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Discussion Starter · #1 ·
44,000 miles. I've got my motor (mostly) apart to replace a bent shift fork. Most of everything else looks good but I'm wondering if piston rings should be replaced as preventative maintenance. They appear to be in good shape, no noticeable oil consumption. Cylinder walls have no scuffing or signs of excess wear, cross hatching is really light so I'll of course re-hone them. But I do have everything apart and the pistons are right there in front of me and easy to get to. On the other hand, I didn't have any issues, and I've seen a few DLs in excess of 100k miles and a few past 200k, so they do seem like they last a long time.

I've considered having the cylinders and heads milled a bit for tighter squish and to bump compression if I get motivated enough, but the primary goal here is to just get her running again without too much time and money invested. This bike is primarily a commuter.
 

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I have considerably more mileage on mine, and still going fine. Unless you are doing power up work, leave well enough alone i reckon...
 
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KNEE DRAGGER
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44,000 miles. I've got my motor (mostly) apart to replace a bent shift fork. Most of everything else looks good but I'm wondering if piston rings should be replaced as preventative maintenance. They appear to be in good shape, no noticeable oil consumption. Cylinder walls have no scuffing or signs of excess wear, cross hatching is really light so I'll of course re-hone them. But I do have everything apart and the pistons are right there in front of me and easy to get to. On the other hand, I didn't have any issues, and I've seen a few DLs in excess of 100k miles and a few past 200k, so they do seem like they last a long time.

I've considered having the cylinders and heads milled a bit for tighter squish and to bump compression if I get motivated enough, but the primary goal here is to just get her running again without too much time and money invested. This bike is primarily a commuter.
You are likely fine, but often when I am doing customer engines, I typically send cylinders out for honing and replace rings.

Carefully inspect your transmission. An actual bent shift fork is typically the result of other issues going on in the gears. There has never been a transmission in the hundreds I have done that only thing I replaced was a shift fork. It is in most cases you get rounded/bashed tips on the dogs and slots and the gears separate under load and they forced apart and take a fork with them.

Mill the heads and cylinders for sure but also understand the the cam timing shift you will encounter due the height change.

-ms
 

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Discussion Starter · #4 ·
Missing Link: Yeah, that's what I was thinking, seen lots of miles on these and rings seem fine, only need to hone to ensure the original rings re-seat ok.

Schmidt: Thanks again for all of your valuable insight. Coin toss on the piston rings, maybe I'll do rings if I bite the bullet and mill up the jugs and heads, timing considerations understood.

Understood on the transmission, I do suspect it may be more than just the shift fork, but I'll finish up disassembly and get the cases split tomorrow to know for sure. I poked in there with a borescope and I can see the drum looks ok, gears themselves appear ok but I the shift fork doesn't want to move when changing gears until I spin the shaft back and forth and then it will move into the next gear. This was due to an accident where the bike landed on the shift lever and pushed something somewhere it didn't want to go.
 

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Discussion Starter · #6 ·
Doesn't the cyl have nikasil coating ? If you hone that you need to recoat ?
Schmidt can provide the correct answer so I'll of course defer to him, but in my case I wouldn't be doing a full re-hone since my bores are in good shape overall. I'd just be breaking the glaze and freshening up the cross hatching in order to ensure the rings re-seat ok. Basically just putting very light scuffing on the cylinder walls in a controlled fashion.
 

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KNEE DRAGGER
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Piston Rings should be replaced if they have come out of the bore.
They will never be returned to the exact the same position and will want to re- break-in/wear (two strokes are the exception as they are pin located in the ring land). This practice has always applied in my experience, from industrial (air compressor ,especially high pressure ) applications to automotive / motorcycle.
They will be worn out once rebreak-in has been achieved.
If they are still in the bore you good to go.
 

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KNEE DRAGGER
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Piston Rings should be replaced if they have come out of the bore.
They will never be returned to the exact the same position and will want to re- break-in/wear (two strokes are the exception as they are pin located in the ring land). This practice has always applied in my experience, from industrial (air compressor ,especially high pressure ) applications to automotive / motorcycle.
They will be worn out once rebreak-in has been achieved.
If they are still in the bore you good to go.
Axial piston ring clearance and the cross hatching in the surface finish of the bore allow the ring to change rotational position on the piston as they traverse along the bore. This means that the rings do not break into any specific clocked position. As you mention, two strokes rings are held into a very specific location. This is not for ring to bore sealing reasons, it is done to keep ring end gaps from rotating around and catching on port edges.

-ms
 

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KNEE DRAGGER
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Schmidt can provide the correct answer so I'll of course defer to him, but in my case I wouldn't be doing a full re-hone since my bores are in good shape overall. I'd just be breaking the glaze and freshening up the cross hatching in order to ensure the rings re-seat ok. Basically just putting very light scuffing on the cylinder walls in a controlled fashion.
Yeah, just a prep type honing to deglaze and clean up is pretty much all that is needed as long as your bores are still round and taper is in spec.

-ms
 

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So my question would be, if the piston ring is “allowed“ to “rotate” in the bore because of the clearance from the ring land to the ring, is the crosshatch providing the force to rotate?

Try lining up the gaps on a ring set and running the motor until you can no longer afford the oil consumption. When you pull that piston, the ring gaps will be aligned, same as they were installed..... I learned the hard way.
I would bet a dollar that a microscopic view of the outside edge of a used ring will show a matching wear pattern to the bore it was installed in. Yah, you can get away with it for a while but you don’t want to have to go back in for something you can avoid.
 

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KNEE DRAGGER
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So my question would be, if the piston ring is “allowed“ to “rotate” in the bore because of the clearance from the ring land to the ring, is the crosshatch providing the force to rotate?

Try lining up the gaps on a ring set and running the motor until you can no longer afford the oil consumption. When you pull that piston, the ring gaps will be aligned, same as they were installed..... I learned the hard way.
I would bet a dollar that a microscopic view of the outside edge of a used ring will show a matching wear pattern to the bore it was installed in. Yah, you can get away with it for a while but you don’t want to have to go back in for something you can avoid.
The rings do rotate due to surface finishing of the bore. There all kinds of good articles in regards to this. A really good one I read years ago was in Race Engine Technology. All kinds of engines I have built that I tracked ring gap position on installation, they are all over the place when pulled down. Even after just breaking in on dyno and running a bunch of pulls. Pull engine down and gaps are in different locations.

In the past I have personally performed some scanning electron microscopy, gallium ion beam, and xenon plasma ion analysis on all kinds of parts I was interested in including rings, buckets, wrist pins, DLC coatings to look at wear at the micro and nano scales. Interesting stuff for sure. Let me see if I can find any piston ring images from the past and see there are any unique wear patterns noticeable.

-ms
 
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