Ford has used them on their 3.8 v6 supercharged and n/a. Their 6.0 and 6.4 Diesels are also TTY. Several manufactures use them. No, you do NOT have to replace them with TTY when you buy new. When doing a head swap on the 3.8 fords, Fel-Pro sells the head kit with or without the head bolts. The bolts they include are TTY. Just go to autozones website and browse there parts and see what vehicles need them.