I see multiple problems.
In this pic, is the firering burned thru? or was it torn at removal?
View attachment 1715239908
In this next pic, the cylinder beside it, there seem to be two issues, a burned thru firering and reverse failed area, which I assume happened at teardown?View attachment 1715239910
So I bet those burned thru areas are next to, or close to, the exhaust valves.
In any case, the compression pathway between cylinders is clearly visible.
View attachment 1715239909
So it looks like those two were clearly failing before the compression failure between them happened.
Now, look at those little 1/4 inch holes between the cylinders, do those have corresponding holes in the heads?
Do you have a catch can plumbed to your rad overflow, and was it empty when it overheated?
Next look at the gasket imprinted on the deck. Did you use the proper torquing sequence?
Ok lets look at the pistons; Is that #1 the only one with carbon on top? What happened to the carbon on the other 5?
Can we get a pic of the head, showing the exhaust valves in #1, and also of #6,
and/or 5, plus the area between 5 and 6, with a teensey bit more lighting. And a pic of numbers 1 and 6 plugs showing the porcelains.
How does the hot air from the rad get out of the doghouse? and is the carb forced to breath that hot air?
About what rpm were you running at? If no tach, can you measure the tire diameter or just give me the size so I can estimate the rpm at.....did you say 75mph?, indicated or actual? I'm estimating 3200rpm. Do you have a clue what the advance was at 3200rpm?
Don't put this engine back together just yet, please.
