I had a 502zz motor rebuilt 3 times,by 3 differant builders.The first & second time the motors lasted less than 5 hours.The motor is in a 30 foot wellcraft & is never run hard.The first 2 times the builder said there was to much compression,and heat and some signs of detination.The third time they lowerd the compression to around 8:1 with JE dish pistons,the block is .060 over,cam was changed to a comp 230I 236E .547lift 112 lsa.the water temp stays around 160.Have MSD distributor inital timing 18 total 36 motor worked great, no pingging good idle.After 20 hours I lost #4 cylinder again no compression engine still ran good no noise and started one dab of the key. What is causing #4 cylinder to always to lose compression/
Thank -You for your advise.Lorne

same head on the same engine same side? possibly a cracked head? unless there is other damage you haven't descibed?

Could it be a crossfire situation from an adjacent plug wire causing it to fire way in advance and detonating?

What is the cause of the loss of compression?.... A hole in the top of the piston?... tuliped valve?....

more info on the failed components needed.

Is the standard 11psi pop off valve still in place in the oil system? Seems for marine use this must be switched out to a 30 psi valve. Otherwise the oil cooler is not used effectively.

Are you running Merc dual plane intake? carb or efi?

I have burned #3 & #4 pistons using this type intake, turned out egt was good on other cylinders. Went to Merlin marine single plane intake and dynoed motor, saw very even egt temps cylinder to cylinder + better power

Mark

What ever the problem is you need to find it before you go back or you will be rebuilding the motor again. Lets assume it is not the heads since you could have easly swaped heads from one side to the other. #4 is not one that normally goes away, at least for me. #5,#7 or #2 are the one that I tear up when I run too much boost & on the raged edge. If you are running port EFI than your injector on that cylinder could be bad & leaning out that cylinder. As stated about we need more info. Did all the pistons look the same after teardown. Holes, parts broken off, melted or ???