This is not true. The method outlined by redline racing is the correct way to do it. Your readings could be off by as much as .005" by doing it through the spark plug hole. Try it both ways and see for yourself. I used to think the spark plug hole trick was ok, until I was taught the correct way to do it. Then I compared my readings from the two methods and the spark plug hole reading is very inaccurate.
Tim @ Titan Racing will tell you to do it the same way. Remove the head, place solder across the top of the piston in the center of the bore, parallel to the wrist-pin. Install & torq head down and bump it back and forth at the top of the bore by hand, then remove the head and measure your solder.
- Jared