I was raising the compression question issue because the pistons you are using appear to have more than a +1CC dome?, 005" in the whole and a .020" cut of the head surface and a .041" head gasket will get the compression to about 9.5 to 1 . This engine with an iron head running warm with a fairly moderate ignition lead which may be near it's high at a no-load 1800 rpm engine speed, depending on fuel quality and type could have help create some edge of detonation conditions in that cylinder. Were air fuels ever set up on the engine after the build for jetting and such? What kind of timing was in the engine?
Obviously, there are a lot of possibilities here and I don't think anyone at this point can point to one direct cause or issue. You could have had a exhaust valve seat that was in improperly or with a slight crack after install? The cylinder #7 was oiling a bit too much leading to some deposits that pre-ignited in that cylinder? As others mentioned here you could have had a 5/7 crossfiring condition? Head gasket could have failed between 5 &7? we could go on and on with suppositions here. In any event the physical damage from the valve seat bouncing around in the cylinder did a lot of quick physical damage, but IN TWO SECONDS or so that much edge of piston damage and underside overheating did not occur. That took some time and indicates something else was wrong in this engine maybe long before it broke up.
I know its painful to look back after a loss like this but its obviously really important that you revisit a lot of examination,build and setup issues before you put a lot more good money down and set up the new replacement new engine. Its always kind of scary when you try to put your faith in a new redo when one is not sure what caused the original failure. One thing you can do is make sure that all the parts and new parts are in top flite condition and setup and that you have good specs. and assembly procedures for what you are putting together.
Not sure any of this will help but just trying to help isolate what may have been problems and help you decide where you might want to go from here.

Best Regards,
Ray @ Raylar

Sounding more like the dropped seat was a result of the problem and not the cause.

Agree.. I'm gonna pull the heads apart tonight and take them both into my Machine guy and see what he thinks, check for cracks, maybe have them pressure tested before I make any rash decisions.. if their questionable, their outta here.

wow i ve been following this thread as well.

hate to see this happen to a "true do it your selfer."

i jsut had my new motor built and i installe dit myself and htis makes me cringe! lol

somebody said earlier in this post that filling up on land is dangerous for a marine engine....mine is 9.3 to 1 compression 468 with a holley 750 and i run 89 octane and i fill up on land. am i lookin at a similar problem lurking?

im just as curious as everyone else about the actual cause of this catastrophe....

-lean condition in the cylinder due to low octane fuel and high compression? seems like it would have been occuring in more than one cylinder if this was the across the board cause.

-detonation caused by 5 and 7 jumping fire or a cracked head....cracked the valve seat....flooded the cylinder...then came apart?

as Raylar said...the burnt underside of the main piston in question may be the most telling piece of evidence to be seen. this indicates an ongoing problem from the onset....either a cracked head to begin with ....or maybe the 5 and 7 issue. (im not real sure on how the 5 and 7 issue works as far as the "plug wires jumping from one to another" can someone explain this phenomenon and how to check for and prevent this?)

Actually ethanol suppresses detonation vs non ethanol so it would help in a high compression situation. Also filling on land you can have some confidence that the fuel is "turning" in the tanks, not becoming old and stale. I always wonder about that when getting fuel on the water.

5 fires just before 7 in the firing order. If the wires were failing dielectrically and they were touching each other, the spark from 5 could travel into 7 causing the plug to fire way too soon (90*)before tdc. Your piston would hate that. Looking closely at the piston from 7 I don't detect signs of detonation though, just the heat on the underside...

When you have the head off look at the #7 exhaust valve spring closely. Maybe the spring is broke and was beating the seat to death which caused it to come out.

I appreciate all the input, and can understand the skepticism. I'm just a backyard engine builder and bench racer. people like myself, come on here looking to professionals like yourself for advice and information that we otherwise, don't have access to. Of course some of us are just looking for sympathy for our own short comings...LOL!!

I had a plug loosen up (obviously I didn't tighten it sufficiently) in that cylinder early on. I replaced the plug, and the Boat ran fine for the rest of summer. At first I didn't think that would be the issue, but now I wonder.

Your opinion?

Dave

A sure sign of detonation is loose spark plug(s).

Did you change the cam from your original build spec of 228/242 650/603? and why or who told you to run 180# on the seat?

Still does not explain the excessive heat.

Dave, do yourself a favor, and throw that cam as far away as you can, and then change your risers anyway you can. I assume that you saw what happened when the reasons for your issues were pointed out by someone who knows in an other location. all the correct info was deleted, because they needed to protect their own. You will get and have been getting the correct story here on OSO. stick to this and you will be fine. Ask yourself how you spent $9000 with their help and only gained 40 HP from stock. If you want to rebuild that motor, and make HP with it, ask here, and you will be way ahead in money and no broken parts.