Both Snap-ON and KD sell the "BACK-TAP" internal spark plug internal rethread tool. "Patented design allows for chasing plug threads from inside the cylinder head and working outward...." Slim profile slips inside the spark plug port. Adjusting knob expands the thread chaser to exact thread size. Debris clings to tool...Eliminates chance of cross threading.

Snap-On: 12mm=SPHR3
" : 14mm=SPHR1
" : 18mm=SPHR4

KD: 14mm=KD3545

5280BENZ