My 89 Merc remote oil filter has a brass allen plug that I want to remove and mount an oil temp sender. Anyone know what size NPT sender I need for the thread of this hole?

take plug to lowes and go up plumbing aisle. while there you might want to get an adaptor to 1/8th pipe. it is probably 1/4 pipe.

Mine is an 88 and is 3/8 pipe.

Jeff

3/8NPT and do not get an adapter. Get the right size sender. If you add adapters it will set the sender back and not in the flow of the oil.