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So the big question is, "How do I KNOW DEFINITELY when the distributor is fully seated and fully engaging the oil pump?"
I posed that question to some VCCA members and two (Ron Beam and Chip Sweet) gave me the answer. Although they have done so many engines that they have a 'feel' for it, they suggested measuring the depth of the distributor hole (from the top of the clamp plate to the top of the bearing washer on the oil pump) and then put a mark on side of the distributor body that I can SEE, which will tell me if the distributor body is down all the way or still sticking up above the oil pump shaft. BRILLIANT!!
Photo 16 shows a long screwdriver in the distributor hole and me marking where it meets the clamp plate with a Sharpie pen. Photo 17 shows that the depth of the hole is 2.750 inches. Yes!! This measurement should work for all '26, '27, and '28 engines. Photo 18 shows the bottom roll pin being inserted into the distributor gear.
Photo 19 shows the hole-depth-indicating mark I made on the side of the distributor. Building upon the flat washer experiment above, I put a small strip of paint masking tape on the top of the distributor shaft to show me the orientation of the bottom pin in the distributor gear. This is shown in photo 20.
As an aside, as Ray Holland (AntiqueMechanic) mentioned in his "School is in Session" document, the July 1927 Chevrolet Service News (page 27) discusses the process of installing the distributor. This description, however, does not discuss how to get the distributor shaft to properly line up with the oil pump shaft.
Photo 21 shows the distributor in its hole, but NOT aligned properly with the oil pump shaft slot. As you can see in the photo, the hole-depth-indicating mark is about 3/8 of an inch above the clamping plate. That is proof positive that the distributor is NOT FULLY SEATED AND IS NOT IN THE OIL PUMP SHAFT SLOT.
Drum roll please!!
After correctly orientating the oil pump shaft slot, I dropped the distributor into its hole and (YAY!!) the hole-depth-indicating mark has dropped further down and is now close to the clamping plate. See photo 22.
Now I am confident that I can positively determine if the distributor is fully seated and fully engaged with the oil pump shaft slot. I pulled the distributor out again, put some assembly grease on the gear (photo 23), and dropped it in the hole for the last time (photo 24). I was careful not to get any assembly lube on the inside of the clamp plate hole.
You might have noticed that at the top of this thread, I positioned the crankshaft at TDC (U|C) instead of 25 degrees before TDC and set the retard/advance lever at fully retarded (as opposed to fully advanced), as recommended by the literature.
To my thinking, I want to ensure that when I'm starting the engine, I want to KNOW that the spark is happening AT TDC (not before TDC at all), rather than relying on the levers to get it close when the engine is timed for the spark to be fully advanced (25 degrees before TDC). I've found this makes for an easy starting engine and it seems to perform just fine when I fully advance the lever for driving.
A couple of days after I put the distributor back in, I took Lurch out for a 10 mile drive. Everything went extremely well! In fact, the oil pressure is higher than it was before this episode. I now get 18 psi at cold start and it drops to 5 psi at idle when warm. This is with a gear-type oil pump that I put in during the 2017 engine rebuild. Here's a link to that adventure: Resurrecting a '28 4 Banger
