I had read many times over the years that removing the Miata PPF (Power Plant Frame) from the diff was either very difficult or very complicated or both. So, before doing it myself I wanted to research the job to understand exactly what was involved. Surprisingly, while I found countless discussion about it on the web I didn't find any one comprehensive summary that truly explained it.
In the end, I decided to just do it and see how it went. Aside from removing the bolts (see below) it turned out to be relatively easy so maybe that's why nobody has written a summary of the job. Still, I decided to write up the page that I had originally hoped to find in case it helps someone else out there.
Above are the two 17mm bolts that hold the diff to the PPF. The rearmost (closest to the diff) bolt is nothing special, simply unbolt it and you're done with it.
Now, 'simply' is probably not quite how it'll go because it's very tight. I started by breaking a 3/8" drive socket, so you might as well skip that step and go straight to the 1/2" 6pt socket. You'll also need a lot of leverage.
I had a 2ft breaker bar and a 4ft pipe on it for about ~5ft of leverage. Even with that, it wasn't easy. So yes, it's very tight.
Once you manage to get it going, do the same with the front bolt. Remove both bolts completely.
At this point both bolts are out. The front bolt hole has two sleeves (top and bottom) the are still holding the PPF to the diff. The bottom sleeve is the piece that looks like a big washer. You'll need to pry that out. Notice there is a groove on the side. I used a chisel and carefully tapped it around the 'washer' slowly backing it out. It's maybe an inch tall (or a bit less) so that's how much you'll need to back it out before it's out.
The factory manual then instructs you to insert the large front bolt back on, screw it in about 6 turns and pull down in order to free the top sleeve and slide it down (off of the PPF). You can then insert a small bolt into the hole (#1 in pic) to hold the sleeve in place so you can unscrew the large bolt back out. Conveniently, the ground bolt nearby on the PPF (#2 in the pic) is just the right size.
I didn't have to do that step though. With the hammering involved in backing out the lower sleeve, the upper sleeve fell down on its own so as soon as the lower sleeve was out, the PPF was free.
And.. that's it.
Back to my Miata page.