I purchased a set of heads for a small block that carry the casting number 3991492, cast on 3F17. I think these should be a standard service replacement head but something funny is going on. My Chevy power manual says the plugs should be 14mm flat seal with 3/8 reach. This is not correct. Using a AC 44 the electrode in down completely inside the treads. The plug reach is too short. I measured the length of the tread and it looks like 1/2. A 14mm tapered seams to fit but they are not machined a differently then a set of 461 heads which do work with a 3/8 reach. Also I'm not sure what the cc size of the combustion chamber is supposed to be on these heads, They have the coolant crossover still in them so they are not an off road head. Any ideas. Jerry