Originally Posted by
TRArena
IMHO the P3 is the best plug for the MRT GRP .21. However, once you use a P3 you need to stick with it as any change to the thread pattern on the head will make it will be difficult for another plug to seal properly. I think this is what was meant on an earlier post.
There is the answer, once you use the P3 plug you need to stick with the P3.. as for any other plug. It seems that the GRP head is a little finicky with plug threads.. I am using the P4 and it is working great in Arizona... clocking in at 200 after a 10 minute main.