When you say 18.5 hp, I'm assuming this is a 31P777 model single cylinder overhead valve engine or similar? Is it's different, please let me know, but the following is correct for this family of engines.
Intake - .003"/.005"
Exhaust - .005"/.007"
The setting should be done on a cold engine with the piston 1/4" past top dead center on the compression stroke.
Hope this is helpful!