Well, this finally got some attention. The photos attached show the finished product.
Due to the angle of the machining face I had to make some drill chuck extenders to get the body of the drill out of the way of the cylinder. For this I used standard grade 5 - 3/8" - 24 bolts, one 4 inch and the other 6". I cut the heads off and then used a 3/8-24 die to thread the bolt shaft. I used the drill press arbor to start the die to make sure that the threads were straight.
Next, mark and center-punch the engine pan casting aligning on the center of the hole inside the block and NOT the exterior flat surface. Start with a 3/32 pilot hole and finish with a 21/64". Intermediate steps are not necessary.
The 1/8"-27 tap fit nicely into a 12-point 9mm socket which I attached to a 12" socket extension. I also braced the shaft of the extension against the cylinder shield and a suitable sized block of wood underneath. This made sure that the tap started straight.
Now a word about NPT threads... The "T" means that they are tapered as in the further the tap goes in the bigger the hole gets. Therefore when tapping something like this it is IMPERATIVE that you don't go too deep! From what I read on Practical Machinist website you are aiming for 2-3 turns by hand with the mating piece. In my case this equated to about 8-10 threads of the tap remaining outside the hole. YMMV, just take your time and check you progress periodically.
The other interesting thing I learned about "NP" (National Pipe) threads are that even though designed for plumbing are NOT self sealing. That is why they have pipe "dope" for assembling plumbing. In my case the pressure switch had thread sealant on it. However, Loctite and Permatex also make thread sealing compound for engine/automotive applications.
After considerable searching I finally found what appears to be the correct 3-5 PSI pressure switch. The Briggs part number is 491657S.
When I was calling around a few local Briggs dealers looking for the pressure switch one of them accused me of polishing a "turd". Well I sort of hope that this will help this engine survive longer and at least the operator will know when they are either low on oil or on too steep of a slope and can take appropriate corrective action.
