Hi Mike,
I didn't have my TR4 flywheel drilled to fit the smaller diaphragm clutch from TR4A and later, so I can't tell you anything about the cost of this procedure. I do know it's possible to have re-drilled (while you are at it, it's highly advised to add bolts at the crankshaft - perhaps a total of 8 instead of the original 4 - for extra strength and security.) Replacing the TR4 clutch with the smaller, lighter and stronger TR4A clutch was a common racing modification back in the 1960s. In fact, I think it was recommended in the Competition Preparation Manual. It makes even more sense today because the TR4A clutch is more easily available and generally seems less expensive. The later diapraghm-style pressure plate is probably more reliable than the earlier one. On the other hand, I think the earlier pressure plate can be rebuilt, while the later one cannot. (There are some separate reliability issues regarding the later throwout bearings, clutch shafts and clutch slave cylinders. But, there are some solutions, as well.)
I replaced the TR4 flywheel with a lighter, aluminum TR4A/250/6 flywheel, rather than rework the original, heavy steel flywheel. A bit more expensive route, of course, but I was going to a lightened flywheel anyway, plus there is a ring gear bolt broken off and jammed in the old TR4 flywheel, needing repair before it could be used.
However, I gotta tell you, I think there is more to it than just redrilling the flywheel and throwing it all back together. I don't think your TR4 gearbox will fit up directly to a TR4A clutch. I'm fairly certain the input shaft diameter at the splines are larger on the earlier gearbox. Also, the throwout bearing carrier sleeve (aka, the "front cover") is a larger diameter on the earlier box.
I can't confirm this right now. Although TR4 gearbox is easily accessible in my garage, the TR4A gearbox/flywheel/clutch are all currently installed in the car. You might be able to gather some more information by looking up the different parts numbers at Moss' website, or TRF's. Alternatively, give John Swauger a call at TRF's Level 2 sales line and chat with him. He'll know for sure.
I do know for certain the popular Gunst throwout bearing cannot be fitted to TR4, but fits 4A through 6, which is a clue to the different size bearing carrier sleeves/front cover.
I also know for certain that the input shafts differ from one to the other in another respect. The two gearboxes were stored for a while in my garage, side-by-side, and the 4A (which is actually a sedan gearbox with the heavier & stronger bellhousing flange, modified to TR4A specs some years ago) can sit flat directly on the floor when stood on the bellhousing. The TR4 box needs a couple wood blocks because the "nose" of the input shaft protrudes about 3/8 or 1/2" beyond the bellhousing. It's possible to fit up the shorter input shaft to the pilot bushing in the rear of the earlier crank, but it doesn't fit in the full depth. This may or may not be a problem. Several folks have told me it's not. I have considered having a special bearing made, though.
Hope this helps!
Alan Myers
San Jose, Calif.
'62 TR4 CT17602L
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