Posts
812
Joined
6/12/2017
Location
High Lonesome, NM
US
Edited Date/Time
2/27/2018 6:45pm
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?
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?
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.
Paw Paw
The Shop
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".
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....
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