Motoscrubs.com

Hi All,

Stuck the piston at 60 miles per hour the other day on my monza 250, that was an exciting ride for a moment, until I pulled in the clutch...it locked up the rear tire and got a little squirrelly for a moment.
Turns out I have a broken first compression ring,180 degrees around from the ring gap, and the broken ring ends were digging in to the cylinder bore. When it got hot enough, it gualled the piston side, and stuck it to the cylinder wall. When I came to a stop at the side of the road, the engine would turn over, so I started it up and limped it the rest of the way to work. I pulled the top-end off and found the damage, but it does still run.

A full teardown is next, to clean up any potential mess, etc.

I'm going to need a new piston set, and a bore&hone to fit.
I'm thinking about a standard size 350 piston (76mm) in that 250 bore? It will leave me with just under 2mm thick sleeve measurement. What is the minimum thickness for a sleeve? Has anyone else done that, and to what result? What are your thoughts?

Bruce

I have a British big single engine that has been resleeved with a very thin sleeve.
But, the original sleeve is still in place, so supports the inner sleeve.
As the bottom part of the sleeve protrudes below the cylinder base, you might have a fragile result.

Rather rare to have a ring break like that.....

Hi Bruce

with a 76 mm piston the liner is still plenty thick .

the thing is a 350 piston has the wristpin lower in the skirt . so the dome sits higher above the cilinder deck .

you could machine a little of the pistons squishband and also put a spacer under the barrel .

Eldert

Bruce,

imo you should stay with a 250 piston. 2mm bore enlargement will not give significant gain in power, but has some drawbacks: down-machinig of the piston's squish area will shorten the fire land above the 1st ring, which in turn will be exposed to more thermal power. This might reduce it's durability. Placing a spacer plate under the barrel is the better solution. But this changes the distance between th circlips on the lower and upper bevel shaft, so the connecting bush gets an additional free axial play. I don't know if this might be an issue.
The 350 piston also might be heavier, so the crankshaft would need some re-balancing. Not without cause, the crankshafts for 250 and 350 models use crank disks different in shape (i.e. different balance weight).

Hans

i had J/E make some 75.5 mm pistons made for the 250 . al sold now .

i am thinking about having some more made up in 75.5 and 76 mm when finances permit .

Eldert

Hi All,
Thanks, guys for the comments and advice. A normal build with standard parts would be easier,  but I'm not opposed to going through a custom build... I have to get all my machine work outsourced, but it helps to know exactly  what I want them to do.

Eldert,
Do you know exactly what is the difference measurement between the 250 and 350 wrist pin location?

Hans,
Thinking about the weight of the 350 piston, any modifications to that piston should bring it closer to the weight of the 250 piston. In the end, after any modifications, the 350 piston could then be balanced to the crankshaft.
Wondering about piston skirt clearance between the 350 piston and 250 crankshaft... I guess that will just have to be checked along the way.

It's a journey, I'm in no particular hurry to finish this, just getting started.

Here is a pic of the broken ring. It broke nearly 180 degrees from the ring gap.Notice the half black, half a shiney ring face, and the last fraction of an inch is shiney is where the ring was gouging into the cylinder wall. That carbon build-up on that ring face indicates to me that ring had been broken for a while, not sure how long though.

Bruce

Hi Bruce

the 350 piston has the wristpin about 3 mm lower in the skirt

Regards Eldert .

Howdy,
Did you measure the taper in the bore, and the out of roundness?
And the side clearance of the ring in the groove.
All of these can contribute to ring failure.
And I was wondering if any of those conditions existed in your case, or if it was just a defective ring.

CZ

Hi Eldert,
Thanks for the measurement, 3 mm is something to deal with, huh...

CZ,
Yes, I would like to measure in depth that bore at some point, I had that bored and honed to fit that piston to Ducati specifications about 2500 miles back...
Here's the thing, it was a used but in decent condition piston at the start of this project, so maybe the ring grooves are (were) a little wide, then at the beginning of that engine break-in period, I put that same top ring in upside-down, and it smoked a LOT, turning it over stopped the smoking, and I thought all was well... who knows, may be related to any of that?

Bruce