Can someone tell me how to identify low rise heads on a 390 vs. Medium rise heads? I am trying to find a roller rocker set for my engine, but I was told I might have medium rise heads which none of the kits work for. Any information would be great. Thanks.
ON the medium riser head the intake ports are 2.34 inches.
On the low riser head the intake ports are very close to the bottom bolt hole and the combustion chambers are rough cast.
The medium riser head had fully machined combustion chambers at 88 to 91 cc. Ford does not recommend using these heads on a 390 engine because you have to enlarge the bore chamfer for valve clearance and can break through the cylinder wall.
Ford recommends the low riser head for the 390 engine because the valves are smaller and will fit the 390 bore, the chambers are 72cc and will lower your compression on a 390 about 1 full point, example is from 10.5 to 1 down to 9.5 to 1. you can bring that back up by milling .030 from
the heads and use a thinner head gasket. dont forget to mill the manifold and check pushrod length. The low riser head was a very popular swap for the 390 engine.
What heads you have depends on their casting numbers. These are located between the center spark plugs. And as TomB posted, Medium riser 427 heads won't work on a 390 size bore. Compression ratio is dependent on the chamber size. 72 cc heads don't lower the comp ratio on a 390. There were two basic sized chambers on 390 heads, 68-72cc & 72-74 cc. Post your casting numbers and we can tell you what FE heads you have.