You shouldn't be tightening them until they bottom out. The thread pitch on a 14 mm plug is almost exactly 20 tpi. If you tighten them an additional 1/2 -2/3 turn as is suggested by NGK, that equates to .025"-.033", which not going to change the reach significantly. Besides, the nose of the 5671 plug is a lot shorter than the TR55's anyway.