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Discussion Starter #1
Hello everyone

I'm looking for opinions on my ring wear, I seem to be going through rings too quickly and can't nail down the root cause.
I'll start with the bike...
2007 Honda CR250 that I ride mostly in tight and steep single track / x-country riding.
Engine is leak-free and gets pressure tested every time I assemble it. Jetting is safe-rich, mixed @ 32/1, air filter always clean.

Piston #1 I started the season with a brand new oem cylinder and top end, the bike ran awesome to 60 hours when I changed the top end as a precaution (admittedly a little past my usual 40 hours) At this point the compression was still good, but the ring showed wear on the exhaust side... not shocking for 60 hours. Everything else internal looked brand new.

Piston #2 lost compression at around 10 hours. One day I noticed a lack of compression at the kick starter, and looking back... I seem to recall it had been running a little rich leading up to this.
-ring worn very thin on exhaust side, and has collapsed into the ring groove
-spark plug reads very rich
-looks to me like good piston wash, a little on the rich side
-no scrub marks on the piston sides
-cylinder plating and cross hatch in good shape
With no found cause, I second guessed my ring end gap, and replaced the top end being more careful with my measuring and filing.




Piston #3 just lost compression again at around 15 hours. Same as last time... engine has seemed slightly richer the last couple rides, and I noticed low compression at the kicker yesterday. I will tear it apart this afternoon, expecting to find the same situation. I had done a compression test a week or so ago, and it was still good.


I am left wondering what is causing this. Tight ring end gap seems like a suspected cause, but I was very precise setting it last round. The only other two things I can think of are:

1- Overheating. I do boil my bike over pretty regularly trying to climb nasty hills in the trees, possibly the exhaust side of the cyl is swelling when hot?
I would expect to see piston scrub marks if this were the case, but there are none.

2- High speed transfer sections. We are forced to cover some ground on old roads at times, and I am geared very low so the bike sees a little time cruising in top gear at moderate RPM (5000 - 6000).
Although I monitor my RPM and try to keep it low, I always hate sustained RPM riding like this. Even at rich jetting, is this a possible culprit?

Any insight would be greatly appreciated.
 

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By reading your post you’re a seasoned mechanic who knows what he is doing, welcome to E2S. We need more members like yourself.

The first assumption would be that the exhaust port is too large or you have some sharp edges on your plated cylinder. At least we can focus because of where the rings are giving up. How much end gap are you using? Too much gap can sometimes be as bad as not enough. Adding a dial bore gauge would be a handy addition to the tool box.

Your problem got worse with piston #2
Did you check the cranking compression with each new piston? I find it very handy with my feeble mind to keep track of things on a note pad. Carefully checking the skirt clearance with that dial bore gauge would have been very helpful during piston changes. We can’t assume that because the piston is new & the bore looks good that the skirt clearance is correct. (A common mistake that even the best can do)

If the engine has a bridged exhaust port, enlarging the rib oil holes would be helpful.
I’d guess that there isn’t this kind of a problem on the next later year engines. Sometimes the design engineers get a little carried away when performance overrules common sense.

Just because the cylinder looks OK with a visual inspection doesn’t mean that it is. A nice Dial bore gauge would be a good addition to the tool box. When the piston rocks the rings will snag worse.

Since I’m a cylinder machinist & not a mechanic I may be way off base but, this kind of problem comes in the front door hundreds of times.
I’ll bet that Mr. Flo-tek or Mr. smitty would also have some more good suggestions.
:Cheers:
 

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sounds like your doing well. mine made it barely a couple hrs before the piston front was chewed up. likely the damage was done within the first few minutes but i didnt take it apart until after a few hrs. i must of grinded the port too wide i suppose :Smoke:
 

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Discussion Starter #4
-Whether or not the port is too wide could be debated with Honda lol. All internals are stock on this engine, with the exception of an aftermarket crankshaft I installed 2 years ago. The port is bridged, and I cannot find any sharp edges or flaws.
-I do compression test my engine after every rebuild. My house is at 4000' and my gauge is a cheaper model, so the numbers are mostly for comparative purpose. I usually see 180psi before startup, and 190ish after 2 heat cycles.
-I have been setting ring gap on this engine at .016 which is on the tight end of Honda's specs.
-A bore gauge WOULD be a good addition to my tool box, unfortunately between normal biking and snowmobiling expenses, my budget is usually stretched pretty thin. I'll bump this up a few spots on "the list".

Today's rebuild went smooth. When I got the cylinder off I found what I expected:
-ring worn thin on exh side, but not as bad as last time. The ring had not collapsed yet.
-cyl passes visual inspection
-piston looks perfect, aside from evidence of blow-by
-spark plug and piston wash indicate rich mixture
So, with nothing to pin the problem on I am left to assume there is either an issue with the cylinder that a visual inspection won't catch.... or, something about the way I ride the bike is putting large stresses on the ring.
This winter I will send the cyl out for inspection / plating as part of my off-season rebuild. For now, I am considering bumping up my mix ratio to slow the wear... maybe 20/1.
 

