I'm a bit late to this discussion; but IMO your workaround should be fine as
a long-term fix. Once the transmission has been installed, the pilot
bearing is trapped by the gearbox input shaft. It might drift fore/aft a
bit more (when/if the interference fit comes loose), but I do not believe
that will harm anything as long as at least part of the bearing remains
within the crankshaft. I've been running a TR6 gearbox for about 15 years
that only engages the pilot bearing about half as much as the original TR3
box did, and I've never had any problems with it.
I'm also puzzled as to why you are looking for a flywheel that has to be
modified (again), rather than using a TR4A flywheel (which is already
drilled for the TR6 clutch).
But I've got several later 3A flywheels available if you don't find one
closer to home (or decide to use a 4A instead).
-- Randall
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