Recently a con rod decided it no longer wanted to continue it's function and parted company with both the crankshaft and the piston. This occurred during a race meeting at Coombe and luckily didn't result in any accident for myself or any other racer. Bit frustrating as it meant I missed another race meeting and was planning on changing the bike at the end of the season I'm off to Valencia for a few days later this month so I've already sorted out a good replacement motor from a friend, I've whipped the head off and sent it off to be pressure tested and get the valve clearances checked amended as necessary. I'd be interested to hear from anyone else who may have experienced the same issue as I cannot seem to figure out the cause. The engine was absolutely spot one performance wise, no odd noises or changes in power, then it just let go out of nowhere. The only indication I got, was once or maybe twice during that weekend, and perhaps the one previous, was when starting the bike warm I experienced it bogging down, a good hard rev seemed to clear it and then it was fine. At the time I didn't give it much consideration but now I am wondering whether it was an indication of an issue. I'm very anal with maintenance, and had checked the oil level prior to heading out for the race it let go. As is always the case with the stupid dipstick the oil level is very dependant on how long it's been since the engine has run. I try to stick to what the service manual suggests in terms of checking the oil. I am wondering now whether it may have had too much oil, although I seem to recall being happy with it, plus I feel like an overfill of oil would cause an issue such as blowing the rings or something similar, rather than this kind of heavy mechanical failure, but I am asking a question see if anyone might have an answer. Of course it could just be a mechanical failure and there is no explanation. I've not had the bottom end off, but from the damage on the block it appears the rod has come away at the top end near the piston, smashed through the block and then snapped towards the bottom, lodging a large length of con rod in the gearbox. The crankshaft end I can see through the hole in the block is still attached and looks alright. Given the very sudden failure with no noise I wondered whether it might be possible that there was oil starvation, the piston got stuck and that's why the rod snapped on the smaller end. However the piston moves freely in the bore. Had this scenario happened, would the piston cool down enough that it could then move freely afterwards? I need to check the condition of the bore this evening. Any thoughts or experiences welcomed.
If a piston had seized hard enough to break a rod it's unlikely it would have freed off after cooling. But it certainly seems like the piston caused the rod to break, is it possible it " hydrauliced" the bore filled with fuel or oil ?
Yea so that's what I had imagined, a seized piston would not be free to move as a normal piston. There's nothing on the piston surface to suggest a lack of fuel, no pinking/det marks etc and it doesn't appear that there was an excess of oil. I had friends and family watching the racing all weekend so they would have pointed out any smoke/noises that was not normal. Interestingly, I pulled the sump off earlier and half of what looks like third gear is there. I know these bikes have issues with 3rd, I'm nw wondering whether the gear could have sheared and headed in the direction of the rod (it's directly in front of it) and whether this could have been the cause. I'll need to split the casing to confirm this I think, but it could help explain the sudden onset from totally normally to totally knackered on a split moment.
@TrackdayCharlie. OMG! That looks horrible & you have my sympathies. Have you inspected the bottom of the piston? It's possible the gear could have wedge between the crankshaft balance web & the base of the piston (or gudgeon pin) on the downward stroke and caused, what looks like, a tensile fracture.
Hi I've yet to had time to split the case on the damaged engine down as I'm rebuilding my new engine and shy on time as the bike is getting shipped out to Spain the weekend after next. I think when I open it up I should also see internal damage which might help indicate the direction of gear and help figure out the cause. It's a right mess and the shrapnel has got everywhere so I'm having to bin anything related to the old engine as it's just far too contaminated.
That's bad. Is the cylinder head damaged from the piston? If not, then it probably has let go on a downward stroke. And on the bright side - you may have a head for sale!
Two of the valves were bent but not a mark on the cylinder head itself. Sadly no head for sale as when I opened up my new replacement engine it has knackered cam journal! I say knackered, it wasn't acceptable for an engine that will only be raced. So the new head is junk to me and I've had the old head rebuilt with new guides, valves, seats cut etc
You've not had the best of luck there mate. Perhaps the piston got 'batted' back up the bore on contact with the crank? Still, hope the build goes OK.
Yea I think that might have been the case. I report back with some pictures when I get around to splitting the damaged engine. Right now I'm hunting down basic parts to get the engine finished. Stupid stuff that nobody has on the shelves :-(
