Can anyone help me figure out my two cylinders have chucks missing near the intake side of the piston and shiny spots on all the others by the intake valve? I bought this boat with a "rebuilt" Hp500 EFI and I got 20 hrs out of it and it was smoking bad so I tore it down thinking the rings didn't seat, I was wrong! It looks to be caused by detonation but why??? I re- rebuilt the motor but Im just not sure what caused this the happen in the first place. Any info for a first timer would be great.

100% stock tune? what did the plugs look like, and what plugs were in it ?

Did you do the initial set up and timing when you rebuilt it ,,,,, stock rebuild??

Did you have the injectors checked?

What was the fuel pressure?

That's the weakest part of the piston. I agree with all the statements/questions above

Plugs were ac and looked good. I didn't check anything before I rebuilt it but I timed it to specs this time and yes its a stock tune. Just wondering if I should be worried it will happen again and what caused it to burn up. I can't figure out how to upload a pic. Fuel pressure at valve was 38psi. Im putting in a new pump in the spring just in case. Inj were sent out and cleaned and flowed. bored .030 new pistons rings and bearings. All top end was reused.

Send injectors to be flowed and cleaned. We've seen lots of failures caused by clogged injectors since ethanol fuel.

Is that the fuel pressure at wot under load or at idle?

Idle. I never got the chance to check at WOT before the season ended. Im putting in new pump in the spring and checking the cyl with a scope just to see if its happening again.

what number A/C 41, 43, 45?

Also,,,, how do the valve seats look in the heads

+1 to oxygenated fuel causing issues

Cant remember on the plugs but the valve seats were bad. I had one exh valve that was cracked all the way around and the stem was sucked up almost a half an inch. I replaced all valves and a couple seats. I still have the valve and if you shake it it rattles. lol I have never seen one crack all the way around but not drop. Glad I tore it down.

Did you put new valve springs in?

They are none to be tired at 300 hours.

Wannabe