[Dave in MN]

Purdy is probably correct about the 3/8 by 32 but I have found George Miller's suggestion works for me.

I use a 1/8" pipe thread tap and it always works. I spot weld the 3/8"steel pipe to the cap with stainless wire and find it holds well. I also drill out the oil passage in the cap to 3/8" when doing this conversion. Be careful, go slow, when the tip of the drill bit nears the back of the cap as the bit can bind in the grooves.

The tapped hole is not 90 degrees to the parting line of the cap. I built a jig from a cut-off of 10" structural "C" channel to position the cap to achieve the correct angle. If you place a 3/16" spacer under the front edge of cap with the rear of the cap on the bed of your drill press, the angle of the pipe will be correct. I use a 5/16" drill to prepare the cap for the 1/8" pipe thread tap and while the cap is in the jig, I use the drill press to start the tap to achieve alignment.
Photos below:
This jig is used for multiple functions...so ignore the other holes and knotches.

The bottom row of photos shows the procedure to enlarge the oil passage.
Remove galley plug with a punch...Drill passage with a 3/8" bit....continue drilling but don't drill out the back side. The last photo shows where to stop.
Good Day!