I am putting a Mark IV back together and wondering if people have had experience with plugging the oil relief in the factory merc adapter that bolts to the filter pad. Previous to this rebuild, I installed a considerbly larger oil cooler, but still could get 250 degree oil in the pan and therorized a good bunch of the oil was bypassing and not getting to the cooler.

I just did mine yesterday. I got a 20mm freeze plug from my local auto parts store. Dorman P/N 555-101 It fit perfect.

Mine have been done for years, 1/2npt I believe. Didn't help oil temps though and that is why I originally blocked them. Since I have learned that other things like a/f ratio, bearing clearances and coolant temp play a larger role in excessive oil temps.

Have any of you fixed your high oil temp issue? My Mark IV block runs around 300*