This is a piston stop for zeroing a degree wheel on a chainsaw engine.  It has M14x1.25 threads, the standard size for chainsaws using a BPMR7A plug or equivalent.  To use it, you rotate the crank one way until the piston rests against the stop, read your degree wheel, then rotate the crank the other way to the stop and take another reading.  Then adjust your degree wheel by half the difference in the readings to have top dead center correctly indicated.  After doing this you should get the same degree wheel reading when the piston contacts the stop going both ways.  I wouldn't use this as a stop for removing a clutch or flywheel.  I have another design that works for that.

I printed this in Voxel PLA+ HS.  I used variable layer height to set the threaded section and the top of the dome of the stop to 0.08mm height.  This printed for me with no supports and the threads came out quite nice.