Fomula 353 w/stock 502. engine cut out in east harbor. Started back up and heard loud clanking from what seemed the top (end loud knocking). Shut it right down came in on starboard only. what should I look for when I pull the valve cover? anything problematic w/this engine (98 w/300hrs). Any help would be appriciated.

I would bet the exhaust seat fell out of your head.

Bad valve seat I
pretty sure

would that require new piston? or just top end re-worked?

When pulling valve covers, look for a loose rocker arm, broken spring or bent broken pushrod. If they're all ok, you have to dig deeper.

You need to do a total rebuild when you pull off your valve covers take off the rocker arms and look for a spring that is taller then the rest.

Good luck mine did same thing 95 502 490hrs pulled the motor for overhaul and it had a spun crank and the block is scrap so now will a new 540. Sure hope it is in your top end you can see with a compression check which cylinder s bad.

this happens a lot with the mercury 502 cylinder heads,it usually destroys the head,and the block will need to be bored,invovles a total rebuild.