First, how should I proceed with this?

Second, how do I prevent it from happening again?

It was pointed out to me that I overloaded the motor with a bigger prop (23p mirage +) and was running too lean so it blew a chunk off of the #6 piston and blew the gasket between 4 & 6. I'm running a lightly modified 350 vortec (guessing around 350 HP) on an alpha 1 with 1.65 gears.



#4 and # 8 also have some very light scoring. #4...



Scoring is so light that I can't catch a nail on any of it. I'm getting 4.031-4.0325 on the mikes.

So, what should I do next? Probably a very dumb question but any chance I can just put a new head gasket on and go (after getting it tuned right so it's not running lean)? Is that piston trashed and just waiting to explode more? Can I get another month out of her without doing any more damage if I just cruise around below 4000 RPM with a mild prop? Boating season is almost over so one more month would be nice then I can pull the motor. But, I've already torn half of it apart so it's close to being ready to pull now.

I really don't think I need to bore it with those numbers I'm getting and such light scoring. But, I'm obviously not a pro or I probably wouldn't have blown it up in the first place. LOL. So I'm thinking just a light honing and at least one new piston. Or, do I need to replace all of them? Pretty sure they're Silv-o-lite Hypereutectic if that matters. Also pretty sure they're not a balanced set.

Hone it myself or take it to a shop? It should only be about $100-125 to take it to a shop. Does it even need honing since the scoring is so light and the tolerances tight? Do I need to re ring all pistons or just that one? Or just the ones that light scoring on the cylinder walls? I've read a little about moly rings for making a better seal and preventing blow by. With the light scoring, should I go with a set of new moly rings? Pretty sure the cylinder needs to be honed with the right grit for molys?

Considered stroking it but the rebuild is only 2 years old and the damage is very minor IMO. I'm probably looking at $1500 minimum for parts and machine work for about 35 more HP? I'm also on an Alpha and I'm afraid of tearing it up with what I have now.

As far as what caused it... I was running an edelbrock 1409 600 cfm. It was recommended I go with a Holley double pumper 750 because a SBC with my mods will run lean. Is that damage something that builds over time and finally breaks or is it just based on the current conditions? What I'm getting at, did I cause it by running lean on the edlebrock for 1.5 seasons or not tuning the holly right for 2 weeks? Or is the holly messed up? It is a factory refurb & still under warranty so I don't want to keep it if it's just going to keep blowing stuff up. I though I had it dialed in right as far as the fuel/air but it seemed awful lean to me. When it came to me from the factory after being flow tested the mixture screws were only backed off about 1/2 a turn. When I tuned it, I could only back off the mixture screws about 3/8 of a turn or it would sputter and stall and not want to start again. With the flame arrestor off I could back em off 1/2 turn so I'm assuming they flow tested it w/o a flame arrestor.

When I was taking off the heads, I also noticed that 4 of my intake manifold bolts were loose. All 4 on the opposite side of the detonation if that matters. It still had a tight seal (had to encourage the intake to come off) and I had just had a vacuum gauge on it about a week ago and it didn't show a low or slow dropping reading.

And, I confess I didn't check the fuel pressure or secondary accelerator pump but I had no stumble on acceleration or at WOT.

You have a lot of questions, but for one, you can't reuse that piston. A 600 marine edelbrock isn't too small, and a 23 pitch prop isn't too big as long as you are turning correct rpms. I have a modified vortec 350 and have run both of those with no issue.
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What was the total timing you were running? How are the heads? Damage looks to be from severe detonation.

I said it was a dumb question. LOL. Just wishful thinking that I could get a few more days on the lake this year.

I was only turning 4700 RPM when it happened.

32 degrees total timing.

After brass brush & kerosene...



After a couple of light passes with surfacing stone...

32 total is to much for a vortec head,usually 28 is tops,that with a lean mixture was the issue.take the engine to a experianced machine shop,just changing one piston would not be the correct path to a happy engine.do it right and you will enjoy next year,s boating.

You will want to have that head checked with a straight edge to see if it needs to be resurfaced. Good chance it does having blown between cylinders.

X 2. Time to tear down and do it right. I am surprised all your valves are still in tac. I hear what your wanting to do but look at it like your thankful. I'm afraid if you replace a piston quick to get in a few more runs you will regret it. Have your heads gone through and spare no expense on your exh valves when you rebuild. Just curious what your spark plugs look like?

Please post a pick of hottest one.

No coloring/carbon on heads and pistons.

Hard to see from here, but one piston (1st photo) looks like it has had material removed (detonation) and looks like ring lifted.

^^^^^^ 100 % agree.i think the water from the blown head gasket steam cleened a few cylinders.

Ooops...I didn't catch that. Doh.