[AKmnstr]

My sad story of how I came to once again tearing about my engine was described here, http://www.ducatimonster.org/smf/index.php?topic=58350.msg716422#msg716422
This past weekend I was at last able to take apart the engine and found the source of the problem. On the vertical cylinder, the hole which receives the piston pin on one side of the piston had enlarged, allowing the piston additional play on one side. As a result of this play, the piston had rubbed against the cylinder wall. The cylinder bore had two small spots where I could feel some aluminum had been deposited by the piston. I should mention the engine had a BCM 795 big bore kit. I honed the cylinder and can no longer feel any scratches in the bore although some discoloration remains. So my questions are. Has anyone had similar piston/pin failures? What could cause such a failure and how can I prevent it in the future? How can I determine if the cylinder is reusable?

 

[Speeddog]

I've seen failures like that, not on Duc engines, but that's not really all that significant.

Failure causes I can think of quickly:
1) Piston pin to pin bore clearance too big at manufacturing.
2) Too much high-rpm running.
3) Partial seizure of the pin to the rod, due to inadequate oil supply.
a) Inadequate oil supply due to overheating of the piston.
4) Overheating of the piston, and the aluminum loses enough strength to allow the pin to oval-out the pin bore.
a) Overheating of the piston due to detonation, lean mixture, poor cylinder cooling, etc.
b) Overheating due to excessive piston to cylinder clearance.
5) Partial seizure of the piston in the cylinder.
6) Contact between the piston and the head.

If the cylinder bore diameter is still within spec, I would think it's OK.
It would be good to have someone that's seen a lot of Duc cylinders do a visual check on whether the Nicasil coating is still in good shape.

 

[Jethro]

I would want to check the bore was square to the crank.

 

[Harry1160]

+1 on what speeddog said. I would also measure the top and bottom of cylinder bore to see if they are the same specs.
MM

 

[AKmnstr]

Thanks for the help!!! [thumbsup] I'll check the clearances. Much of what speeddog describes can be linked to hard running and I was running the bike hard when it failed. Is there a way to check the oil supply under normal conditions?

I would want to check the bore was square to the crank.

Jethro, I don't understand what you are referring to. I can only see one way to install the cylinder into the crankcase. Could you explain more? I may be missing something important.

 

[Jethro]

I did not mean that you had fitted it wrongly but that it had been machined not square as it sounded like piston was tipped along pin axis. Worth checking before you rebuild it unless you can find something else definitely wrong.

 

[Speeddog]

I did not mean that you had fitted it wrongly but that it had been machined not square as it sounded like piston was tipped along pin axis. Worth checking before you rebuild it unless you can find something else definitely wrong.

+1 on this, especially since it's an overbored cylinder.
It is possible to bore 'em crooked. Sloppy workmanship, but it does happen.

The pin to bore clearance is usually quite small, it's tough to measure accurately without high $ equipment.
If you oil up the pin, put it into one side of the piston, and tilt the piston so the pin is vertical, it should *slide* through under it's own weight.

The oil supply to the small end of the rod/piston pin comes from splash, the fog of oil in the crankcase, and the oil that gets scraped off the bore by the oil ring and ejected through the holes in the bottom of the ring groove.

*Some* pistons have holes drilled from the oil ring groove to just above the pin, and then a cross drilling to connect that with the pin bore.
That really helps lube the pin.

Some engines also have oil squirters that point up from the crankcase towart the bottom of the piston.
Those are primary for cooling the piston, but they also help with the small end of the rod/piston pin.
My S4 engine has those squirters, not sure about the small Duc motors.

 

[AKmnstr]

I want to thank you dudes for your replies!!! [thumbsup] You've been a huge help. I'll be making several checks before replacing the pistons. I haven't check my log book but I think I had about 8k miles on this piston/cylinder combo before it failed.

 

[Mark1305]

Don't forget to check for a bent rod. A lateral bend could lead to your type of damage.

 

[AKmnstr]

Don't forget to check for a bent rod. A lateral bend could lead to your type of damage.

There is no obvious bend and since the engine has Corella rods I think it is unlikely, but it is a good suggestion and I would like to check it. Besides a visual inspection and a comparison to a straight edge, is there an accurate what to check for a bent rod? Preferably without splitting the crankcase. Also, how does one check to see that the bore is not crooked? Again, a visual inspection reveals nothing, but I know that tolerance are close in these engines and my eye is not likely to detect all problems.

 

[Mark1305]

Checking rods is one of those tasks I have to leave up to a reputable engine builder or machinist. Having Carillo rods is a big plus for durability, though.