School Time: Two Stroke Ring Gaps

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2/26/2018 8:03 AM
Edited Date/Time: 2/26/2018 8:06 AM

Bike is an '02 RM134. I put a new Wiseco piston in and Wiseco recommends .004" x bore size for the ring end gap. That's a 56mm piston, so 2.20472" x .004" = 0.008".

RM 125 manual says 0.018-0.024" and service limit of 0.031". (for stock 125)

KX125 manual say 0.0138 ∼ 0.0217 in for new ring, service limit is 0.033"

The gap of the removed ring (hours unknown since bike was used) measured 0.028".

Obviously, the Wiseco spec is MUCH smaller that either the stock KX or RM pistons.
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Correct me on any of these:


1) I know the hypothesis of a larger bore requiring more clearance. I figure the 134 is 7% larger than the 125, so increasing the RM manual specs by 0.07 = 0.019 - 0.025"

2) The purpose of the ring gap is to provide room for expansion.

3) Too much ring gap will allow more blow-by, but theoretically, I could have a huge ring gap on a new ring and the only negative would blow-by (which could cause the piston to seize from carbon build up, maybe?). Too little ring gap, though, and there's no room for expansion and the ring can seize from that.

4) Therefore, the Wiseco spec seems dangerously small.

5) The stock ring was 0.014" and that made me nervous. I set the gap at .020".

6) If you want to say anything about the ring end gap of a used ring, you really need to know what the gap was when new. I mean, I could've opened that gap up to 0.028" (same as the one I removed) and you'd think "Worn out!", when it really was a new ring. Me, I write down the ring end gap of all my newly installed rings for reference when I do the next teardown.

How'd I do?

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2015 Kawasaki KX 250F
2015 GasGas TXT Racing 125
2015 Husqvarna TC 125
2018 KTM 150 XC-W

2/26/2018 8:32 AM

IMO you are solid. I don't like the super small gap and have used high teens at the very least (I'd think the one at .014-.020" was fine). I've done my own top ends since riding 80s and never had a top-end issue.

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2/26/2018 9:59 AM

Ive always gone between 8 and 12 thou per inch of bore the wiseco spec seems dangerously tight.

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2/26/2018 10:38 AM

DTHA70 wrote:

Ive always gone between 8 and 12 thou per inch of bore the wiseco spec seems dangerously tight.

So, 2.20472 x .008"-.012 = 0.018 - 0.026"

That sounds reasonable. At 0.020" I'm at the tight end, and at .028" for the old ring, I wasn't too bad.

Bike actually was running great.. I'm just suspicious of used bikes. This one "needed nothing" but I found a loose spark plug, a torn air filter, bent bars, a new chain but old sprockets, and a broken throttle housing. Fixed all of those things, rode it awhile 'cause it was starting 1st-2nd kick every time. Got to know the bike a bit and then when my son broke his collarbone, decided that was as good of a time as any to get the new piston/rings in.

Obligatory naked 2-stroke picture. Photo

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2015 Kawasaki KX 250F
2015 GasGas TXT Racing 125
2015 Husqvarna TC 125
2018 KTM 150 XC-W

2/26/2018 5:56 PM

After you do get the ring gap set up as you want....Pull a compression test and record that number. When you see that number drop off with use, it will be time for new rings.

Paw Paw

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2/26/2018 8:46 PM

Paw Paw 271 wrote:

After you do get the ring gap set up as you want....Pull a compression test and record that number. When you see that number drop off with use, it will be time for new rings.

Paw Paw

I just bought a Motion Pro compression tester for this reason. How important is it to have the bike warmed up?

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2015 Kawasaki KX 250F
2015 GasGas TXT Racing 125
2015 Husqvarna TC 125
2018 KTM 150 XC-W

2/27/2018 2:16 PM

Cold should produce the proper reading. If not try it warmed up. It should go up somewhat i.e., everything is expanded at that point.

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2/27/2018 2:32 PM

HighPlainsSquid wrote:

Bike is an '02 RM134. I put a new Wiseco piston in and Wiseco recommends .004" x bore size for the ring end gap. That's a 56mm piston, so 2.20472" x .004" = 0.008".

RM 125 manual says 0.018-0.024" and service limit of 0.031". (for stock 125)

KX125 manual say 0.0138 ∼ 0.0217 in for new ring, service limit is 0.033"

The gap of the removed ring (hours unknown since bike was used) measured 0.028".

Obviously, the Wiseco spec is MUCH smaller that either the stock KX or RM pistons.
------------------------------------------
Correct me on any of these:


1) I know the hypothesis of a larger bore requiring more clearance. I figure the 134 is 7% larger than the 125, so increasing the RM manual specs by 0.07 = 0.019 - 0.025"

2) The purpose of the ring gap is to provide room for expansion.

3) Too much ring gap will allow more blow-by, but theoretically, I could have a huge ring gap on a new ring and the only negative would blow-by (which could cause the piston to seize from carbon build up, maybe?). Too little ring gap, though, and there's no room for expansion and the ring can seize from that.

