99-02 Lightnings, Harley's have 4 Threads and under heavy boost( Plugs not torqued to 13 lbs have blown out) 03-04 have 8 thread and will still blow out if not torqued properly, These trucks are really nice but can hurt your bank account, I have put together many 10 secound trucks that are daily drivers, you can keep a stocker safe with not to much timing and a great Tune boost up to about 15 lbs is fine If tuned properly, Good Luck and have Fun,