I happen to have a nice Nikken metric boring head that was designed to fit a
26mm BT-flange holder. This boring head has a 26mm dia x 26mm straight
cylindrical shank with two radial holes at 90-deg to accept locking set
screws or pins.

Of course, my mill doesn't have a BT spindle; it has an R8 spindle. So my
idea was to use an end mill holder as an adapter for this boring head. But
that presents its own problems as I have yet to encounter a 26mm end mill
holder. (Anyone know a source for these? I find it amazing that the US
machining industry is still so firmly entrenched in the English system.)

Short of finding a 26mm holder, my next thought would be to buy a 1" end
mill holder (25.4mm) and try to modify it to accept the 26mm shank of my
boring head. I just don't know how successful that plan might be. For one
thing, I suspect that the bore of an end mill holder is hardened and ground,
so I will probably have a tough time trying to bore 0.6mm off the inside
diameter in my lathe. Another possibility would be to use an abrasive
cylinder hone to enlarge the bore of the holder. But that may not be a very
accurate operation as I only have the low-cost, sprung-arm hones that are
common in auto parts stores.

Any thoughts or advice?

Thanks,
Michael