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im sure youve checked the PV isnt protruding into the bore during any part of its normal travel ?

i see no good reason to have ring gap on the tight side. in the middle or looser end will cause no harm or noticable power loss

did you run a straight edge down the bridge and be sure its recessed slightly ?

since your port hasnt been modified i highly doubt its width or shape is causing the problem but take a look to be sure

whats the chamfer on the top edge look like

ive had problems with bridged exhausts but it was nothing that i didnt create myself from modifying the port.
 

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190 /14.7 = almost 13 to 1 corrected sea level, maybe a little too high? It could be blowing the ring out the exhaust?
My Kart buddies ran 150psi on alcohol.
.004" per inch end gap would be .010" for a 2.5" bore, maybe your gap is a little too loose. This may be splitting hairs, but it doesn't take much to cause your problems.
 

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Discussion Starter #7
You bet, I scrutinized the exh valve and the exh port area.
The exh valve has plenty of clearance, the bridge is slightly recessed, and the chamfering looks ok.

I'll expand on my description of my ring gap being "on the tight side". My Honda shop manual states the ring gap to be set at .016 - .020" during installation, with a service limit of .026".
I have always set this engine at .016 which should be plenty, and the abnormal wear is a newer problem this season that must come from another variable in my mind. The installation of this new cylinder seems to match up with the start of the problem. I wouldn't expect to get a flawed cylinder from Honda but who knows.
 

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Discussion Starter #8
190 /14.7 = almost 13 to 1 corrected sea level, maybe a little too high? It could be blowing the ring out the exhaust?
My Kart buddies ran 150psi on alcohol.
.004" per inch end gap would be .010" for a 2.5" bore, maybe your gap is a little too loose. This may be splitting hairs, but it doesn't take much to cause your problems.
Good idea on the 13/1... although I don't have that much faith in my compression tester feeding me accurate numbers, like I said I use it for comparative purposes.

Its funny... using Wiseco's math for end gap, I end up with .010 like you just did. I have always just followed my shop manuals like a good sheep lol.

This engine in the most modern CR250's (02-07) are known to have a loose squish band, and are prone to det in stock form. I dont know if any of this might also play a part?
Wanting to run an internally stock engine, I admit I have never checked squish... fearing that I will find I need to have machining done. I hear the odd decel det but there is never any evidence of it on my piston or dome.
 

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if your using honda piston and rings i would disregard wisecos recomendations for ring gap as the materials may very well be totally different


john im not understanding your compression calculations. ive grinded enough cylinders to know if the exh port is modified you typically see a decrease in kicking compression while the compression ratio remains the same, atleast if were refering to the uncorrected method. i dont see how it can have any accuracy in this scenario. does it only work for predicting a corrected comp ratio ?
 

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(does it only work for predicting a corrected comp ratio ?)
This exercise is the corrected compression ratio, no matter where the exhaust port is. I think that the math in 2 stroke work is what makes it so interesting to me. pi*r^2*H= volume, the math never lies if the equation is correct.

As I have seen in the last few years; all 2 stroke piston rings are made from some kind of plated spring steel. I have no idea of what kind of plating is used since chrome rings used to stick to plated cylinders. Anyway, no mater what brand of piston is used the ring expansion rate is the same. Also over the years, we've always used .003" to .004" per inch of bore diameter for end gap calculations.
Back in the early days; Wiesco pistons used a moly flled cast iron ring, which almost buried that company. Fast forward a generation; Wiesco is now the Best.
 

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if honda and wiseco rings are the same material as you say then why such a large difference in recomended gap ? unless the ring thicknesses are far different. i cant think of any other reason why the gaps would be so different


ok so your comp calculation method is only valid for predicting the corrected method ? what does the 14.7 represent ?

danbots engine is a '07. ive got the '02-'04 manual. it states the corrected comp ratio is 8.5:1 for '02-'03. 8.6:1 for '04, this must be with the exh valve closed im assuming. if i recall the '07 had a cylinder similar in design, atleast as far as we're concearned on this subject at hand. maybe danbot can chime in with what the manual says the corrected ratio is for the '07 model and see if it matches up with the 13:1 that john posted
 

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I can’t answer your question about the different ring end gap from different manufacturers. When I look at piston ring boxes from various piston manufactures, they all appear to be made in the orient.

A good test for all of you home mechanics; every time you replace a set of rings, give the old set a hard tight twist. If the ring snaps it’s probably made from cast iron. If the ring tightly bends while twisting, It could be made from a spring steel or ductile iron. Your further questions could only be addressed by an automotive engineer.

14.7 pounds per square inch is our standard atmospheric pressure at sea level. If the corrected compression ratio is say 10-1 then the cranking compression would be 147psi. The corrected comp ratio is all I ever use since true compression is in valid in a 2 stroke engine with all of the ports.
Now I’m getting things straightened out. True compression is for a 4 stroke & corrected is for a 2 stroke; I hope that helps.

Take a look at pg51 “How it’s Done”; You can see a more in depth explanation on cranking compression. Don’t feel like the Lone Ranger here; It takes all of us some time to figure it all out.

