Parts bin special

[luke2152]

Things went a bit pear shaped today. Balancer shaft broke and the counterweight tried to escape in dramatic fashion. I ended up covered in hot oil - fortunately at ouch temperature but not burn your skin off temperature. Back wheel was covered too so was a bit of a handful bringing it to a stop.

No idea how the failure occurred, was doing approx 9000rpm, full throttle, had just changed from 2nd to 3rd. Rev limiter is set at 10500rpm and I wasn't hitting it at the time so I don't think it was over revved.

Only thing I can think of is that the engine is tilted forward 13 degrees from original position so maybe that allowed the balancer shaft to hit the oil under certain conditions - but that seems like a very unlikely scenario to me and 13 degrees is no different than you experience riding downhill anyway.

Could have been a fault already present in the used engine but I've done a few thousand miles on it and would have expected faults to present themselves earlier than that.

[AP996]

Wow, glad you managed to get to a stop without crashing. Not heard of one doing that before.

[Pursuvant]

37 minutes ago, luke2152 said:

...Balancer shaft broke and the counterweight tried to escape in dramatic fashion... ended up covered in hot oil... was a bit of a handful bringing it to a stop...

While it's a short term disappointment, nothing can take away from your accomplishments, mental and physical delivery on your exceptional dream. This is a bike build, nothing I've done has been anything  other than "fitting parts". 

That sheared demolition looks like a hole in time and space

[luke2152]

Well I've finally had time to pull the engine to bits and try and identify the source of the destruction. It was an interesting exercise but generated more questions than answers! What is clear is that the balancer shaft broke into 3 pieces and number one conrod broke, one of these caused the other and both of them caused much destruction to both sides of the engine case. Less clear is which one was the cause, I'm leaning towards balance shaft for reasons I'll go through with photos.

More surprising was how much of the engine wasn't damaged. The cylinder head and cams were perfect. The gearbox was undamaged too as far as a brief inspection goes although was littered with broken bits from elsewhere which may have caused damage under closer inspection. Despite all the broken bits the oil strainer was clean so it was very much a sudden failure. The crankshaft had minor damage to the webs where it had hit the balance shaft but clearly won that battle. The unbroken conrod looked ok but was slightly distorted because the rod bolt was very hard to get out. The bearing on it showed some marks more consistent with a violent stop than with wear.

Edited September 22, 2023 by luke2152

[luke2152]

First clue is that the cylinder head is perfect, valves straight, not a mark on it. That makes me think the conrod failed on the downward stroke. I'd expect a normal conrod failure would happen in tension not compression - so that makes me think something else failed first.

[luke2152]

2nd clue is the piston, it has been flailed by the broken rod causing considerable damage but one part of the skirt points inwards, the broken rod would surely push it outwards. So it has clearly hit something else. This is less conclusive because whether the rod broke first or not it could have still been hit by the balance shaft when no longer attached to the rod.

[luke2152]

Going against my previous statements is the very clear conrod shaped impression in the balance shaft which could indicate the conrod failed first and smashed into the balance shaft. Of course it could have been the other way around and only made contact as it flailed round after being broken by the balance shaft.

[luke2152]

Another clue, this is the balance shaft on number 2 cylinder where the conrod didn't fail. It's been hitting the crank web as well but in a less destructive manner. The bit that caught my interest was the wear and bluing on the edge which indicates it was rubbing for more than a few seconds. Maybe the failure wasn't as instant as it appeared.

[luke2152]

One final clue but I can't make sense of it. The swirl marks on the piston crown (this is the piston with the broken rod). I have no idea how they got there and as the head looks to be in perfect condition. I thought I might have made them when I pulled the piston out by they are a bit too symmetrical to make by accident. So I have no idea how they got there. Very shallow but can feel them with a finger nail. 

[luke2152]

Anyway all that said I was hoping to find a clear cut cause of the problem and I didn't find one.

Con rods tend to break when over revved and my tacho displays max revs I ever reached 10700 RPM (10500 soft limiter and 10700 hard limiter). And while I rode it hard I didn't often hit the limiter, not more than a couple of times per ride and never intentionally.

If it was the balance shaft which failed first (which I think is more likely) I struggle to think of how that would happen. It's possible the engine being mounted 13 degrees further forward than in an MT07 caused the oil pickup to run dry and therefore starved the bearings of oil, but the crank journal bearings and cams were perfect so I don't think that was the case.

