Let's talk pistons and rings...
- Thread starter pscook
- Start date
pscook
Platinum Level Site Supporter
Questions- How does the piston look? What about the underside evidence? Any concerns? Jetting was a NOZG in a 36mm Keihin, it's now a Lectron (as of 6 motor hours ago). Comments?
Now for the specs-
Piston is 2.6(ish)mm above the floor of the exhaust port at BDC. Base gasket thickness of 0.65mm. Squish unknown. Ring gaps are at 0.4mm top and bottom ring at halfway down the bore. Hours on this piston are unknown. I am the fourth owner. The third owner installed an hour meter which currently reads 108.4 hours, previous hours unknown.
I am going to have the bore and piston measured on Monday when the machine shop is open. What do you all think of the ring gap? Plating looks nice, no visible wear or flaking. Ports could use a little love to get matched and smoothed, but everything else looks decent.
Thoughts?
Now for the specs-
Piston is 2.6(ish)mm above the floor of the exhaust port at BDC. Base gasket thickness of 0.65mm. Squish unknown. Ring gaps are at 0.4mm top and bottom ring at halfway down the bore. Hours on this piston are unknown. I am the fourth owner. The third owner installed an hour meter which currently reads 108.4 hours, previous hours unknown.
I am going to have the bore and piston measured on Monday when the machine shop is open. What do you all think of the ring gap? Plating looks nice, no visible wear or flaking. Ports could use a little love to get matched and smoothed, but everything else looks decent.
Thoughts?
Jakobi
Super Moderator
+1 ..Looks as close to perfect as you'd want to be. Small hot spot on the underside is OK.. If the whole underside was as black as the top she'd be too lean. Right now you look like you're on the spot.
pscook
Platinum Level Site Supporter
Yeah, I noticed that, too. No heavy lean build up and no "clean" rich wash pattern. But it's so even that it seemed too good to be true...
Wiseco. Wossner (and I believe Vertex) are coated.
Yep yep, just what I wanted to hear. Surprisingly, I run my bikes leaner than "everyone else," for whatever reason. I want to just get rid of the off-idle burble and have the mid-throttle "on the pipe" snap, with a reasonable top end.
So, how's about them ring gaps? I see 0.5-0.8 as the range, with Wiseco recommending 0.004" per inch of bore, which brings me to .0114", which is 0.28mm. Such a deviation, and I'm almost 50% above Wiseco range yet still under OEM recommendations. Do I drop the complete package back into the hole, or replace rings (and possibly piston)? So confused...
Jakobi
Super Moderator
I run my bikes the same way Phil. Can't handle an overly rich bottom end, or the dullness in the mid range that comes with fatter jetting. I'll be interested to see what the pistons out of the new bike looks like once I open her up.
In regards to the new rings I went with wiseco specs, plus a little extra. Put them in at the tighter end of the spec, and use the 0.9mm max spec as worn out, and kind of gauge the time line it took to get there and use that as a baseline for next service interval. On the 250 I find 85-100hrs is about the right time, and do the rings, piston and little end all at once.
In regards to the new rings I went with wiseco specs, plus a little extra. Put them in at the tighter end of the spec, and use the 0.9mm max spec as worn out, and kind of gauge the time line it took to get there and use that as a baseline for next service interval. On the 250 I find 85-100hrs is about the right time, and do the rings, piston and little end all at once.
pscook
Platinum Level Site Supporter
I must run the bestest air filters ever, because I don't believe that Steve installed the new piston, and he put well over 80 hours on the bike before I got to her. It's at 100+ hours and the ring gaps are almost to OEM specs. Maybe I need to measure them at all depths of bore and see if there is a weird cone or hourglass shape to the bore.
Crap. Should have performed a compression test. Double ding against Phil. And I wasn't even drinking when I tore it apart!
Looks pretty good.
The rings can be changed 2-1 raitio to the piston,standard.
80-100 hours on piston,40-50 on rings, depending upon how hard you ride is a decent benchmark.
If you cant maintain that interval then the bore,or plating is out of spec.
As long as you haven't had any jetting issues or dust and dirt pass through the filter.
The rings can be changed 2-1 raitio to the piston,standard.
80-100 hours on piston,40-50 on rings, depending upon how hard you ride is a decent benchmark.
If you cant maintain that interval then the bore,or plating is out of spec.
As long as you haven't had any jetting issues or dust and dirt pass through the filter.
pscook
Platinum Level Site Supporter
Got it, thanks for the input. What do you think about my current ring end gap? If I get new rings, then end gap them at 0.28mm (per Wiseco)?
For each specific engine and bore and piston manufacturer some of that will vary.
