FRPP's hot Two-Valve M-6049-D46 cylinder heads use a full-thread-reach spark plug, a tidbit the FRPP catalog won't tell you. All other modular engines-GT and Cobra-use the half-threaded ...

TIP For a cylinder below 100 psi, pour 1 teaspoon of engine oil into the plug hole and retest. If the reading jumps, the piston rings are worn. If not, think valve problems.