Is there a common tap size to ream my GP threads - used to have a Mercedes glow plug reamer - cannot find it. I've heard M10x1.0? I was able to back them out with PB blaster and lots of back and forth movement. Both GP's were a little loose which surprised me so no doubt a fair amount of carbon must have moved up into the threads. Installing the new GP's, I found they would go down a fair amount but met some resistance and I thought it best to back off a little vs forcing them because breaking a brand new GP in a head would be just plain stupid. Actually, it just occurred to me that I can take a GP to the hardware store and match thread with a metric nut to determine size and pitch and go from there. Both GP's had way too much resistance and lil 6000 was firing on compression alone - going to be nice to have working glow system soon!