ORIGINAL: pe reivers
... If you want to guild the lily, make a bolt with sheath that has a scale divided in 10 (metric). This will give you very exact piston stop positions for ignition timing purposes.
For a soft and solid piston stop, just stuff some twine or thin rope down the sparkplug hole.
Re the graduated stop, that is what my motorcycle piston stop has built into it.
Re the rope trick, we used to do this so we could replace valve springs/seals/keepers on car engines way back in the day. The rope kept the valves from falling into the bore while we worked on the other valve bits.
Regards, Richard/Club SAITO #635