1995 7.4 Merc KNOCK
 

Hi Guys,

Last night I took my boat out. Started both motors and observed 45psi on both, warmed up and we headed out. After about 10 min on a plane I noticed my rebuilt motor (less than 10 hours on it) started to drop oil pressure slowly. It came down to 30 psi so I thought i should stop and check. No oil in the bilge and the dipstick was reading full. Started it back up and low oil pressure alarm sounded after startup for about 8 seconds. Shut it down, went back into engine compartment to check remote oil lines etc. All looks good so i start it back up, getting about 25psi of oil pressure, I rev the motor up and NOOOOO. i hear a nasty clanking/knocking sound.

Limped back to the marina on other motor. Started it up when i got back to port. The clank/knock is not noticable at an idle and I now have 40psi at an idle. As i rev the motor up i can hear the clank/knock. Any ideas?

Id pull the valve covers and take a look but it sounds like its in the bottom end, maybe a spun bearing, who rebuilt the engine

It was rebuilt at a local machine shop. A guy on another forum just suggested the same thing. What would cause this to happen? I did just remove break in oil and put proper merc oil in with new filter last week but ran it to warm at Marina with no issue.

Not sure till its opened up, hopefully its in the upper valve train, but I wouldn't get your hopes up

Pull oil filter, cut it open and have an oil sample sent for analysis, if metal is present good luck.
Again, as somebody else recommended pull the valve covers and look closely in the oil puddle areas of the heads for metal flakes/glitter.
Sorry to hear the problem

As said above.
And get that fork ready. Sorry. :(

What was the reason for the rebuild in the 1st place ?

Spun rod bearings usually make a unique "double knock" sound. They also tend to be quieter when cold.

I'd cut the oil filter open and see what's in there.

If journals where not machine correctly, it can spin a bearing. Currently rebuilding a Merc Reman (note Merc Reman) with a spun bearing. Got about 100 hours on it before it happened. Was number 8 rod

Thanks for all the info boys. Took the filter to the machine shop on Thursday. We cut it open and there was a good mound of goldy color metal flake in the bottom. Crane is coming today to pull the motor and load it in my truck. Lets just hope the machine shop can turn and burn this thing so I can get back to boating... Of course the nice weather just showed up for us.