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The proper differential for this job is a 89-97 Ford 8.8 diff with ~3.55 gears. I was able to snag one out of a Cougar that did not have limited slip, for ~$100.
Then I ordered a rebuild kit to include the bearings, posi clutch, and seals. I knew nothing about Diff's. Zip Zero. How hard can it be? This turned out to be a problem, but we were not afraid. We took the day and started in. Disassembly.. No problem.
Assembly.. Well little more difficult . Wes poured over the instructions. We started in.
We put the yoke in the freezer, heated the bearings to 500 degrees in the oven, and while trying to push the bearings on with a hammer, we busted a bearing. It was at this point that I had to put my ego aside and take it to a professional. Wes sand blasted the cases and painted em. And Wallah, we have a diff. I have about $600 in this now with the new parts and assembly. Not too bad. Here are some of the completed assembly pictures. The diff mount parts from BF were great. No problems.
Ric at BF suggested replacing the factory shocks with Bilstein PSS9 Complete
coil over replacements. He offered up
Good-Win racing as a source. $1550 shipped. They arrived. One of the
bushings was split on arrival in the box. I was surprised by this and also
surprised by the excessively tight fit. Its why they split I believe. There is a
difference between the front and rear coils. Dont screw this up like we did!.
Front spring is the coil marked E4-FD1-Y365A00, rear is the coil marked
E4-FD1-Y366A00. You will need a coil spring compressor rent or buy from your
local auto store. First pic is the split bushing.
The ford diff attaches to the Miata spindles via the "half shaft." Thanks to the guys on the Miata forums and Ric at BF, for steering me to a cheaper half shaft solution from The Drive Shaft Shop. They make a bolt in set of half shafts, 400hp rating, for this conversion. The part number is RA6245L2. Costs was $680 shipped. The only modification I had to make was to loosen the clamp on the blue boot near the shock, slide the boot back a bit, and retighten in order to keep the boot from rubbing the new larger Bilstein Shocks.
The drive shaft I had made from again The Drive Shaft Shop. They needed 4 pieces of info. HP, Tranny, rear end, face to face distance between the diff and tranny. Mine was 35 3/8". Cost for this delivered to my door was $328.