Question for all you gearheads out there.
I have a Mercury Racing 600 SC motor with 20-25 hours since complete rebuild. Oil has been changed 3 times including all filters. Ran boat last summer, no problems. First time out this summer ( 2 weeks ago, but that is another story) and I am getting a lower end knocking noise. Seems to go away when rpms come up, but comes back when she idles down. All the best parts went into motor (diamond pistons, inconel valves, etc...) when built 2 seasons ago. Can't reach the builder, my local mechanic thinks possibly a thrust bearing ???
Can't hear the noise with hatch closed, but definitely noticable as soon as you open the hatch.
Any thought or suggestions?
Frustrated Mike

Mike:
Sounds like trannies to me. Does it knock in nuetral or when in gear? My Twin discs did the same thing when at idle, just the play in the driveline. It would go away with the slightest rpm increase. Hopefully no big deal. Hows your oil pressure?
Good Luck

Josh:
Trannies were just rebuilt completely, all shifting is fine. Noise is there in either neutral, forward or reverse. I was even thinking a connecting rod bearing? Oil pressure is good.
What a f#@king summer it's been. I've been on the boat twice all summer, about 30 mins each time. Hope your doing fine. How is she running?
Mike