Broken Crank Shaft.

[Mo]

A friend broke the crank in his engine I think. It will turn a bit then locks up. Click the starter and it will move a little bit again with the crank handle..not a full revolution and I don't see anything moving looking through plug holes at cylinders. All pistons are top dead center by the looks of it.

He was just heading back to the dock and doesn't think he hit anything with the prop etc. His prop is the proper size for his C&C 29 MkI as well.

Anyway, I had a good crank there that I gave him. He's going to pull the engine sometime this week and do it.

Thoughts on how this happened??

[Dave Neptune]

Take a good look at the broken area. I have seen a few broken cranks on duty rated engines. Most often they are an existing crack that finally let go. Look for dissimilarities on the surface of the break in surface finish and color.

Timing can influence crank shaft cracks too. Many older castings were not as good as today and many a small internal crack got through but most won't break in these duty rated stuff.

I doubt a prop strike could break a crank but it would do the rest of the drive train no good.

Dave Neptune

[Al Schober]

Was the engine over-revved? This engine only has two main bearings, one at each end. There's no crankshaft bearing between cylinders 2 and 3.

[Mo]

Hi Dave.

I had to time the boat about a month ago for him. She was in rough shape and so off that she had lots of blow-by in the cabin. She ran much smoother after that.

Hi Al,

as for over revving I'm not sure. I know he is very mechanically inclined, so I doubt that. That said, I don't know when the last time it had an oil change. I told him a few months back to do it every 25 hrs because it did have some blow by and that oil would need to be kept clean.

[Mo]

Alden just hauled the engine out of the boat this afternoon and pulled it apart
Number 3 piston....number 4 in firing order, connecting rod failure at bearing / crank shaft journal with the lower piece of the connecting rod deformed...Looks like a bolt backed off. Removed the crank and I had a piston with rings and connecting rod that I gave him. I put that piston in and it went in with about as much trouble as a proper fitting set of rings should go, so I know that block wasn't bored.

Crank is scored pretty good on both inner journals but I had a good crank sitting there that I gave him. He put a gauge on the one that came out of it and the journals are smaller than the one I gave him. So the crank shaft coming out of the boat was machined and I believe the one I gave him came out of an original engine without ever being turned. I do know that engine he has was rebuilt about 10 years back and the crank must have been machined.

So, he's going to get in touch with MMI and get some bearings. Also told him he should get a machine shop measure both cranks so that he buys right bearings. Sounds like a major, but if we had the bearings for that it would go back in the boat in the morning. Sounds a lot worse than it was and he had that engine out of the boat in minutes today.

[Al Schober]

Your friend got very lucky. First that the damage wasn't worse. Think hole in the block. Second that he had you for a friend to help out.

[Mo]

We hauled up each piston, not a score on anything in the bores, no cracks in block...he needs that crank I gave him measured for a set of bearings as there are various sizes.