Rebuilding a 9 bolt posi unit.
 

I recently discovered my cones were about to bottom out so I proactively had some reconditioning done to them.

There are several excellent writeups on the internet for this, so I followed those and disassembled and figured it all out. This video is more intended for the people who are in the boat that I was in where I just wasnt sure if it was worth it.

It's going to depend a lot on how much machine work costs in your area. My machinist did it for $40. Said it was a pain to figure out the best way to put these parts in his fixtures, and that next time it would be $60, but he said it was otherwise a pretty simple job and any machine shop should be able to do it. Some places are going to charge a fortune for this, some places wont. For those that will charge a fortune, i recommend Mark Pikas' blog where he walks you through how he did it. I went a slightly different route, but I hope my explanation in the video is informative enough to those of you who arent very familiar with these things that you can make a better decision about what you should or want to do. :)

It's my first time doing one of these, and I generally like things in print so I can go back and fix them when I discover new information or learn where i was wrong, so I apologize for any misinformation, but I hope it helps someone. I wouldnt know nearly this much if it werent for the help and advice I've gotten from the internet and guys like big gear head and mpikas. big gear head, the gears in this 9-bolt are the ones I bought from you years ago. ;)

http://mpikas.blogspot.com/2008/03/r...rner-cone.html

But I just remember having a hard time understanding what people were writing, and figured that a video might help people understand what these guys are saying a little better. :)

Re: Rebuilding a 9 bolt posi unit.
 

I've cut them in my lathe several times and it was no problem at all. I just removed the bearings and chucked up the bearing journal in my 3 jaw chuck and used a boring bar to remove the material from inside the case. I check run out with a dial indicator before making the cut so that I am removing the same amount of material all the way around.

I have to disagree with the video as far as setting the clearance on the gears. I assemble one side with no springs and check the backlash in the gears and shim it until the backlash is about .010 or less. Then I take everything out and assemble the other side and do the same thing. Then I assemble everything with the springs and it's ready to install. If you get the gears too tight you can break them, and if you don't get them tight enough they will not run the way that they were designed to run and may flake, chip or break.

Re: Rebuilding a 9 bolt posi unit.
 

That sounds like a great approach to this actually. Just didn't occur to me at the time. I never came across anything stating spider gear backlash for these, either. So that's great to know! I went ahead and added that to the vid.

Re: Rebuilding a 9 bolt posi unit.
 

This is your video? Not trying to be picky, but for future use the side gears are the axle gears and the 4 gears in the center are pinion gears.


The way that the Borg Warner unit is made it's very hard to get inside to measure the backlash on the gears. You only have one small window to access the gears once the case is bolted together. You can do it with the case separated, but you might not get an accurate measurement without the pinion gears being held in place by the other half of the case.

I haven't seen anything that says to replace the springs. I'm not doubting you, but I've never replaced them unless I was going to a higher preload spring. The springs are only there for very low traction situations anyway and could be left out completely for street and drag racing. The main pressure that is applied to the clutches or cones comes from the pinion gears pressing at an angle against the axle gears. The angle of the teeth pressing against each other tries to force the gears apart and puts pressure on the cones. If there is very low traction such as in snow or mud then the gears will not apply enough pressure to lock up the clutches very well and the springs would be needed to help.

Re: Rebuilding a 9 bolt posi unit.
 

Yeah I cant find it now, but I found some service manual pdf somewhere that said to replace those springs any time it was disassembled. It may be bad info, I didnt question it at the time and I cant find it now, so, meh. Information on 9 bolts is so difficult to find and even more difficult to verify. I scoured through a whole bunch of stuff over the past 2 months and I have no idea where I read that.

So whats the difference between spider gears and pinion gears? I think I recall them being called "pinion gears" before, but i always think of pinion gears as complements to ring gears... So I just stuck with terminology i was comfortable with. Like I said in my other thread... differential terminology drives me crazy. It seems like half the terms that are used are reused for other connected parts. Axle, carrier, housing, case, pinion gear, gear....Then there's the whole posi vs lsd thing that people start wars over. Makes my eyes glaze over... LOL.

But yeah it's my video. I've never messed around with differentials much, but I figured anyone else who wanted to give it a go but had a hard time understanding the articles and pictures of how to do this out there, they'd appreciate seeing it apart before they committed.

Im likely going to be doing this again in a year or so, so I'll see if I can figure out a good way to approximate the backlash in there... Maybe a weight of some sort on the cross shaft to seat it it into the carrier better?

Re: Rebuilding a 9 bolt posi unit.
 

That would be pushing it in the wrong direction. The cross shaft that the pinion gears run on would be pushing against the other side of the case. There would be a little clearance in there. I guess it's 6 in one and half a dozen in the other. Whatever works... I've seen people pay much less attention to this stuff and it worked well. I sweat the details and stuff breaks. :(

Re: Rebuilding a 9 bolt posi unit.
 

Well in my case, I know the cones are not bottomed out yet. So the backlash is at the farthest extent of what would have been accepted from the engineers that made it. So I cut metal off of it so it can farther, so I guess you would, in theory, just add enough shims to cover that extra distance plus a little extra. Mine is tighter than it was before I had it remachined, so I figure I've got time. Although thinking about it more I do feel like maybe I should have put .040 or .060 worth of shims in it. I didnt have nearly enough to do that, though. But next time I do it I'll probably do something like that.

It would be nice if we knew what the cone depth was when it was stock.

Re: Rebuilding a 9 bolt posi unit.
 

I've added as much as .060 to them. The last one I did took .030 on one side and .025 on the other. It was not bottomed out yet. Yes, it would make things easier if you had a measurement to go by.