that is really strange, when i measured mine it was the same diameter that i found in the specs.
i guess one option at a risk is, (unless a return is possible or wanted),
if you know someone with a lathe, bring it in and the block in and have them turn it down,
if you are feeling mechanically inclined
if you don't best would probably figure out how to chuck the head squarely in a drill and use a very fine tooth file (a knife sharpening block or a nail fail even may work, after taping the flat surface where it impacts the top of the block/top of sleeve.
last chance scenario would be using a file/sandpaper (like 400 grit or more) and doing it by hand, this would be very difficult to keep it perfectly round though, and again tape the deck of the head so you wont dig into that causing air leaks
either way if this is the route you take test fit very often and go very slow.
my thoughts on that anyway, so take that for what you want.
seems thunder tiger is really hard to find parts for on older models without buying a new engine.
