The amt of psi to blow out the spark plug porcelain like that is super high. Detonation and/or preignition is probably the culprit.

Last time i saw plugs like yours was when nitrous was in it’s infancy. Damagecwasn’t usually limited to the plugs unfortunately. Those where confusing and costly days. Lol.

Also, buy your AcDelco plugs locally, not from internet, just in case that is an issue.

Tom,

Thanks for digging that up, yes all pistons resembled number 2. In that picture I was in the middle of cleaning things up for install. Now at the second tear down, the piston tops looked to be in good order with Cylinder 7 showing some dings from the electrode.

Yes

SB,

Appreciate the input! Understanding the costly part, it’s more aggravating climbing in and out of the boat more than anything. I wanted to at least get through the 2021 Season before pulling these engines for replacement but it seems the odds are against me here. I have seen the AC Delhi differences you mentioned from Local to Internet. Also learned my lesson there before.

Talking out loud, roundtable style…..Well, melting electrodes is too much heat for that plug too handle. Typically what I was digging into but could be cooling issue like Bill K was digging into, I guess. And this coukd explain why same bank each time. Wrong headgaskets for gen of head/block or a flow issue on that side or ????? Hard to fathom lno piston top loss of aluminum too, but here we are :)

I agree with SB possible wrong head gasket or blockage with no water flow around the cylinder wall.

Thanks F2, I am using 17046 head gaskets. During the initial disassembly it had 27-13218 gaskets. Now, when I pulled the heads originally, I found they were 188 castings with 2.19 intake valve sizes, not the 2.25 that the 420s should have. I was able to get a set of used, fresh 188’s with the appropriate 420 intake valves.

Talking out loud was what I was hoping for, I’m dying to get to the bottom of this but it sounds like I should pull it all apart to get another look at things and thoroughly flush the block. Damn…

Im not qualified for much of an opinion but these guys seem to be on the right track.

one bank tends to run leaner than the other but blowing up spark plugs is a bigger deal than that. Water blockage on that side of the block looks highly plausible. Is there a way to measure water flow from side to side? I don’t think measuring pressure would pinpoint anything.

I may be able to use a series of short flanged bolts with rubber washers to plug the head bolt holes on the block and rubber plugs for the water passages then force water into the block from the circulating pump passage.