For a mower, use the
BPR5ES. Set the
gap to 0.7-0.8mm (0.028-0.031 inch).
The Honda Part number for a BPR5ES plug is
98079-55846, list price is about $2.98 or so. Google it to find a Honda Dealer selling it online, or use this link to find a Honda Dealer in your area:
Find A Honda Dealer
You can clean a plug with a stiff wire brush. If the tip or side electrode are worn, or if there are any cracks in the insulated (body) section, replace the plug. When installing a used plug back into the engine, tighten with a plug wrench or socket fingertight, then another 1/8-1/4 turn to compress the sealing washer. For a new plug, tighten about 1/2 turn.
Torque setting is 14 ft-lbs if you have a torque wrench.
FYI, a loosely-installed plug can get very hot and damage the engine. An overtightened plug can damage the threads.