BadDog

Gen 6 502 has developed a loud sharp metallic clack from the #8 area. I am not able to isolate it with a stethoscope. It almost sounded like a bolt on the coupler had backed out and was striking something in the bell housing at the 10 o'clock position. Removed engine and nothing loose on coupler or flex-plate.

The noise is not directly timed to engine RPM. If you hold rpm steady at fast idle the sound is consistent. Blipping the throttle would increase the rate but it sounded like at certain points during rpm increase or decrease it would go away momentarily or change its pitch.

Stethoscope place on directly on rocker studs revealed no culprit. Pressing on the rockers did nothing to quiet it down. Oil pressure is 60 at warm idle. Compression check showed 160 lbs on all holes. Plugs look fine.

The sound was more of a sharp metal to metal contact sound that is why the mechanic and I suspected a bolt had backed out, it was not a dull thud like a rod. Since we found no bolts or coupler trouble we are suspecting it may be a valve seat rocking around it the head but are puzzled as to why we could not hear it localized on a rocker stud.

Other possibility is wrist pin or piston skirt?

If a valve seat is getting ready to drop would I be able to spot this visually when I get the head off and valve out? What am I looking for in this area?

Here is the kicker. Heads were just done by reputable machine shop. New lifters and rockers. Ran fine for 8 hours then the noise developed. I suppose it is possible that a wrist pin or skirt is bad but usually when something goes wrong it is the last thing you messed with as in the heads.

Engine is out of the boat bolted to a jet ski trailer. I can drop the pan to look at the bottom of 8 or 6 but not sure if I could spot a cracked skirt. I want to isolate what the problem is before sending the short block to the machine shop. If possible I would like to avoid tearing the engine apart if I can confirm a problem with the head.

Any tips for physically finding the cause of noise? What can be spotted by visual inspection?

formula31

I know this is too simple but this sure sounds a lot like a flapper in the exhaust banging. What exhaust system do you have?

Magic Medicine

I was thinking the same thing.

Roger 1

I had Corsa on my checkmate and the first time that I heard it the hair stood up on the back of my neck! It didnt take me long to isolate the problem though. I hope that this is the noise you are hearing and that it is'nt a problem at all.

Roger

BadDog

Folks, I am aware of how a flapper can sound. This noise is coming from inside the engine. I have a Merc Master Tech looking at with me. This is the guy Merc sends customers to when many other dealers cannot fix things.

Anyway got the head off this morning. Cylinder walls are fine. Not a scratch in them. Unable to get the triple valve springs off because the tool has morphed legs and scooted under a bench somewhere. Tie bars on lifters are fine. Lifters ae pumped up solid. Unless it is a valve seat (will know more when the valve comes out), I am looking at wrist pin or piston skirt. Might be time to send off for full rebuild and throw the old stock 454 back in for a month or so.

formula31

Well, we didnt know what you knew and didnt know. Sometimes those flappers can be un-nerving. Hope its nothing serious but please keep us informed what you find. I know wrist pin noises pretty well and this doesnt seem to fit. The top of the pistons looked ok huh? Damn. If you set a dial indicator on the deck and measure the piston movement of say #6 against #8 with the same degrees of crank rotation each way you might be able to pick up a problem on 8 if there is one. My quess is still in the head.

BadDog

Well the heads were the last thing touched so I am taking them back to the machine shop for inspection. I did spray carb cleaner into the intake and exhaust ports and nothing came out the combustion chamber. Still possibly a seat? My worst case scenario is to take it all apart and not find an obvious cause. I plan on pulling the pistons early next week. Sort of busy right now. I really hope I find a bad skirt or pin. Then I have to decide to either replace the piston and go on with it or do the full rebuild. Probably close to time, 550 hours but walls, oil pressure and compression is fine. May as well do it while down but is very late to start that type of work.

I need luck...
Of the correct kind...

formula31

Valve sticking maybe, check guides and stems real good. Another thought, windage tray or intake valley splash tray. Ive heard of a windage tray coming loose and hittng making strange noises that were inconsistant. The thing about it not being timed to the motor is wierd. Did you use a timing light on #8 to see if the knock was related to the top end firing and valve events of #8? I know, too late now.

Ted G

Have you actually pulled the lifters from the bores? I have seen something like this with a failed roller.

axapowell

That's what happened to me. Could NOT locate the noise last fall. Tried everything, then finally a lifter came apart and not only did the noise stop...well you can use your imagination on what happened next!

Good luck

Dave