HI I AM CONSTANTLY DEALING IN NEW AND VERY VERY LOW TIME HIGH PERFORMANCE GLO ENGINES I CAN NOT TELL YOU HOW MANY ENGINES I HAVE PURCHASED THAT HAD THE WRONG GLO PLUG IN THEM-YOU SHOULD RUN A SHORT GLO PLUG -ONLY IN GLO ENGINES THAT ARE SIZE 15 AND SMALLER-A LONG GLO PLUG SHOULD BE RUN IN ANY ENGINE LARGER THAN A 15THE SIZE OF THE GLO PLUG COMPLETELY CHANGES THE TIMING OF THE FIRING OF THE ENGINE SO THEREFORE WILL EFFECT PERFORMANCE AND RELIABILITY VERY MUCHALWAYS SAFE TO GO WITH A MEDIUM HEAT RANGE-THIS INFO MOSTLY APPLIES TO TWO CYCLE GLO ENGINES AS THE FOUR CYCLES LIKE TO RUN ON A LONG OS F PLUG-WHICH I THINK STAYS HOTTER AS THE ENGINE BY DESIGN ONLY FIRES EVER OTHER STROKE THIS INFO IS PRETTY BASIC STUFF-BUT NEED TO GET THE BASICS RIGHT TO ENJOY OUR DAYS OF FLYING-MOVED TO FLOIDA JUST TO FLYTURNING OUT TO BE A GREAT IDEA-WE FLY AND BQ ALL YEAR ROUND IN CITRUS COUNTY - CENTRAL FLORIDAENJOY REGARDS TONY "the omega man " think pink !!