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Kohler L600, J8 vs H10 spark plug

Kenny Crenshaw

My Kohler L600 calls for Champion J8 spark plugs with a 3/8 thread, but, the threads are nearly 1/2 inch deep. Champion H10 (same heat range as J8) spark plugs have a 7/16 thread depth and seem to be a better fit. They reach down into the head a little better, but not seemingly too far. It would seem to me that the H10's would be better. It's not like it is going to raise the 6.4:1 compression ratio to a dangerous level. My thinking is that the little extra bit the plug sticks down can only help igniton. The J8 actually is recessed into the head. Am I overlooking anything?

Andrew Mackey

Last Subscription Date
Not sure of the original plug for that engine - you need to pull the head and install the H plug in it and see if the threads on the plug are exposed to the combustion chamber. If plug threads are visible, then you need the J series (3/8" thread length-14mm diameter). If you do not want to pull the head, use the J series plug. Having the head threads exposed (short plug) won't hurt the engine, and it can actually help prevent oil fouling. If the H fits better thread wise, make sure that the electrode and side bar does NOT extend beyond the internal surface of the head. There may be an interference problem if it does (valve or piston head contact) The J series is 3/8" reach 14mm body. UNDER NO CIRCUMSTANCES use an N series plug (1""reach 14mm). You will either bust a piston or bend up a valve if you do!:eek:

Looking on the internet, the Champion J8-C (copper core) is recommended. The J-8 will work just fine. As I stated before, the H may be too long.

Kenny Crenshaw

Thank you. As I said in my first post, the manual calls for a J8. I have the head off and the H10 still does not use all of the threads available. The spark plug is situated in the area betweem the piston and the valves. No possibility of interference.