Thread: Merc 25 4s Spun Bearing- What Next?

I have a 2001 Mercury 25HP 4S Bigfoot that was running great until late last summer when she suddenly lost power and wouldn't plane. I thought I found the problem when I discovered the fuel line from the tank was deteriorating. Replaced everything in the fuel system but a knock/ticking noise developed that I couldn't figure out. Finally after 9 months I had time to dig into the problem which I thought was valve train ticking. The valve clearance checked out so I pulled the head and found the cyl walls were scored a little bit.



I dug a bit deeper into the motor and found the lower cyl had spun the connecting rod bearing. I think the fuel problem was causing the engine to run lean and it overheated and spun the bearing.



Now the question -

I've run the parts and can put it back together for about $400 in parts (rings, bearings, connecting rod, gasket kit, etc).

The machine shops are closed for the weekend so I'm hoping to get some ballpark estimates on honing the cylinders (2 cyl) and honing the crankshaft. Based on previous auto engine machine work I'm estimating about $500 to do the work but I've never had to hone a crank. It just sounds expensive.

Depending on how much the machine work may run I might consider a manufactured powerhead but I can't seem to find one for the 25HP Mercury 4 stroke. Similar sized engines seem to run in the $1200 to $1500 range for the powerhead (new is 2k from merc)

The boat is rated for up to a 40HP so a re-power is also an option but the 3-5K is a bit heart stopping

I'm in the Washington DC region so if anyone knows a shop that deals with remanufactured powerheads please let me know.

Unless you can get undersize bearings, the crank will need to be welded and ground. Even with undersize bearings, the crank will still need ground. Not much difference in price either way. Its been a while, but the last time I had a 2 cylinder crank ground it was $250. I would do no more than a quick ball hone to the cylinders. That should be enough to clean up the walls without removing material. Any more than that and you are looking at oversize pistons(if available) and a bore job.

Open the oil pump and check it. anything more than very light scuffing then replace it. Lean running can heat up the cylinders but it is not responsible for the bearing damage. What I can see of the mains look great. Something stopped or slowed oil flow between the mains and the rod. Flush the oil galleys and check the pressure relief valve for a broken spring or stuck open ball.

While its open check your water passages and impeller. Once its all back together, change the oil and filter after the first trip out, then change the oil again after the second trip.