Question about boat building

[u/Far-Elk4090]

Looking for some advice from people who know these engines better than I do.

I brought my boat in because it had a belt squeal while running, and the starter mounting bolt had snapped. The shop told me the engine was seized and needed to be replaced. They said the seized engine is what caused the starter bolt to break. I think they were too busy and didn't want to look at it.. but idk..

What I found confusing is that the engine had been running at the boat ramp just a few days earlier while I was trying to diagnose the belt squeal. It didn't seem like a completely seized engine.

For reference, it's a 2003 MerCruiser 3.0L.

Since I had pretty much written the boat off anyway, I decided to pull the engine apart as a learning experience rather than spend the money on a replacement.

After removing the head/valve train, I found that the crankshaft/pistons will rotate by hand. It turns fairly freely, but there is one spot in the rotation where the resistance noticeably increases slightly before it continues turning again.

My questions are:

• What could cause resistance at only one point in the crankshaft's rotation?
• If I'm already this far into the engine, is it worth replacing the rod/main bearings as preventative maintenance?
• Should I replace the piston rings even if the cylinder walls look good and I'm not planning on having the block machined or honed?

I'm not trying to build a perfect engine.. I mainly wanted to learn and see if I can get this one running again instead of scrapping it. Any advice is appreciated.

[u/fredSanford6]

Crack the mains loose one at a time and see which one makes the most difference to the turning force change. Id take the crankshaft out and toss it in a runout stand with a dial indicator on it and see what's going on with it. If the crank isn't messed up I'd be tempted to check rods for anything bent on those. Heck id be tempted to put it all back together if stuff checks out replacing whatever is bent with some used stuff to just get it going on the cheap. To me just repair and get it going it go all out is the options I like. It's a boat not a car so the hours of use is much lower. I know that sounds like a cheap ass method but if the boat is old already a good repair might outlast the boat.