This info is for engines with CAST IN HEADS

Many folks just use soft solder threw the spark plug hole > Unfortunately this cocks the piston in bore and loads up the bearings giving a FALSE measurement.

In a HIGH performance motor you need to set it up by knowing the actual Mechanical clearance with piston nice and square within bore.

To find "Actual" clearance you must remove cylinder & piston. (Remove ring from piston.)

Using a depth Micrometer or "** Dial Calipers **
Insert piston into cylinder and put it up into head
until it stops at TDC , Hold it there & measure threw plug
hole the distance from center of piston (High spot) to
edge on spark plug hole ... Write down this distance.
Next put piston back onto rod w/o ring or a base
gasket ***
Lightly tighten cylinder back onto cases.
Next move piston up to TDC & remeasure distance from
plug hole to top of piston. Write this distance down.

*** Now subtract the two #'s from each other. The # you are left with is the "ACTUAL" mechanical clearance
within the engine.

You adjust as needed by adding/decreasing gasket thickness or machining
the base of cylinder.