look on the front of the casting of the head, where the exhaust port #4 is, there should be a set of symbols in circles just above it. The first one (from the left), should have a number, than a "|" than another number. In example, (8|9) but it will be sideways when you look at it. That will tell you the year head it is from. I have a 1st Gen, pr3-2. Its from a 91.