Jerry B

Need to buy new cam and lifters for 502 Mag MPI. Twin application so I need a factory spec replacement. Also looks like I may have to buy one new head. Spark plug was rusted in #7 so bad that there may not be any plug seat left by the time I get the old plug all out. I work for a GM dealer so I can get the heads at a good price but not sure which heads to get. Does anyone know what the valve diameter's are on this engine. I'm hoping to buy a bare head or heads and use my old valves.
Thanks for anyones help.

Trippin

Valve Diameter's on the 502 are as follows:

2.225 (56.515) INTAKE
1.880 (47.754) EXHAUST

The margin is: 1/32 (.79)

Jerry B

Thanks Trippin. I thought that would be the valve size. I think I will buy a new set of heads. GM has them with the springs and valves. They are square port with open chamber so I think they are the correct heads. I sort of hate to buy only one just in case there are any differences. Do you think the springs will be safe to use with a factory spec cam for this engine?
Also Crane cams show a camshaft in their listing that is supposed to be standard for several Mercruiser engines but the 502 is not on the list. I will have to make a few calls.

Trippin

Factory springs will be fine as long as they are an engineered set matched to the camshaft. Factory automotive parts may not offer roller lifters, stems or cams as an option. Almost all current marine applications are roller oriented. Keep that in mind when ordering or building your project.

509 SC

I'm not sure what year motor you have (or if their different) but I believe the intake valve is 2.19" . I believe the HP 502's are 2.25" on the intake. Steve