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Piston sides

Knoxes

Veteran Member
Joined
Jun 22, 2020
Total Posts
257
Total likes
1
Location
Chapel Hill NC
One looks fine, but the other is pretty significantly scored. Are these garbage now? And what happened here?

Pistons 1.jpg

Pistons 2.jpg

The cylinders look similar to each other, but both are showing some wear on the front and rear:
Cylinder 1.jpg

Cylinder 1b.jpg

Cylinder 2.jpg

Cylinder 2b.jpg

The engine probably has only a couple hundred miles on it. And I don't know how well you can see the cylinders, but the cross hatching is still pretty visible.

I tore it down because of shifting problems, but I've been concerned about the compression since I did the latest rebuild. I checked and came in pretty consistently around 135 on both sides, but don't have a lot of confidence in the gauge. My concern was that it seemed far too easy to turn over manually (like when doing timing and lash sets). However, it ran pretty well until I had the shifting issues.

Also, during this rebuild (thread "Sigh..."), I had to buy a new set of rings and it really wasn't clear at all which side was up on the compression rings. They didn't have a marking and if there was a bevel, it was impossible to see with a magnifying glass.
 
Can't tell anything about the cylinders from the picture, but the piston that is badly galled is junk. Hard to say why it did it. Running long and hard at high rpm has been an issue for the 350 and 450 engines, some believe the oil clings to the crankshaft after a while causing less splash to the bottom of the cylinders, and in other cases of extended high rpm running it can be based in oil loss caused by the engine burning some oil while being ridden hard. If the clearance was average before this event, it will likely need to be bored and first or second over pistons used (or larger depending on what is in it now).
 
From the look of things it did get a bit warm, is that the right side or the left? Looks like a partial seizure, from running lean or overheating from too much timing advance. I've seen it happen when a rider runs out gas, while running down the highway at a good clip. The carb fed by the shorter gas hose runs out first, and leans out that cylinder, you don't notice it, running down the road with the throttle open until it suddenly loses power(briefly tries to stick that cylinder). Once it cools for a moment and the gas is on reserve it will start back up, with a new rattle from the extra clearance of that collapsed piston skirt, still running well with maybe an occasional wisp of oil smoke out that side. Run a hone through the cylinder to get the aluminum smears off the bore, and see what you have. I would replace both pistons to make sure they weigh the same for balance issues.
 
Thanks for the responses. Unfortunately, I'm not sure which side but I think it was the right. You can't really tell from the cylinders either. And these are +1 pistons.

I'm not sure what to make of it - going back to the build thread provides some additional information. I switched over to Mikuni 30s for awhile but couldn't get it tuned properly. I think it was running way too rich, but I could be wrong. There were certainly times when I couldn't get it to fire on the left side due to the mixture being too rich (I think). But the right side plug was always really dirty.

I don't recall it ever getting very hot, but I didn't get any long rides at all. I may have over-revved it early on because I didn't have a tach hooked up.

The static timing was great, but dynamic tended to bounce around. Never ran out of gas.

Heading back over to the build thread with some new developments that *may* resolve the issue.
 
My first thought is the piston clearance was too tight, next is overheated. Has nothing to do with the 2nd set of rings.
So fresh cylinder and new pistons needed.
 
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