If it was me,Id found out what the manufacturer recommends,then what Wiseco says if that is what piston and rings you will be using.I saw a lil w on the piston Im guessing you will be trying to run that piston with some new rings.Then Id take one more random, manufacturers specs just to be sure everybodys pretty much on the same page.I like a little tighter end gap myself,but I dont recommend it.In many cases that requires buying an oversize set and grinding them down.Youll know more after you mic the cylinder.
btw that piston looks a little "washed" to me.De carbed.
If it was me,Id found out what the manufacturer recommends,then what Wiseco says if that is what piston and rings you will be using.I saw a lil w on the piston Im guessing you will be trying to run that piston with some new rings.Then Id take one more random, manufacturers specs just to be sure everybodys pretty much on the same page.I like a little tighter end gap myself,but I dont recommend it.In many cases that requires buying an oversize set and grinding them down.Youll know more after you mic the cylinder.
btw that piston looks a little "washed" to me.De carbed.
pscook
Platinum Level Site Supporter
Good info, thanks. When you say "washed," you mean by a too rich condition? Pics were taken as it sat. My last race was a 50 mile lap of a 100 mile race, and the bike has a Lectron. I set the needle a bit lean to give it a little pep, but maybe it's still a bit rich. I'll know more when I get it together and ride it at my usual altitudes.
I am going to get a set of rings and give the piston another couple of hours before the next inspection. I'll keep a compression log as well, just so I can track any deviations.
Thanks for everyone's input.
The piston looks great,Id agree with most here looks like you have it jetted fine,in the ballpark.I try to stay on the richer side of that for power characteristics that I like,but it doesnt leave much leeway when conditions change and can make for a messy bike,stuck valves lots of carbon build up etc.
What oil are you using?
Some guys try to de carb the piston and some of the powervalve by squirting some water down the throat of the carb while the bike is running.
Excellent on older non nikasil aircooled 2 strokes if done right.Not so good on the newer designs.
What oil are you using?
Some guys try to de carb the piston and some of the powervalve by squirting some water down the throat of the carb while the bike is running.
Excellent on older non nikasil aircooled 2 strokes if done right.Not so good on the newer designs.
pscook
Platinum Level Site Supporter
Decided to bite the bullet and replate with a new slug. Going for a Wossner. Started checking the rings at 1/4" depth intervals and didn't like the numbers I was seeing. $600 (parts and plating) is piece of mind to know that I am starting from new on the top.
The crank, well, I probably just jinxed myself. I'll cross that bridge when I burn it. :/
The crank, well, I probably just jinxed myself. I'll cross that bridge when I burn it. :/
pscook
Platinum Level Site Supporter
It's a former race bike, unknown hours. The ring gap went from 0.018" at the top, to 0.017 1/2" down, to 0.016 at 1" depth, to 0.017, to 0.018. Much variation in ring gap over the length of the stroke. Is that okay to have that much change?
The third owner put an hour meter on the bike, and it's at 109 hours. That doesn't include the second owner, or the fact that it was owned/raced by Airgroup/Radiant Racing for the 2011 season.
Should I just toss a new "A" slug at it with new rings gapped at mfg specs and disregard a replate? Serious question.
Jakobi
Super Moderator
From the shop manual. Piston to Cylinder clearance (measured at smallest area of cylinder, and largest of piston)
300cc
STD: 0.06 mm
Limit: 0.10 mm
Your variances range between .457 and .406 mm so a total of say 0.05mm
How does the skirt of the piston look? Hows the engine been sounding? Any piston slap? Remember at 0.05mm with feeler gauges it could even be human error (or at least a factor of).
300cc
STD: 0.06 mm
Limit: 0.10 mm
Your variances range between .457 and .406 mm so a total of say 0.05mm
How does the skirt of the piston look? Hows the engine been sounding? Any piston slap? Remember at 0.05mm with feeler gauges it could even be human error (or at least a factor of).
pscook
Platinum Level Site Supporter
Okay, I ran a quick calculation (14 hours later, he said non-ironically) to determine bore diameter deviation based on ring end gaps.
Biggest deviation is 0.016mm diameter, smallest is 0.008mm. This assumes a round bore. I'll see if I can get the cylinder and piston to the machine shop tonight and have them give it the old once-over.
I purchased a top end kit as well as a set of rings, just because. If I can get away with using the bore as it is, then the rings will give me another 20-40 hours. If the piston has to be changed, then I'm set for that. And if the plating has to be redone, well, I'm prepared for that.
"Loaded for bear," as my old man would say.
Biggest deviation is 0.016mm diameter, smallest is 0.008mm. This assumes a round bore. I'll see if I can get the cylinder and piston to the machine shop tonight and have them give it the old once-over.
I purchased a top end kit as well as a set of rings, just because. If I can get away with using the bore as it is, then the rings will give me another 20-40 hours. If the piston has to be changed, then I'm set for that. And if the plating has to be redone, well, I'm prepared for that.
"Loaded for bear," as my old man would say.
