Suspected bad rod bearing - 2000 Merc 125HP

[frozenokie]

Just found out that I have a "suspect" bad rod bearing in #3 after my visit to the mechanic. That cyl also has the lowest compression at 100. The #1 & 2 are at 120 and #4 is at 130.

Could a bad rod bearing cause that? Is it possible to inspect and replace the rod bearings and crank without removing the head? Like say from the frontside - remove carbs, cover, reeds, then I should be there - right?

 

[j_martin]

Re: Suspected bad rod bearing - 2000 Merc 125HP

Just found out that I have a "suspect" bad rod bearing in #3 after my visit to the mechanic. That cyl also has the lowest compression at 100. The #1 & 2 are at 120 and #4 is at 130.

Could a bad rod bearing cause that? Is it possible to inspect and replace the rod bearings and crank without removing the head? Like say from the frontside - remove carbs, cover, reeds, then I should be there - right?

It's possible. Pistons have been replaced on the fly at the races, but it's not for the timid or clumsy for sure.

The rod cap is a broken casting match. It has to be reassembled perfectly or it will fail. Difficult to do with the little space available through the front half, but not impossible.

If you take the rod cap off and find a damaged crank, you'll have to tear the whole thing down anyway, so nothing is lost. If you have low compression in that cylinder, it needs to be fixed anyway.

If you turn the engine over by hand till the piston in that cylinder has gone over TDC and is halfway down the stroke, then reach through the spark plug hole with a screwdriver, make contact with the piston, then press it in harder. If you feel it give before the flywheel turns, the rod bearing is way gone for sure. Crank will be bad also.

hope it helps
John