If I blast air into the small hole (red arrow on my drawing), coolant splashes out all over the place from the large hole (green arrow).

If I place the air blaster in the main opening with a rag over it to seal it in and then let go after a few seconds, then the coolant just rushes back out. Expansion tank too low to get a good sense of what's going on in there.

I really only think we are dealing with a coolant breach, not combustion gas.

You can make a mold for both holes out of wood and the 2 drill bits you used. See this video https://www.youtube.com/watch?v=Ep0z710Zl5k

You can shape the lead plugs with a file to fit the hole snugly before inserting. To test the hammering to expand the lead plug, make some extras. Drill a 1/2" hole into a block of aluminum, insert lead plug and use a 1/4" diameter punch to hammer it in all around to expand it in. Tap a screw into the lead plug and try to pull it out of the aluminum block to test.

That's what I have been saying repeatedly.

Have you listened at the valve cover oil fill hole while blasting compressed air into the 1/2" hole? Do this with the coolant expansion cap on. Is your broken fuel heater leaking coolant? If so you need to plug that up before the test.

NOT Yet. Sorry. You did ask me to before I got bedridden last weekend.

Yes, understood and already did this.

Yes, purpose of the bolt is to exert force on the lead plug and lock it in place and allow it to be adjustable.

The reason for no pipe thread is to preclude the possibility of cracking the aluminum head.

You'd want to use something like this:

https://www.fastenal.com/products/details/40852?r=~|categoryl1:%22600000%20Fasteners%22|~%20~|categoryl2:%22600047%20Set%20Screws%22|~%20~|cat egoryl3:%22600049%20Socket%20Set%20Screws%22|~%20~|sattr01:^Metric$|~%20~|sattr04:^%22M12%20-%201.75%22$|~

I'd keep it metric so your same hex sockets work :)

I'm still not sure trusting carbon and Epoxy (or lead) to seal out combustion gasses and coolant is a good idea. You'd be better served by removing the fragments, installing a plug/set screw with lots of red loctite to seal combustion gasses, and then another set screw with blue loctite to seal coolant. Assuming that there is not a breached oil and coolant gallery.

since you have drilled the hole 1/2 inch and some what larger, tap drill the hole to
9/16 then tap with a 3/8 pipe tap test fitting every few turns with the plug
https://www.fastenal.com/products/details/426317?term=3%2F8+hhp
until you get three to four turns of engagement. use teflon sealant and tourqe appropriately. your choice of filler below the plug

NO pipe threads in a cast aluminum head with thin sections. Pipe threads cause a lot of radial force, which may crack the head as it is tightened.

NEW INFORMATION
 

Leak Down Test Results:

I am attaching pic again with colored arrows for reference:

http://i194.photobucket.com/albums/z...%20plug/GP.jpg

I blew air into the small (red) hole. This is the ONLY place that has a coolant breach. While blowing air in there I was able to hear air rushing into the expansion tank and splashing coolant with the cap off. Furthermore, when I stopped blowing air and removed air gun the coolant back flowed out from that small hole and ONLY that small hole, not the hole depicted by the Green arrow.

Next I plugged the small (red) hole with a hardened bolt. (just snugged it up). Then I blew air into the opening again so the air would only go into the green arrow. There was NO air rushing into the expansion tank this time. Instead what happened was that it blew off my paper towels that was plugging the first 2 ports where the intake manifold attaches. (see photo below). So this breach is into the airflow section of the head? As we all know, these ports get very gummed up with carbon. So as a further test, I took a 12 gauge copper wire and probed around in the green hole. When I removed the wire it indeed had carbon buildup on it.

At no time did I hear any airflow rushing into the oil cap (which I had tested while blowing air in both red and green holes)

http://i194.photobucket.com/albums/z...55478DBA72.jpg

The two ports without paper towel is where air was rushing out of when I blew air in the Green Hole. No other IM ports to the right had air flowing from them.

I am confident now there is no breach of the pre chamber. Do you agree? I am confident that with a self tapping hardened bolt I can close that up for good. (The red hole).

The question now becomes: How hot does that air get flowing through the IM and the head? If I pack the High Heat JB Weld (or lead) and close the whole thing up with a threaded plug can it withstand the heat? The JB weld product holds up to 400 degree F. And even if it can't, if I successfully seal up the red hole, where will it go anyway?

Thoughts? Questions? Did I miss anything in these tests?

Looking at that last (zoomed out photo) it is obvious that the large breach goes into the intake runner. I doubt the air on the intake side gets hot enough to damage JB weld.

ran the numbers -- at 14.7psig of boost and a 150F intake air temperature, the temperature at the outlet of the compressor would be 284F -- but you should never be at 14.7 psig of boost, so it will probably be less.

Thanks RM - You've all been a big help.

Have you accounted for where all the lost coolant went? Have you turned the engine over by hand a few revolutions?

It's not as low as I originally thought. It is just below the breach apparently because as stated above when I took the air trigger away, the coolant was high enough that the back pressure caused it to back flow out. And even though I can not see a level in the expansion tank, there was enough to cause splashing while blowing air through the red hole.