Let's start over --
What RCIGN was explaining was simply clearance between closest possible point between piston and top of combustion chamber -
The shape of the chamber is quite varied-but most are symmetrical being flat / slightly inclined,outer portion with a raised dome in the centre.
Don't use piano wire - that's the hard way to do it (rimshot)
He shoves the lead wire into the hole and angles it to one side
then hand twists the crank to bring the piston up, smashing the wire flat. remove and measure thickness of the tiny lead wire.
It is a very time accepted practice.
Some elderly designs used a wedge shaped chamber (similar to those in auto engines) and you had to feel for the "low spot. Others had weird shapes with slots for piston baffles - and the pistons-depending on who designed the thing, have been all kinds of shapes -concave /convex, center dished , domed -etc..
Most current stuff - squish band and center dome