My engine's out of the truck and I'm surprised that the threaded coolant drain plug requires some odd tool to remove it. The plug looks like it should be removed with a square 3/8" socket wrench, but my 3/8" is too big and my 1/4" is too small.

Does anybody know what tool will fit these plugs?

What I really want to do is remove the same size threaded plugs in the rear of the block in search of that spring-loaded oil cooler bypass valve. Just want to make sure that valve is functional before reassembling the engine and I figure its hidden behind one of those threaded plugs that match the coolant drain plugs.

The oil filter bypass valve was easy to check. With the filter adapter removed from the block it was easily accessible.