differently/suggestions:

Disassemble: unbolt the axle retainers, pull the axles, pull the center section cover, remove the bearing caps (make sure that you remember what side everything came from so you can put it back like it came out), pop the carrier/ring gear assembly out of the housing. Note, this will require some prying and turning and make sure you save the side preload washers/shims so you can reinstall them like they came out. Be careful with them, they are machined out of a cast piece and can be brittle.

To perform the rebuild you need some kind of fixture to hold an axle. I took a chunk of wood, cut 2 shoulders to sit on top of my vice jaws, drilled a hole through the middle and then sliced it in half on the table saw to make some clamp/vise pads for the job:



Through the whole deal use the axles as alignment tools.


Before you disassemble it check this spot out:


It’s hard to see in that pic, but if the end of the cone is contacting the end of the case there your case is worn out of spec and you’ll either have to do some machining to it to make it work or get a new case.


With it sitting on the bottom axle, housing bolts up, unbolt the smaller, center bolts and pull the end of the housing/ring gear off.

When you do that you’ll see this (except you’ll have it in the case side, not the ring gear side like in this pic):




If you lift the spider/cross/preload spring assembly off you’ll get this:



If you lift that side gear out of the end of the case you’ll see the pocket in the side cone where you put the shims, there will be one on each side of the case between the side gear and the cone (unfortunately I don’t have a good picture). You can use any shims that are small enough to drop into the machined area between the 2 and still fit around the axle. I usually use 10bolt or ford 8.8” pinion shims. Actually, the last time I think I just stopped by the local machine shop and got a stack of whatever left over shims they had approximately the right diameter.

* This is where I differ from the instructions on the site or in the FSM, forget measuring clearances or anything else, just load both pockets with as much shim as you can get in there and still reassemble the case completely. I’ve never seen one of these get so tight it doesn’t work right (if anything you’ll want it tighter, not looser) and you’ll be kicking yourself if you do any less and start spinning one wheel again sooner then you had to. The shim packs will probably have to be the same thickness on both sides.

From there the reassembly is the opposite of the disassembly, just make sure that you have both axles installed and fully bottomed before torquing all the bolts, if you don’t line up the splines in the side gears and cones before then it’ll be next to impossible to do with it assembled.

Now if the case is worn out of spec, you can still usually get some life out it. What happens is that there are some rings machined into the end of the cones and the case to contact if the case is worn down too far, if you cut them down you can get some extra life out of it.

This is what it looks like if the cone and case ends are worn to the point where they’re grinding into each other, case side. The numbered parts were originally put in for something else, but they do point out the points that are contacting and wearing that aren't supposed to:



Cone side:



What I’ve done before is machine those down till they no longer contact (as a matter of fact, I think I machined them down till I had over .125” clearance). Again, case side:

Cone side:

FWIW, I did the inside of the case on a drill press with a surfacing disk and I did the cone side with an angle grinder by hand, just played with them till I got them within about .005 of flat… If I remember right I ended up cutting close to 1/4" total to make sure that everything fit (yes, this thing was way outside of spec, and yes, it still worked well after all this).

Eventually, after a few more rebuilds the cones/case will wear enough that they get too thin, and I started getting cracking around the opening that I mentioned to check above to see if it’s worn past spec. When that happened I pulled it all apart, bevel ground the sides of the cones, centered them up in the housing using the axles and welded them in, like this:


3 comments:

Kyle said...

Are you saying that you welded the cone to the carrier at the end? Doesnt that basically make it function as a straight welded axle?

Mark said...

If you read through you'll see that I rebuilt it multiple times. Eventually, even with cutting back the ends of the cones it wore to the point that the spot that I pointed out in one of the pictures wasn't only contacting but the case was starting to crack. When that happened I decided that it was on it's last legs and that's when I welded the cones into the end of the case, essentially making it a spool (I'm pretty sure that the last paragraph expains all that).

FWIW- I drove it like that on the street as well as drag raced and autoXed it for I think almost 2 years, and for the most part it was streetable and didn't cause any problems. The only place where it really got in the way was in tight parking lot situations and then you could work around it, it just did some skipping around trying to go straight if you had the wheel turned tight.

It pretty much lived like that till one night at the dragstrip I both twisted up one of the axles and blew up the transmission.

rlopez018 said...

yes i got the same rear end but i dont welded i do some different,anyway i got 89iroc-z 355 engine,casting 492 full porting,victor Jr intake,750 carb with trick,5500 rpm converter,350 trans,3.70 gears strange,i run 12.23 at 108.48 at 1/4 mile,i forgot the cam is for heavy car flat tappet lunati.