one of the lessons I learned.........

once motors are running get them to temp, shut them down and pull oil sample (Go to NAPA and order kit, ~$14). Pull sample and see what it says.

Change oil and do it again but run motor for 15 minutes or so.

Compare notes, call analyst for his insight.

If all okay, change oil and run again for about an hour, then pul lsample again.

Pull a sample EVERY time you change oil just accept it as part of boat maintenance. OVer time lab will develop trend lines and be able to see a LOT of info.

Then you will know very early on if a problem exist. It adds 10 to 15 days to engine install to do this, and $14 to a oil change, but it is smart, you spend less than $50 at point of install, and KNOW if there is going to be a problem. If there is builder can rectify fast, if everything here checks out, and you later have a problem installer is place to look.

longer term you will know your motor far better tha n99% of boaters who will ever live...

Bill gave you best advice. You are having a bearing failure. Most likely due to improper clearance. I also have seen the GM cam bearings deterate. I always replace them on rebuilts with 1 piece bearings.

What's the advantage of one piece bearings, especially if the clearance is off?

Q?: If the copper is from the bearings... what is the most logical answer to the silver flakes?

The silver is the outside lining, the copper is the bearing that is being ground down
Sorry I have been in your shoes also....

The cam can also have a copper like appearance when it grinds down! It happened to me at 20 hours oil pressure was low because a lifter came apart and took out a lobe in the cam. The low oil pressure was the space between the lifter and the block.

Put a magnet against the flakes. If they are magnetic, cam material. Non magnetic, silver is the babbit of the bearings and copper is the core of the bearings and you have already wore though the bearing surface. If it is not knocking now, you dodged an expensive bullet.

BT

I think we all are learning the expensive way why Merc motors cost so much. I'm on the third engine in my little Donzi, and I have maybe 15 hours on it in over three years.

Both motors start and run great. No noise, knocks, pings, etc at all. They produce great power, but I'm gentle on them. Is there any possible chance that some of this crap could have been trapped in the oil cooler from before and is just now showing up? Or is this Wishfull thinkin? The oil cooler, Crank and carbs are the only items I didn't replace during the rebuild. If it's a cam bearing...is it most likely due to a clearance issue? Meaning... as long as I replace the bearings and get the clearance correct... I should be OK? I want to be sure I understand this before speaking with my assembler. Don't want to tear this out and fix it, just to have the same problem in another 8 hours.

I did replace the stock cams with crane 731's in the copper color (ordered it myself) If a lifter or spring came apart and started tearing into the cam... I'd for sure hear something wouldn't I?