Isn't it located on the side of the head on those engines? I also looked but didn't see any plug holes, certainly not in the customary spot on the top of the head. Holes in the side wouldn't show in the posted pics I don't think. Then again, Brian may not have made those holes yet.
To...
Both yours and Sids comments on the spark plug hole are excellent BUT did Brian already machine the spark plug hole? If he did, I highly doubt he would remake the heads. The rocker post is another matter.
There are excellent tutorials on youtube teaching the advanced features of solidworks. I used it heavily in building a gatling gun I recently designed and built. Using the animation features I was able to "operate" everything on screen before cutting metal. It sure saved a ton of issues...
I researched Oatley product #48319 out of curiosity. In the Q and A section Oatley responded that the silver content is less than 2% for that product.
https://www.rona.ca/en/product/oatey-silver-solder-kit-483191-1120099
Just a guess.... but..... L is the standard abbreviation for a inductor in electronics. Rather than a capacitor discharging I am guessing it works on "inductive kick" principle using a inductor.
The pushrods on the rear cylinder (furthermost from the flywheel) look to be bent near the rockers when zooming in on the picture. Is this just a illusion from the camera view?
appears to be separationin degrees between intake and exhaust cam lobe centers. Also, the lobes lobes for the two cylinders appear to be identically in line, that is the intake/exhaust timing is identical for both cylinders.
A normal human can blow a balloon up much faster than in that video AND a human can only generate about 2PSI !!! In short, totally worthless test. 2PSI ain't going to get a engine to run !
Does not appear to be one in his leak down test setup. Possibly there is one (brass) at the cylinder head but if so it is in the wrong location for leak down tests.
You have the line from your compressor disconnected when watching for leaking down, right? Otherwise the regulator is doing exactly what it is intended to do, hold pressure at 60psi regardless of the airflow.
As a test, hold a valve open during your leak down test, does the gauge drop to zero?
I made a set of helical gears for my gatling gun build using Gearotic Motion software on my cnc mill. First time using gearotic and first try the gears came out great. Made with just a 1/16 straight endmill. I believe there is a free trial version.
I am a hack compared to your beautiful...