.040 over pistons are legal if they'll fit in an .040 overbore motor. If one overbores by more than .040 to give the extra clearance necessary for racing IT motors and especially forged pistons, then I would argue that this is illegal according to a strict reading of the rules. It all depends on the factory specification and allowances for a stock bore + .040 to it.

For example, what if I put .040 pistons but the cylinder's have worn to .045 or even .050 overbore. Rings are unrestricted so I get same thickness but wider rings made to fill the cylinder. Yes, I'll have excessive piston slap but there is some limit where I can get a wee bit more displacement and more power.

.040 overbore may not give enough clearance for .040 forged pistons which need about .005 minimum clearance.

Now, what if I build a legal .040 motor. After a long season, the tops of the cylinders have worn so I now have up to .043 overbore. Am I legal or ?