Honda TRX420FPA Owner's Manual page 149

Spark Plug
9. With the plug washer attached, thread the spark plug in by hand to
prevent cross-threading.
10. Tighten the spark plug:
If the old plug is good:
1/8 turn after it seats.
If installing a new plug, tighten it twice to prevent loosening:
a) First, tighten the plug:
NGK: 3/4 turn after it seats.
DENSO: 1/2 turn after it seats.
b) Then loosen the plug.
c) Next, tighten the plug again:
1/8 turn after it seats.
An improperly tightened spark plug can damage the engine. If a plug is
too loose, a piston may be damaged. If a plug is too tight, the threads
may be damaged.
11. Connect the spark plug cap. Take care to avoid pinching any cables or
wires.
12. Install the remaining parts in the reverse order of removal.
140 Servicing Your Honda