A true 240 is .285" over or 7.25mm over, making the bore 73.25mm. .4 over would be 10.17mm over and would make the bore @76mm which would be @258.6cc on a stock stroke(57mm) blaster. The 240 kits come with a 72mm piston and produce 232cc on a stock stroke crank and 245cc with a +3mm crank.
An easy formula to figure displacement is (bore)x(bore)x(stroke)x(.0007854)x(# of cyls)=displacement