502 Mag.
Looking in the end with the end cap off there are 3 chambers,center bolt hole.
Can you use a solid disk with full disc gasket as a new end cap?Doing it this way seals each chamber from one another.
Is water suppose to flow around the ends into the different chambers perhaps original end caps are
slightly bevelled?

When I had mine off and cleaned out from a sea pump failure, my end caps off it looked like someone just traced a disk of rubber material and cut a hole in the center. I have never had a problem.

Yes that is what i thought.I had a cracked end and used a stainless disc.
Now have a overheating problem and am starting to wonder how the water passes through.
There was a oring on the center bolt that maybe acts as a seal and spacer allowing flow around the end cap.

yes full gasket is fine

Unhook the raw water hose at the pump and transom. Adapt a garden hose up to the pump end and back flush out the hose. Do the same from raw inlet side of heat exchanger back to raw pump. May be nothing in there, but now you are sure...