Q: I have a 1989 5.0L Mustang, and I recently purchased a Ford Performance X303-grind cam for it. I have some questions concerning valve clearance. Can I run stock pistons with this cam, or will it be necessary to alter the piston to achieve the proper piston valve clearance. How much clearance is recommended with this cam?
A: Your valve clearance is going to be very close with that cam and stock pistons and cylinder heads. The only way to know for sure is to install the cam and do a clearance check by laying some clay on one of the pistons, rotate the engine through one complete cycle. You will need at least .080-inch of clearance on the intake valve and .100-inch on the exhaust. If you decide to switch over to aftermarket heads with larger valves (which is what the X303 cam is really designed for), you will definitely have clearance problems with the stock pistons. You might want to consider a less aggressive cam grind, like Ford Performance Parts’ E303 or B303 cam.