I don't think you can go wrong with a MC9.
MC-4C: 4-cycle engines
MC-55: The MC-55 is considered a hot plug and is designed to run on fuel with a low nitro content. Applications for this plug would be planes, tether cars, etc.
MC-59: Like the MC-55, the MC-59 is considered a hot plug. The difference between the 55 and 59 is that the MC-59 is designed to run on fuel with a higher nitro content (low to medium nitro content) than the MC-55. Applications: Boats, planes, heilcopters, and cars.
MC-8: The MC-8 is a medium to cold plug, deisgned to run on fuels with a medium to high nitro content. Applications: Boats, cars, and planes.
MC-9: The McCoy MC-9 is considered a cold plug and is designed to run on fuels with a medium to high nitro content. Applications: Cars, boats, ducted fan and Giant Scale planes.
MC-14: The MC-14 is a hot plug, designed for helicopters and 4-stroke engines.
From reading just these bits of info on McCoy's plugs, you can easily see that general rule...if you're running fuel with a high nitro percentage, you should be using a glow plug that's considered to be a "cold" plug. Conversely, if you're running a low nitro content in your fuel, you should be using a "hot" plug.
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