4) Therefore, the Wiseco spec seems dangerously small.

5) The stock ring was 0.014" and that made me nervous. I set the gap at .020".

6) If you want to say anything about the ring end gap of a used ring, you really need to know what the gap was when new. I mean, I could've opened that gap up to 0.028" (same as the one I removed) and you'd think "Worn out!", when it really was a new ring. Me, I write down the ring end gap of all my newly installed rings for reference when I do the next teardown.

How'd I do?

If im correct the forged wiseco piston will expand less than the stock cast type piston. Perhaps thats why they spec what seems to be a small ring end gap. The tolerences of the wiseco are tighter. I think in the wiseco instructions they slso specifically instruct you to use a wiseco ring set with their pistons. I am asuming because of the metalurgical differences between the cast / forged materials and process. . .Weather is getting nice, slowly but nice. Can't wait to ride !!!

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2/27/2018 2:43 PM

DTHA70 wrote:

Ive always gone between 8 and 12 thou per inch of bore the wiseco spec seems dangerously tight.

I agree. In an absolutely new fresh PROPERLY bored clearanced etc... cylinder, the specified wiseco ring end gap should work. But, and but, it does seem dangerously small. In the world of brush hones and beer bottles 8 to 12 would be nice and give me more piece of mind. (And yes indeed i am talking about myself...☺)Seizures are not good...

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2/27/2018 4:29 PM

Stupid question here, but when you guys talk about setting the end gap how do you go about that? I was under the impression that you just measure it and it is what it is. Or are you talking switching rings/taking material off the cylinder?

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RPM Performance
CT
783

2/27/2018 4:36 PM

murph783 wrote:

Stupid question here, but when you guys talk about setting the end gap how do you go about that? I was under the impression that you just measure it and it is what it is. Or are you talking switching rings/taking material off the cylinder?

File the ring ends. Dont bore or hone a cylinder to adjust the ring end gap. Take an equal number strokes. Make nice cuts. Measure as you go. Keep putting the ring back in the cylinder at the specified location, check the ring end gap with good feeler gauges. One swipe at a time...

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2/27/2018 4:39 PM
Edited Date/Time: 2/27/2018 4:40 PM

murph783 wrote:

Stupid question here, but when you guys talk about setting the end gap how do you go about that? I was under the impression that you just measure it and it is what it is. Or are you talking switching rings/taking material off the cylinder?

Btw, the "Stupid" question is the one that causes a failure because it was not asked...☺

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2/27/2018 6:21 PM

Thanks! Makes perfect sense when you put it that way

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RPM Performance
CT
783

2/27/2018 6:45 PM

murph783 wrote:

Thanks! Makes perfect sense when you put it that way

YW ☺

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2/28/2018 6:08 AM

kruspy378 wrote:

File the ring ends. Dont bore or hone a cylinder to adjust the ring end gap. Take an equal number strokes. Make nice cuts. Measure as you go. Keep putting the ring back in the cylinder at the specified location, check the ring end gap with good feeler gauges. One swipe at a time...

Yup. That's what I did.

That's why I said that measuring a gap on a worn ring seems useful only if you know what it was when new. I _could_ have filed the ends of that brand-new ring down a bunch to create an "out of spec" ring end gap, but it still would've been a new ring. Conversely, if I'd started with Wiseco specs, it might be a long time before that ring wears enough to show "out of spec".

I'm gonna pressure test ASAP. Again, it seems like the thing to do is be as consistent as possible. To start with, I'm at 6,000' elevation and I know it's going to show less pressure right off the get-go, so it's going to be the change in that pressure that's important. Cold engine seems like it might the best way because cold is cold, but "warmed up" could be vary from "barely warmed up" to "very warmed up".

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2015 Kawasaki KX 250F
2015 GasGas TXT Racing 125
2015 Husqvarna TC 125
2018 KTM 150 XC-W

2/28/2018 6:30 AM

murph783 wrote:

Stupid question here, but when you guys talk about setting the end gap how do you go about that? I was under the impression that you just measure it and it is what it is. Or are you talking switching rings/taking material off the cylinder?

kruspy378 wrote:

File the ring ends. Dont bore or hone a cylinder to adjust the ring end gap. Take an equal number strokes. Make nice cuts. Measure as you go. Keep putting the ring back in the cylinder at the specified location, check the ring end gap with good feeler gauges. One swipe at a time...

HighPlainsSquid wrote:

Yup. That's what I did.

That's why I said that measuring a gap on a worn ring seems useful only if you know what it was when new. I _could_ have filed the ends of that brand-new ring down a bunch to create an "out of spec" ring end gap, but it still would've been a new ring. Conversely, if I'd started with Wiseco specs, it might be a long time before that ring wears enough to show "out of spec".

I'm gonna pressure test ASAP. Again, it seems like the thing to do is be as consistent as possible. To start with, I'm at 6,000' elevation and I know it's going to show less pressure right off the get-go, so it's going to be the change in that pressure that's important. Cold engine seems like it might the best way because cold is cold, but "warmed up" could be vary from "barely warmed up" to "very warmed up".

