I didn't expect to get anything done on the Cobra this weekend. But after spending all morning outside in the hot sun yesterday (and all day outside on Saturday), the kids were really tired and wanted to veg. My wife had some cleaning up to do, so that gave me an opportunity to work on the rear axle. I'd been hoping to get a full afternoon to work on this because I've never set up a rear axle before, and I'd expected it to take a bunch of iterations.
When setting up a Ford 8.8 (and I think most other rear axles) it's commonly an iterative process. You make an educated guess as to the shim you need under the pinion bearing to set its location correct. Then you put the differential in with its shims to get its location correct. The possible issues are that the pinion is too low or too high, or the differential is too far left or right. In the end you have to get the backlash correct and the engagement pattern.
Changing the backlash really isn't too big of a problem. You've got the 4 bolts that hold the differential carrier in place and then you pull it out and mess around with the shims. At the end of it, you have to check the gear tooth pattern and if it's good, you're good to go. If not, you have to at least change the backlash, but might need to change the pinion shim. That's a lot harder and more involved, more than anything because you then need a new crush spacer (those are one time use) and you also have to pull the bearing off the pinion, which often results in destroying it.
Oh, and here's the other fun thing with the pinion installation. You have to torque it down based on a preload, so specifically how much torque it takes to turn the pinion. This is a moving torque so you need a beam style torque wrench, which I bought. And, as I found, it's extremely sensitive - only a couple of degrees of turning can make a big difference.
Because of all of this, I decided I wanted to try to get the pinion together once and hopefully get it right. Most people set things up with the plan of taking it apart, putting it back together, etc. etc. I had two crush washers and two new pinion nuts so that would give me one additional opportunity if I had an issue.
When I took this rear axle out I immediately noticed that it felt like it had too little backlash (essentially zero) and also way too little resistance to turn (also essentially zero). While I didn't know the official spec on those, it didn't feel right to me - it felt like everything was freewheeling. Of course these were older, original bearings and old, leaky seals, but it still didn't feel right. When I took everything apart, the pinion shim was 0.030" (which I suspect was the stock shim) and the shims for the differential carrier were sticking out. What I'm now realizing happened was they set this up for zero preload, and probably reused the crush spacer on the pinion. They definitely didn't set anything correctly. Because of this I was basically setting it up blind.
However, most people say that the stock shims are usually what you end up needing with new gearsets. I also saw another video where someone from Ford Performance said that 0.024-0.028" was the typical pinion shim range.
So, I picked out a pair of shims that were good for 0.028" (a hair less than what was in there) pressed the bearing on, and crossed my fingers that it would work. Tapped in the forward seal, bearing, slinger, and got ready to tighten it down.
But before that, I had to figure out some spacers for the differential carrier and this time set it up so there was some preload on the bearings. I was going 100% from scratch on this. There's not good documentation (and no spec) on preload for the differential bearings, but Ford Performance gives you some thicker spacers (2 per side) and you put the thinner shims between those. I more or less guessed on a preload where it felt like the shims and spacers could get in and out easily enough, but also that they were solidly in there and wouldn't slip out like the did before.
On my first try of tightening down the pinion I ended up over-tightening it. Spec is something like 16-28 in-lbs of preload. I wanted a hair on the high side, had it at 20 in-lbs (right in the middle). Then I torqued it a hair more and it went up to 50 in-lbs. Well crap. Also for some reason when I moved the differential back and forth I heard a little metallic "clink". So took that all apart, put it back together. I couldn't figure out what the clink was, so I just put more oil in the bearings figuring maybe I didn't pre-lube them enough and maybe having it overtightened was causing something odd. Put it all back together, got it to about 22 in-lbs preload, called it good, and crossed my fingers that I wouldn't have to mess with it again.
I then split the shims between the left and right side on the differential carrier and put it in, which resulted in zero backlash. So I put both spacers on the right side (moving the ring to the left, increasing backlash) and that resulted on about 0.015" backlash, a bit above the spec of 0.008"-0.012". A few more rounds of playing and I ended up with the backlash right at about 0.009-0.010". Then I did the gear paint.
The gear pattern looked good. I'll admit I had a hard time really seeing it and I didn't get any of the super pretty patterns that you see on videos from people who've done it a bunch of times before, but I could tell that none of the patterns looked like the examples I found that said "need to [increase/decrease] pinion shim". So I got the backlash set, pattern looked good, and then put in the axles and called it a day. The setup has a good, appropriately level of resistance but is also super tight. The limited slip looks like it works correctly and well.
Now that the axle at its core is back together, it's time to put on the lower control arms, the 3rd top link, the coil/over assemblies, brakes, and then put in fluid, put the cover on, and put the whole thing under the car and attach it. I'll probably do that tomorrow or Wednesday night when my wife is back on shift. At that point I'll have a "roller" although I'm not putting wheels on it yet.
Today my intake manifold setup arrived, and the short block is supposed to arrive, too, although that's LTL freight and I haven't received a phone call to schedule delivery. I'll need to be home for that one.
On Friday I heard from the machine shop working on the heads and they confirmed that the springs were of the correct pressure (as recommended by Isky) so they're all back together. I'll pick those up probably tomorrow. Reality is until I have the cam, those heads don't do me a whole lot of use since I can't torque them down (or at least won't torque them down) until I have the cam installed and can check the piston-to-valve clearance. I can put them on with the head gaskets and snug the head bolts down, but won't do much beyond that.
In the end, the heads are a savings of about $1200 vs. if I bought them new, so I think that's a pretty good number.
There's a lot to do, although part of me feels like I need more parts still.
