Cases aren't bored out until you get to 105mm pistons and cylinders. All T-4 motors have the same "spigot" hole size. This is the hole the cylinder fits into. 1.7L and 1.8L used identical cranks and rods, the cylinder bore diameter is different (90mm vs.93mm) and the outer cylinder head side sealing area is different (1-2mm difference, not sure). So you 'can' bore out the 1.7L heads to accept either 1.8L or 2.0L pistons, but it is faulse economy to weld and cut 1.8 or 2.0L heads to fit 1.7L cylinders.
Problem is the 1.7L heads have small valves. Best bet is to either stick to a stock 1.7L (80 hp) or rebuild the 2.0L bottom end, use 914 2.0L P/C's (1.8L P/c's won't work with a 2.0L crank because the wrist pin is in the wrong location), and the 1.8L heads (914 2.0L heads would be better, but cost money).
Stay FAR away from 2.0L bus pistons and heads. The pistons are dished, and the heads have smaller valves than the 914 1.7L. A stock 2.0L bus only made 67hp.
P.S.......
\BUY TOM WILSON'S BOOK!!!!!
It tells you everythinh you even wanted to know..
[This message has been edited by JP Noonan (edited 07-28-2001).]