Erik, that is way too much clearance. That number would be OK for a big piston of 85 mm in an ALL cast iron cylinder with less heat expansion. Greg´s number of max. 3.5 thou is about right. You can compare a Vincent cylinder with that of a Yamaha SR 500 or XT, same size of piston, shrunk in cast iron liner, reaching way down into the case and not much more oil flow. The SR oil pump is small, all roller crank, no oil jet for the bore, so no factor here. They go for 3 thou clearance. Same story with two valve /5 or / 6 series BMWs , iron liner in alu cylinder.
I am running 5.5 thou clearance in the iron block 3.8 Jag E-type with that sort of pistons so, no, 5 thou is ridiculous. You just have to see the very different conditions in iron or alu engines while looking at the same types of pistons without any extra features like slots or cast-in steel inserts for expansion control.
All statements are to be seen on a basis of correct engine work, meaning perfect geometric honing process and no distortion of the liner in poorly fitting , too tight engine case.
Vic