I got a RB header from a friend but he didn't know what it would fit. My question is: How can you tell if it's for a 12a or 13b? We do know it came out of a 1st gen.

 

13-B bolt holes are 4 5/8 inches apart

 

Thanks!

 

Check for the part number engraved on the engine flange. Sometimes racing beat will engrave it on there and you can cross reference it with their website. I've had success with this method a couple of times too....