Super Tigre (Como) produced a .60 a .75 and a .90 in the same basic case by adding a little metal on the bottom of the case to clear the longer rod(s) and then boring the cylinder and thinning the sleeve to accommodate the larger bore pistons. Evolution increased the .46NT to make the .60 NX. I am guessing that most of the .50 and .55 engines were once .46 size and the .46 likely .40. Bigger is better, right? My facts on some of these may be wrong, but of good intention.