I'm going to have to assume you are being sarcastic here. :) Sounds like you are going to have to tear it down and rehone #6 at the very least. After that, you need to figure out what made that cylinder go lean and melt the piston. Did the rest of the cylinders look like they were running lean as well? If you just slap it back together like it was before, you are just going to repeat the same results until you correct the issue that led to the lean condition.

"My engines are 502 MPIs with 500 EFI cams, larger intake and exh valves, crane roller rockers and the boat has stellings boxes on it. "

Well, if the ECU was not remapped after the cam change, and you had the lean issues that you reported in an earlier post, then the results you had are not real surprising. Unless you want to go to the expense/trouble of remapping the ECU, I would go back to a stock or near stock cam with the rebuild.

Well, this is a really long story and the above is just the abbreviated version. The MEFI 1 computer was re-flashed for the cam / head and exhaust updates. On the first trip out the boat was running rich (the transom was black) but the trip was cut short by a broken input shaft on one drive. I called the person who flashed the PCMs and he told me to install adjustable fuel pressure regulators and set them to 36 psi to get rid of the rich condition. This was done over the winter along with prop updates. The next summer the boat never made the power I expected (based on speed) but the soot on the transom was gone, so Labor Day weekend I hooked up my lap top to record some engine parameters. I found lots of spark retard... There was my lack of power. Since I had stock compression and had the fuel pressure set to the recommended level for the PCM program I looked else ware for the cause of the spark retard. I found threads on this site which stated that the drive shaft for stellings extension boxes could set off the knock sensor causing the PCM to pull spark. With this I decided to disconnect the knock sensors (bad move). After two high speed passes with the knock sensors disconnected the center of the spark plug in the # 3 cylinder shot out of the head and there was a tapping sound in the engine. Again, the taping sound turned out to be the melted piston in the # 6 cylinder... I again contacted the person who flashed my PCMs and told him the story. This time he said 36 psi fuel pressure was too low but was a good starting point for tuning. He also said when I get the engines back together he would be glad to dyno tune the PCMs for me (for a price of about one days labor). I can hardly wait to run the leak down test on my second engine...

Hmmm, interesting. Did you have to purchase special cables and software to download the data from the computer? I have a friend with a 454 MAG MPI that seems down on power. It would be interesting to see if he is getting the same timing retard.

Good luck with your build!

Yes, I purchased a Rinda Scan disc and cable set up to monitor my MEFI 1. You can look at what is happening but cannot change any parameters.

502 knocking
 

Problem found

No. I reached a point where I just gave up. We pulled the motor, checked the flywheel bolts but they were all tight...Reinstalled the engine and the tapping noise continued. I ran the boat the rest of the summer with no issues (still full power and full oil pressure).
I decided to sell that boat in October and took quite a hit on it because of the motor issue. I just had so many issues in 2009 with the motor rebuild, a bad ecm, then a drive blowing up (yes, all in 2009) and then to have all this headache in summer of 2011 it just pushed me into selling. Furthermore, the bolsters needed to be recovered and I was still making payments and dealing with all this!

Im probably going to regret it but Im buying another boat this weekend but this time with cash. I live in Seattle where we only have 3 months of good boating weather so having a boat payment has always been an issue with me...Having to repair one, make payments and watch it sit on the trailer for 9 months a year is torture!! Ill cross my fingers that I have better luck with my new (to me) toy.

i feel the same way living in the north short boat season pay cash no payment hurts a lot less all winter all boats need to have upgrades and repairs maybe with out payments there is a little cash left over to do them

Ive heard this noise before, its definitely a Muffler Bearing! Change the Flux Capacitor at the same time while you`re in there!