I thought some might want to know what caused detonation of these pistons. In the interest of full disclosure I humbly present the facts, hoping it may help someone else someday.

I finished tearing down motor last night and this is what I found.
I noticed a rusty stain next to a freeze plug, and thought it a good time to replace while accessible. I popped the plug and immediately found my problem. Sand.... and a lot of it. And yes, directly under the piston that had the meltdown. In fact the entire side of block was loaded with sand. The # 4 cylinder was almost buried in it. there was no circulation happening on that side of motor. If you recall, my #8 cylinder was showing sign of detonation as well. Apparently I must have done some good clammin with the outdrive the week before, during the night. I never knew how much sand i actually sucked in. Running in river up here, you sometimes get a lot closer to the bottom than you would like. I never ran it hard until the following week, when damage became apparent.

As for symptoms or lack of :

-Still had good water pressure ( yes I will still replace pump)
-Never showed hot on gage. Sending unit is next to thermostat and was still getting cold water flow
-The only thing I noticed was right side valve cover was getting hotter than other side.

So there it is in a nutshell. My bad..
I am still looking to get back in water with reasonable budget. I need pistons, heads, and will also replace valvetrain. I am trying to track down a set of 781's locally. If anyone has some hardware for sale, I will listen. Would like to stay with ovals. I would also appreciate some ideas regarding cam/piston/compression combo's that's been proven to work well. Thinking about staying with hydraulic flat tappet for budget reasons.I may upgrade rockers though. I have performer intake with 750 carb and EMI's to bolt up.
Thanks guys
Rich

Curious about the picture in the 1st post. Did you take the picture upside down? If not, pistons are upside down. Just a thought!

I'd be willing to wager...that with 150 hrs on the motor....the pistons are not upside down. :drink:

The engine wouldn't even turn over with the pistons upside down... The dome would hit the cylinder head.

ran into the clogged cooling passages before as well...but make sure you keep total timing at 30-32 total max !!! to much will do the exact same thing !!!

Rich,
As stated, the root cause of this failure is evident. The event originated as a localized hotspot, in the thinnest part of the crown. The detonation intensified further as the shallow, sharp edged area of the crown increased in temperature. The land area adjacent to the relief continued to lift until contact with the cylinder head was initiated, resulting in the loss of the ring land. The melted aluminum is clear in the photo. It is my understanding that a second piston was experiencing land deformation also but had not yet failed. The lack of heat discoloration outside of the area of the valve relief would indicate that the detonation was low-grade, localized near the sharp valve relief area of the piston crown. At this point the positive that can be taken away from this analysis would be how to move forward in the rebuilding the engine.

You stated your pistons are Speed Pro/TRW part number, L – 2465 – 030. They are a VMS – 75 high silicon piston, very similar in composition to a hypereutectic 4032 alloy, containing about 11% silicon. Hypereutectic pistons are relatively strong; however, in your instance it exemplifies one of the alloys main shortcomings. While the hypereutectic forgings display exceptional thermal stability, they lack the ductility and 2618 alloy. When detonation comes into play, the 2618 forging, a more malleable alternative, has a tendency toward plastic deformation. This can mitigate crack migration, potentially delaying/preventing eventual failure of the piston. A hypereutectic piston has a tendency to fracture immediately upon exceeding these limits. In the Marine environment, where prolonged high stress, high temperature, potential detonation, and sustained wide open throttle operation is routine, the hypereutectic piston is very unforgiving. While it is impossible to determine whether or not a 2618 alloy would've survived this particular event, you can make a case that it may have provided the necessary margin against catastrophic failure. This is a textbook example of why many engine builders specify the 2618 material exclusively for the marine environment.

Bob

Well I am glad there is sombody else on the same page as me I mentioned the timming earlier and there was no response.

Was not overlooked, and indeed extremely important. Last I checked, I had it locked in at about 33. It was already checked off on my list. My lack of response on this topic was purely time, not indifference. I appreciated your input for sure. Just so busy now hustling around after work, trying to save some summer. All steel is currently being checked for cracks and straightness, and I was able to score on a virgin, uncut set of 781's locally, for easy money. Depending on how the crank looks will determine which route I take if i need to replace it. (stroke time??) We'll see. Have been in contact with Bob Madera, and he will handle the valvetrain components. What a great guy to speak with. You really need a tape recorder when talking with him, so you can go back and listen again, to make sure you heard it all. He is a wealth of knowledge.
Updates to follow. Thanks,
Rich

Rich, one good thing about finding the sand is, atleast you know what caused the problem. Keep us posted on the rebuild and be sure to take lots of pics.



Darrell.

One year later...
 

Was pretty hard trying to keep my boat from turning into another back yard project with all the non boating distractions that popped up this year, but here we are...
Motor is now in and alignment is done. Should hopefully hear it come to life in a day or so... Wish me luck..

Also attached a pic of root cause of last failure. Discerning eyes will find the culprit. Mea Culpa.