Why is pinch important?

[RCModelReviews]

There are no real rules in respect to pinch.

I've had strong-running engines ABC engines that show no pinch at all and some with lots of pinch that weren't worth a bean.

This is probably explained by the fact that different manufacturers use different metals for pistons and liners and those metals have different rates of expansion.

If a manufacturer uses a liner with high thermal expansion rates then it will benefit from more pinch than when the liner is made from a metal with a lower expansion rate.

You can get a liner redimensioned but I wouldn't bother -- if it's that bad then you ought to get a new piston/liner set anyway.

If an engine started out with strong pinch but has lost it then either the piston or liner has worn. If it's excessively worn then you will likely find that it won't idle reliably and may be hard to tune -- due to excessive piston blow-by.

Adding extra oil can compensate slightly by creating a better piston/liner seal -- but that will also reduce your top-end due to viscous drag.