Correct Spark Plugs

[farleyjd400]

Looking at my service manual and cross referencing the Champion spark plug numbers to NGK I see some differences and am a little confused

My 1973 400, and 1973 JDX8 calls for the short thread (electrode) style plug just different heat range

My 1974 JDX4 special, and 1974 JDX6 calls for the longer thread (electrode) style plug

Does this seem right or am i missing something? I know when I put new longer style plugs in the 73 400 i measured and made sure the plug didnt hit the pistons, and they didnt, but the engine ran a lot better with the shorter style plug.

Thanks

[HoosierDeereMan]

It sounds correct to me. Sometime during the 73 production runs the KEC engines were upgraded with the newer version /5 engine. They had a thicker taller head to aid in cooling. So the plugs are different lengths. Installing the longer plug in place of the shorter one is not a good idea. Yes the plug may not hit the piston but it will have exposed threads protruding from the head. As the plug gets hot, it can distort those exposed threads and carbon will build up on them. When you try to remove it, it can strip the threads right out of the head. Your manual should list the correct Champion plugs to use. You should be able to cross it over to NGK fairly easy.

Troy