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.

the pistons look like what came out of my 502mpi.bad gas =detonation.that was my problem.

In reference to your block filling with sand assuming from the area you run in it may be a better idea to run a circulating pump on the front of the engine instead of the cross over. Just my 2 cents.

Wow, reading this thread was like a case of Deja' Vu for me - my 462 was also running domed pistons, although mine were forged and had been flycut and had a little bit of the domes shaved off to help with compression. I had a nearly identical ringland failure as you had. I actually might not have known about it if a piece of shrapnel had not taken a small chunk out of an intake valve. This was causing a backfire under heavy load, which led to a compression test and disclosure of the problem. Like you, I briefly flirted with trying to cobble it back together to salvage the season, but a couple of lifters had stopped spinning, and when I pulled the other head I found another piston broken in the exact same spot! Looks like domed pistons have no business in a marine engine.

I ended up building a 489 stroker based on a gen VI block. Still dialing it in. Make sure you have the right distributor gear on there if you are running a roller cam - see my thread. Little details like that can get you.

a couple of sets of 781's on ebay. done set near me in s. jersey. guy wants 800 plus 60 ride. little steep. another set in cincinnati guy wants 375 plus 125 ride. 500 makes the done set look better.

Batteries on charger now. Maybe tonight she fires. Still finishing up some minor details, like exhaust flappers and such.

Budman,
I saw your thread about the distributor gear wear and now shakey oil pump. Hope it works out. At least if pump is going bad, or vise versa,, you found the problem before things turned real ugly. I do have full roller set up now, and correct gear has been provided from Bob M. when I got the cam. After reading your thread, I stared at my new distributor gear for a long time. LOL

Piclkenjim,
To your point about recirc pump, are you saying that with a circ pump this would tend to keep sand in suspension longer and give it a better chance of getting flushed through?

Explanations like this are why I feel good comfort in knowing that every part inside my engine was from Bob Madera. Sometimes it seems like hes speaking Greek but you should get your specs and parts from Bob. That is the best way to make reliable power on a budget. He is a pool of knowledge and will be happy to talk things over with you on the phone at any time.

Then when I go a little bigger. I'm dropping it off at Young Performance! Another reason I've had good luck with this build. Eddie helped me with fixing the main oiling issues with these engines. And setup after getting my rotating assy back.

Get with Bob on pistons, cam, etc. Set it up with the power of OSO and have some fun boating. Just look at your cam card when you set up your firing order. (Bob knows what I'm talking about :lolhit:)

Yep. If your running in sandy areas and parking on beaches regularly. As stated though that's just my 2 cents.

Finally got it wet.
 

Went out yesterday for maiden voyage. Felt like an anxious father with fingers crossed. Had nobody to blame but myself if this one craps the bed. My first "complete" assy, Motor is running great so far. About an hour on motor now..Have some old gas in there mixed with some new, so taking it easy for now, and haven't been above 3K yet except for a couple quick revs in neutral to 4 k or so. Timing is set to 34 degrees for now. Oil pressure at 65 cold, and 50 hot cruising at 3K. Down at idle below 1K, pressure never drops below 25 hot, then climbs back to 30 or so.. Seeing about 210 on oil temp when cruising for an extended period, and back down to about 160 at idle. Water pressure at 25psi when cruising.
Fuel presssure steady at 6-7psi.
So..... I'm anxious to change oil and filter to have a look, but perhaps too soon? Want to get all the assy fluids and whatever out ASAP. Running 20-50 dino oil now. Plan on going back to mobil 1 15-50, but maybe after 10 hours or so? What do you all think? This is full roller set up.
Also, opinions on how long before I start moving up the rpm band.
Thanks guys.
Also thanks to Bob M. at Marine Kinetics, for his advice, and his patience listening to my many questions and answering every one of them as I went along with my home build. His service extends well beyond any purchases made.

Change the filter now. This will let you know if any problem is in the begining stages.

You could probably change the filter now and cut it open, but I doubt if you would see much beyond the usual initial start-up debris - a few stray metal flakes from initial machining, maybe some lint from shop towels, etc. This would tell you if something catastrophic was about to happen, but personally I would not feel bad letting it get a few more hours on it first. I waited until I had about 10 hours on my engine before I let it sing much above 4K, but others on here are more adventurous.

FWIW, my engine builder recommends a full season (50+ hours) before switching to synthetic, but as stated before, you will hear differing opinions on this. You might get Bob's or Eddie's recommendation on what oil to run.

I'm curious to know what oil pump you went with and what your bearing clearances are. You oil pressure numbers sound good for 20W-50, but with the same weight oil mine were considerably higher - more like 65 - 70 cold idle, and 60+ warm. I ordered a Mellings 10778c pump to replace the one that has apparently failed on my engine.

I have an M77HV pump in there.
As for clearances, .003 on the mains and .0025 on rods was my target.