It is all about the ignition timing
R/C Tech Forums - View Single Post - How do you adjust engine timing on nitro engines?

With this knowledge there is no universal list of what size engine using what headplay and what amount of headshims

Basically all RTR engines use a plug in the same range, all ,12 racing engines use a plug in a same range, all .21 offroad competition engines use a plug in a same range and all onroad .21 competition engines use a plug in a same range.

And yes, basically this counts for any glowplug engine. How lower the compression the warmer glowplug you have to use but as a side effect, the wider range of nitro percentage you can use. With higher compression engines using cold plugs that range is more tight and critical