Crossposting from RD.com, in case any of you are interested in how to do this:
ok, so far in this thread I have basically only built a rotary engine, but now I'm gonna get into the miata-specific stuff required. This stuff is important and it's tough to find info for it online.
Today I have made some good progress. I've been working on the 1999 transmission I bought for this swap. First thing I did was wrestle the bellhousing off. Not easy. You undo all the bolts that go through the gearbox, and all the bolts on the inside of the bellhousing, then there are a couple of metal C clips that are a huge pain in the *** to get off. Then you can stand the transmission up and rest the input shaft on a block of wood so the bellhousing is off of the ground. Then kick the **** out of it with a rubber mallet. EVENTUALLY it will come off. Then you get this:
Because of the metal C clip that holds it together, it is impossible to use a spacer like I have been planning. (I was hoping I could use a spacer between the gearbox and bellhousing because of the longer miata input shaft) I figured this out a couple weeks ago. The tried and true fix is to cut the input shaft down a little. This picture is from mazdatrix, showing the measurement specs they use. This is done on rx7s if you want to swap the miata gearbox into an rx7, using an rx7 bellhousing and tailshaft.
They do the fabrication by completely disassembling the transmission so they can remove the input shaft and put it on a lathe.
Well, that **** costs ******' money, and is a giant pain in the ***. Because the input shaft is relatively small and lightweight, I figured if I did this modification by hand, my slight imperfections aren't gonna be enough to throw this out of balance where it's gonna **** anything up.
What you see here is the result of about a half hour with a tape measure, sharpie, careful use of an angle grinder, 80 grit sandpaper, and a scotchbrite pad:
I'm confident it'll work just fine. I don't recommend trying this if you don't have experience using an angle grinder. To remove the length off the front of the narrow part of the input shaft, i simply measured and accurately marked it, then used a cutter wheel and slowly cut that **** off. then I switched to a grinder wheel to make a nice radiused edge. I got it level by holding the angle grinder with one hand and spinning the gears of the transmission with the other.
the splines were trickier: I held the angle grinder as flat as i could and began grinding on the area i had to remove while spinning the gears with my other hand. This takes patience, but it's best to go slow. once the splined area was ground down level with the narrow part, it gets even harder.
After that, you mark an area slightly longer than 1/8" further back on the splines. you need to grind this at roughly a 35 degree angle. The angle is probably not important, mazdatrix probably just wanted to match the OEM angle. so, carefully using the angle grinder, i would spin the gears and take material off at an angle. as you can see I cut a tiiny amount inwards on the narrow part of the shaft due to holding the grinder too close. that part is super tricky.
All in all, I'd recommend that for the average person doing this swap they should probably take the input shaft off and do it on a lathe, unless they are a badass like me
. After this modification, I can simply bolt the rx7 bellhousing on, and the transmission should work 100% with the rotary. We'll see tommorrow when I get around to that part.
I'm glad I had to end up cutting this, because the engine will sit about 1/2 further back in the engine bay than I had planned, and I won't have to order new gearbox bolts.