spar plug

[tubman]

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Originally Posted by joe 1950

i always run champion H- 10 in my 50 cpe 26 years and never had a problem with them good luck

Just about all aftermarket flathead aluminum heads I have ever seen use longer than stock spark plugs (the threaded part). The reason for this is that cast iron is stronger than aluminum which needs more threads to hold the plug. Running stock reach (3/8") plugs in aluminum heads risks stripped threads. It also shrouds the electrode in the plug hole and makes the spark less efficient.

It's not that lots of people haven't done it; I'd say the majority of used heads I have come across have the bottom 2-3 threads all carboned up. It's just not correct. Most use 7/16", 1/2", or even 3/4" reach plugs.