| Here's the story of making an N.A. manual transmission fit around a T.T. flywheel and pressure plate, part of my recent 5 speed conversion. I have read that in some cases the N.A. bell housing is big enough so it's not an issue, and might have seen somewhere that for some housings, only a bit of grinding is necessary but for this one, a fair amount of grinding was needed. Before you get too far into the job, make sure your patient has similar anatomy to the one in these pictures. This particular transmission was a JDM import, year unknown. The flywheel was an F1 Racing X-Lite Chromoly, P.N. 51024FX. It had 126 teeth and the outer diameter (maximum dia. of the teeth, not pitch diameter) was 12 5/8" (320.675mm) O.D. to the outer D of the gear teeth.
1) One thing you will want to know is how far rearward into the bell housing the flywheel goes once the transmission is installed. That is how far in you'll want to grind to accommodate the larger gear teeth. There are a few ways to figure this out even if you've never seen a manual transmission car, but you can just cheat and look ahead to this picture:
The rearward face the gear teeth turned out to be about 1 3/8" in from the mounting surface of the front of the bell housing. 2) Here's a plastic bushing so the flywheel could be centered on the transmission input shaft. The orientation is like you're looking at the back of the crankshaft. The spline goes into the opening and the flywheel seats onto the outer shoulder as if it was the crankshaft. 
It is very helpful for figuring out the starter motor shimming and also seeing where the bell housing is going to rub against the flywheel and pressure plate. Note that it should not be your only source of info; there is some play in the input shaft. 3) A blurry picture but you can see a couple of obvious places where metal is going to get removed. There are others. The majority of your grinding will be on that thin web near the starter. 
4) Spray paint the bell housing first if you really want to be able to see where you're grinding. Give the paint a chance to dry completely or sticking aluminum dust is going to ruin the effect as you will see... 
5) Start grinding. Here's a look at the lower passenger's side. Sometimes the grinding wheel was too big or it wandered, those are "incidental/accidental". This is probably far more grinding than necessary but I was not sure about the pressure plate, pressure plate bolts, or the true final location of the flywheel (remember the input shaft can move around a bit). The most common thing you'll be going after are bulges where a bolt head is on the outside. I just knocked them down to flush. 
5) Rotating around, here is how the lower driver's side of the inner bell housing might look when you're done.
6) Keep rotating, there's one more spot a little past the top. 
7) The different views and rotation of the camera don't make it easy, but with the flywheel and pressure plate in place, here's how it fits inside the bell housing now. All the ground spots are marked green. The pressure plate goes a lot further in but is not as big diameter so you want to push the flywheel in a little farther than you know it should be and spin to check for snag spots. 
SHIMMING THE STARTER MOTOR. 1) Again, the plastic bushing can't be the only thing you rely upon. For example if someone could count the number of teeth on a N.A. manual flywheel, there are formulas for what the change ought to be. I'm still figuring it out because my starter sometimes grinds but it did that before the conversion. I will post a follow up if I change the shimming.
Here are various shim thicknesses and what they look like in terms of gear meshing.
For now, I am going with the .089" or 2.24mm shim. I think the magic number is .100" or 2.54 mm so I might make that a little larger. Look for a follow up post. 2) Future owners or mechanics will thank you if you make the shims a permanent addition to the transmission with epoxy. Bonus points for stripping the bottom hole and putting an insert into it. For some reason this picture was blue... 
Thanks to Midlife Z, twinturbineZ, SlicktopTT, Guapo, Sebastian & Greg, BOOZTD 3, SpartanZ32 and EricTTZ for help with the 5 speed conversion. As a newly minted 5 speed converter, I would be happy to answer anyone's questions on that job. Wish me luck on the smog. Such fun!
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Grinding and shimming an N.A. M.T. for a T.T. flywheel. - 