I could be wrong but i assume a warm or bettet yet normal heated up temperture would be a more realistic / real world riding condition to assess the actual compression the motor is actually putting out. I.e. the motor is in its "stride". Its spending most of its life running (Hopefully... ☺) not sit4ing cold. Thats my line of reasoning, but i am not an engineer, or a mega talanted wrench. (Always wanted to be though) I am just commenting and contributing any advice based on my world experiences. I am enjoying the dialog in this thread and forum. Such a wealth of good information and one can truly pick up something new every day from this community. Hope ANY of my posts or responses have helped somebody get that little bit further ahead with respect to their projects and thanks for listening and putting up with me... Sorry for long post and cheers!☺

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2/28/2018 6:39 AM

kruspy378 wrote:

I could be wrong but i assume a warm or bettet yet normal heated up temperture would be a more realistic / real world riding condition to assess the actual compression the motor is actually putting out. I.e. the motor is in its "stride". Its spending most of its life running (Hopefully... ☺) not sit4ing cold. Thats my line of reasoning, but i am not an engineer, or a mega talanted wrench. (Always wanted to be though) I am just commenting and contributing any advice based on my world experiences. I am enjoying the dialog in this thread and forum. Such a wealth of good information and one can truly pick up something new every day from this community. Hope ANY of my posts or responses have helped somebody get that little bit further ahead with respect to their projects and thanks for listening and putting up with me... Sorry for long post and cheers!☺

I agree with the "most realistic indicator of output", but it's the consistency part that worries me. I used to design biological field experiments and it's always the X-Factor that you have to watch out for.... the thing that you didn't know about... that bites you in the butt. Therefore, I would want to know what factors change as the engine warms up and how those affect the compression.

The smart thing would probably be to check the compression BOTH at cold and after warming up. You'd have to do this while new and after some wear. I suspect that if you see a big difference between cold/warm with some age on the motor, then you're really looking at wear. While new and tight, it's probably going to give good compression while cold AND while warm. When warm, I'd expect to see less compression while cold and then much better (comparatively to the new engine) when warm. Make sense? Of course, by then I'm probably replacing parts anyway.... smile

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2015 Kawasaki KX 250F
2015 GasGas TXT Racing 125
2015 Husqvarna TC 125
2018 KTM 150 XC-W

2/28/2018 6:49 AM

HighPlainsSquid wrote:

Yup. That's what I did.

That's why I said that measuring a gap on a worn ring seems useful only if you know what it was when new. I _could_ have filed the ends of that brand-new ring down a bunch to create an "out of spec" ring end gap, but it still would've been a new ring. Conversely, if I'd started with Wiseco specs, it might be a long time before that ring wears enough to show "out of spec".

I'm gonna pressure test ASAP. Again, it seems like the thing to do is be as consistent as possible. To start with, I'm at 6,000' elevation and I know it's going to show less pressure right off the get-go, so it's going to be the change in that pressure that's important. Cold engine seems like it might the best way because cold is cold, but "warmed up" could be vary from "barely warmed up" to "very warmed up".

kruspy378 wrote:

I could be wrong but i assume a warm or bettet yet normal heated up temperture would be a more realistic / real world riding condition to assess the actual compression the motor is actually putting out. I.e. the motor is in its "stride". Its spending most of its life running (Hopefully... ☺) not sit4ing cold. Thats my line of reasoning, but i am not an engineer, or a mega talanted wrench. (Always wanted to be though) I am just commenting and contributing any advice based on my world experiences. I am enjoying the dialog in this thread and forum. Such a wealth of good information and one can truly pick up something new every day from this community. Hope ANY of my posts or responses have helped somebody get that little bit further ahead with respect to their projects and thanks for listening and putting up with me... Sorry for long post and cheers!☺

HighPlainsSquid wrote:

I agree with the "most realistic indicator of output", but it's the consistency part that worries me. I used to design biological field experiments and it's always the X-Factor that you have to watch out for.... the thing that you didn't know about... that bites you in the butt. Therefore, I would want to know what factors change as the engine warms up and how those affect the compression.

The smart thing would probably be to check the compression BOTH at cold and after warming up. You'd have to do this while new and after some wear. I suspect that if you see a big difference between cold/warm with some age on the motor, then you're really looking at wear. While new and tight, it's probably going to give good compression while cold AND while warm. When warm, I'd expect to see less compression while cold and then much better (comparatively to the new engine) when warm. Make sense? Of course, by then I'm probably replacing parts anyway.... smile

Absolutely makes perfect sense. Totally agree. The x factor, the unknown.... Thank you for your insight and taking the time to share methodology and science based ideals with me. Best wishes and enjoy a great riding / wrenching season to come. I usually dont post ANYTHING this much. I am at home nursing a lumbar injury. Daytime tv just doesnt cut it... ☺ This been good therapy for me because i am going stir crazy. I want to go in my garage and i cant...boo hoo.....

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