The Stovebolt Site Forums The Shop Area Engine Shop Flywheel specs 1939 JD (3/4 ton)

OK, here are some measurements of the flywheel I have. It's used, but no idea whether it may have been resurfaced.

Thickness at rim -- 1.17" (may have originally been 1-1/4" rough casting before factory machining.)
Flywheel mounting (crankshaft side) is ~1/4" below rim.
Flywheel mounting (clutch side is ~1/2" below wear face
Thickness at flywheel mounting ~0.45"
Back surface of flywheel mounting to clutch disk springs - 0.95" (from previous dimension, that leaves ~0.40" from clutch side of flywheel mount to clutch disk springs.
Disk lining (used disk) is about 1/16" to the rivet heads. That much could be allowed for wear before the flywheel to crankshaft bolt heads would hit the disk springs.

Per the attached picture (sorry about the teeny tiny text, you'll need to blow it up to see well,) there's 0.258 between the wear surface and the bolt heads and the disk springs take up about 1/16" of that space. And probably where you should think about replacing the flywheel, there's a machined ring just inboard of the clutch disk wear position that's 0.015 below the wear surface. If that ring starts getting cut, that might be a good stopping point for resurfacing.