On-road guidelines
1- .12 to .18 engines using 20-percent nitro fuel need a medium glow plug
2- .21 engines using 30-percent nitro fuel need a medium-cold glow plug
Off-road guidelines
1- .12 to .18 engines using 20-percent nitro fuel need a hot or medium glow plug
2- .18 to .21 and up engines using 30-percent fuel need a medium glow plug
These guidelines are for factory stock engines that are unmodified.