14mm short reach will work fine.

Be sure and hold the throttle wide open while you kick until the gauge stops climbing. Both cylinders need to be within 15% of each other. Min pressure needs to be over 100psi. More compression is gooder.

Thanks! Very helpful. Much appreciated.

The adapter should have a fat o-ring to make the seal at the spark plug gasket surface. So the length of thread engagement is meaningless. Therefore, the short tool will work on short OR long reach heads.

I get that the short reach should work on any head.
My concern was that if you stuck in a plug that was too long, or an adapter that was too long, that you'd potentially damage the piston.
Is that not the case? A long reach plug in the wrong head won't get clobbered?
It would be nice to know there's always forgiving room in there for such a mistake.

My concern was that if you stuck in a plug that was too long, or an adapter that was too long, that you'd potentially damage the piston. Is that not the case? A long reach plug in the wrong head won't get clobbered?

If you placed a 3/4" reach plug in a 1/2" reach plug hole, then you probably wouldn't get an opportunity to damage the piston. The piston would either be stopped dead in its tracks during cranking, OR the plug gap would be closed to zero. In either case the engine would never run to do any major damage.