951 Crank Questions

[Ryan727]

Well guys, after 3 weeks of rebuilding the ski, I blew a hole in the crank an hour in on the water this weekend. Check it out (PTO side under carb. Blew out both hoses, one was pulse line.. that's how I found a big 'ol hole in her):



Couple of questions. What could have caused this? I ran for about 40 minutes or so. 10 minutes at an idle, and the rest between 1/4-3/4 throttle. Not once did I get to full open throttle, it was a bit too choppy out. After she broke down, I towed it back and put it on the trailer. Pulled the plugs, MAG side was a little dry, and coffee colored. I dropped the other plug right when I pulled it so didn't get a very good read, but seems it wasn't colored very much, and almost looked scratched on the surface (could have been from the drop, or running horribly). These were brand new BR8ES gapped to 0.22-ish (or 0.02 - whatever the specs are, I gapped them based on manual)

Either way, I've got to pull her out to evaluate. So a few more questions:
1. How do you tell if rods need to be replaced, or are they usually good, but can use some new bearings?

2. I've got a spare '99 crank case. If this crank is salvageable, are they directly compatible? This is a '98 engine.

3. I may end up needing a new crank. Any reman companies to stay away from? Any you have good experience with?

 

[racerxxx]

Just a top end rebuild?

Well, how many hours on the engine, before the top end rebuild I guess?

Get an OEM reman crank.

 

[Ryan727]

Top end was rebuilt 20ish hours ago, I didn't do it. Currently has 122.8. Was pumping out 150 on both cylinders just before I took it out.

 

[racerxxx]

Well, regardless the hours, it's done. Now you need to POSSIBLY get the cases welded or find good used ones. They must be a matched pair. The 951 usually gets more life than that.

 

[Matt Braley]

The 99 cases you have are the same so those will work.

Usually it's piston slap that gets ignored until a piece of skirt breaks and gets caught in between. Actually some don't make it that far because the pipe loves to spray water up the carbs.

With only 20 hrs on the top end it could be a failed lower end bearing or a RAVE valve that was not trimmed or something.

 

[Ryan727]

Well, regardless the hours, it's done. Now you need to POSSIBLY get the cases welded or find good used ones. They must be a matched pair. The 951 usually gets more life than that.

I've got a spare case, not going to try and do a patch job on it. As long as a 98 crank can easily fit and be compatible with a 99 case.

 

[Ryan727]

The 99 cases you have are the same so those will work.

Usually it's piston slap that gets ignored until a piece of skirt breaks and gets caught in between. Actually some don't make it that far because the pipe loves to spray water up the carbs.

With only 20 hrs on the top end it could be a failed lower end bearing or a RAVE valve that was not trimmed or something.

I almost thought about piston slap because of the sound it was making. But again, I was pumping 150 PSI in both cylinders before I ran her for an hour. Wouldn't the slap give me a chopped reading on the PSI?

Edit:
the "sound"was much higher pitched after she broke down, not during my initial tests. Could have just been a ticking timebomb waiting to go..

 

[mikidymac]

Well, if you were at 150 psi it could have been detonation that got it as a 951 should be at 130 psi new especially if you were running 87 octane on dirty carbs.

You can't rebuild the cranks, just replace with OEM, cheapest is Pro Caliber at $394.39.

Make sure you rebuild the balancer too.

 

[Ryan727]

Had freshly rebuilt carbs, 40:1 premix with 93 & API-TC oil. Someone told me there was a chance my head was shaved to achieve the 150, but another guy told me it wasn't. So I ran 93 in it anyways.

I had also pondered detonation, but figured I would have heard it beforehand. She was running beautifully, and just came off a plane and shut down.

I'll be pulling it soon, and I'm sure whatever the cause, it will present itself with the tear down.