I'm using the engine as a stressed member just as was the case in an MT07 but maybe the TRX frame or the exact mounting positions causes more stress than the cases can handle. In other words maybe the failure was from the outside in if the cases flexed and cracked.

Anyway I'd love to know if anyone else has other ideas because it's too easy to narrow ones focus and miss something obvious.

[M, Hausknecht]

The engine cases are known to flex. The stock rods are wimpy. I have a stock bottom end race motor that has slightly over 90 rwhp but I set the rev limiter to 10,200rpm. No problems but it doesn't have more than 500 miles as a race motor. I have another motor with Carrillo rods that I'm comfortable revving higher. 

The marks on the top of the pistons suggest it hit the head. The stock cams, pistons and valves have substantial clearance. I suspect strongly the rod broke and that wiped out the balancer shaft. The later just doesn't have much load on it but if the crank was flexing sufficiently (you have some evidence of that) to hit it, perhaps the balancer broke first.

Why rev the motor so high? You using higher duration and lift cams? If not, all those revs are just noise with the stock cams.

 

[luke2152]

30 minutes ago, M, Hausknecht said:

The engine cases are known to flex. The stock rods are wimpy. I have a stock bottom end race motor that has slightly over 90 rwhp but I set the rev limiter to 10,200rpm. No problems but it doesn't have more than 500 miles as a race motor. I have another motor with Carrillo rods that I'm comfortable revving higher. 

The marks on the top of the pistons suggest it hit the head. The stock cams, pistons and valves have substantial clearance. I suspect strongly the rod broke and that wiped out the balancer shaft. The later just doesn't have much load on it but if the crank was flexing sufficiently (you have some evidence of that) to hit it, perhaps the balancer broke first.

Why rev the motor so high? You using higher duration and lift cams? If not, all those revs are just noise with the stock cams.

 

Thanks, that's interesting info. I thought the stock rev limiter was 10500 which is why I used that number, indeed I thought that was being conservative. That is certainly what is displayed on my stock Tenere 700 although it's possible that Yam displays a higher number than it is actually revving to stoke peoples egos or whatever.

The marks on the pistons do indicate it hit the head but there's no marks on the head to correlate which I'm very confused about, especially as they seem to show rotation.

Edited September 22, 2023 by luke2152

[AP996]

Swirl marks look like the edge of valves hitting piston, I would bet on the rod breaking first.

[luke2152]

5 minutes ago, AP996 said:

Swirl marks look like the edge of valves hitting piston, I would bet on the rod breaking first.

You could be right, they certainly look like the right radius although they don't really line up well with the valves. Also don't see any marks on the valves but perhaps that's because they are pulled back into the head with spring pressure. I'd like to believe it was a rod failure as it means all I have to do to prevent it on a new engine is reduce my RPM.

[M, Hausknecht]

The stock motor lays down after about 8800rpm. The long intake tract has a lot to do with that but so do the cams. I think you'd be happier adding some length to the intake tract (before the TBS), some good bellmouths would help too, and run a 10,000 rev limit. I suspect you'll have peak power below 10k, so the rev limit won't limit your maximum HP. Then adjust your gearing if you can reach redline in 6th, if top speed is a priority.

Edited September 22, 2023 by M, Hausknecht

[luke2152]

Well apologies everyone for the long delay. I'm not really sorry, life has a way of delaying hobby projects. Anyway I've got a new engine in now and running nicely. It's a 2022 model MT07 one (or so I was told) and seems identical in all respects apart from one: the flywheel is bolted on 180 degrees offset from previous engine (ie. the missing tooth is on the other side). Why I don't know. That really confused me and took a while to figure out what the problem was as it had fuel, spark and compression but wouldn't start. Easily fixed with aftermarket engine management as you can change the settings but would be a problem if you were swapping a new engine into an old MT07 using stock ECU (although you could probably just swap the whole flywheel).

Anyway it's running nicely now and I had a little spin around the block but it's bloody freezing and the roads are covered in salt so I won't be having a proper ride for a while.

[sweetscience]

I would imagine the 3rd gen MT's, R7's  are restricted to Euro 5, EPA laws, thus the tweak in engine management and resulting in a slight loss in power and torque.  In one aspect, the future grows bleaker..