:Cheers:
 

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sooner or later i may understand it

the 8.5:1 is straight out of the honda manual for '04. i assume thats with the valve in the closed position but it doesnt say. if that eqauls 125psi then how is danbot ending up with 180psi on his '07 (unless his gauge is way off ), but 125psi doesnt sound right ? i believe theyre nearly the same engine between '04 and '07 . the numbers arent jiving it doesnt seem like . maybe danbot can chime in and shed some light on this ??
 

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If Honda says 8.5 – 1 I’d love to talk to one of their engineers & have them explain this. Maybe one is reading our discussion.

Is the valve you’re talking about the power valve? Power valves are so sloppy that they won’t affect the cranking compression much, if any.

I did make a mistake which will really mix things up; I’ll check “How it’s Done” tomorrow.
The corrected compression ratio formula which I listed is correct, pi*r^2*H; however there is a 3rd ratio. The volume of the combustion chamber divided into the corrected volume times 14.7 will give the actual cranking compression.
I think Graham Bell’s book says V1/V2 * 14.7.

I’m sorry for all of the confusion, you folks really a person on their toes. :Dope::Dope::Dope::Dope: :Blackeye:
 

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dANbOT, I'm not sure I should ask, since it's so basic you surely have considered it right away, but is it possible you're looking in the wrong place? Seventy or eighty years ago, when rings were all grey iron, you could order ones that were chrome plated, not for any performance reason but because the "air filters" of those days were not very effective, and that not for long. Is your engine ingesting dust? K&N filters, properly cleaned and oiled (with K&N's cleaner and oil) out-perform most or all others, both in initial flow and in capturing dirt and still flowing pretty well. Could there be any leaks around the boot that holds the filter?

As to end-gap, this is a perpetual source of discussion, but I suspect along with KTM550 that as long as the ring-ends never actually contact each other, the gap is a lot less critical than people think. Back in my remote youth, McCullough Corp., which was selling a lot of the early kart racing engines as well as chainsaws, ran an experiment on ring end-gap. McCullough reported seeing very little loss of power with gaps up to a tenth of an inch. I wish I could remember if that was with a one- or two-ring piston. But the point is that if the ring conforms well to the cylinder wall (properly broken in ring face, and good gas-pressure atop and behind the ring), a somewhat large end-gap shouldn't hurt noticeably. But no one wants to believe this (and maybe there have been contrary tests I don't know about)

A couple of other ideas. First, don't ever clean a piston skirt by glass-beading (unless you are prepping it to be moly-coated, and even then, salt-blasting or sode-blasting are safer). Some of the glass gets stuck in the aluminum; IIRC, it was Hastings or maybe Sealed Power (rings-maker) that sent out a service bulliten warning about this. You can actually see the tiny, shiny glints if you look through a good optical comparator.

Nuther idea: when you hone a cylinder, get it hot first. If there is an exhaust bridge, heat that even more with a propane troch right before honing. Who knows if the bridge will bulge into the cylinder rather than some other direction, but in case it does, hot-honing should take care of it. NASCAR shops have been hot-honing for years (and some outboard racers have for many decades, heh, heh). To really do it right, you should fabricate yourself a torque-plate. Also torque down the exhaust stub and anything else which might distort the cylinder. In other words, do whatever you can to get the cylinder block to approach its running condition. Not perfect, but better than honing a cold cylinder with nothing bolted on. With some cylinders this makes less difference than with others. Finally, for a finish-pass, get the special honing stones for "platform honing." You could google this term; basically it takes off some of the tops of the microscopically torn metal surface and pulls out some of the leftover bits of abraisive. I think it was Smokey Yunick, famous auto racing mechanic and experimenter, who sold everybody on platform honing.

Well, one more thing, when you're done hot-honing, platform-hning, yadda yadda, do a final hand clean-up of the cylinder surface. I do this by immersing the barrel in fairly hot TSP in water, and scour the cylinder with an SOS pad. This should get more of the stuck pieces of abraisive from the hone stones, assuming there are any. Pull the finished barrel out of the TSP, blast it with a garden hose, and immediately rub it down with WD-40 to get the rest of the moisture, then oil it. (There will be other techniques for final cleaning, maybe better than this one).

Gotta be some reason for your ring-wear . . .
 

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yes im talking about the power valve. exh valve= power valve

the power valves ive seen generally have a fairly large range of movement up and down depending on rpm. so i would think it would change the corrected comp ratio quit a bit depending if you calculated it with the valve open or closed. 8.5:1 im sure would be with the valve closed but i dont know. the book doesnt specify. i cant imagine any 250cc race engine having only 125psi with the valve closed unless maybe you were on the top of pikes peak. with valve open perhaps theres a chance you could see 125psi even at a fairly low elevation. i guess danbot has gotten cold feet and wont chime in. interested to here what his honda manual says for a corrected comp ratio but i suspect it wil be very similar ,if not the same, as my '04 manual
 

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It goes to show us that opinions & numbers are all over the place; I guess that a person should know & understand as much as they can & then decide for themselves.
 

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its all good john. just had me wondering why none of the numbers seem to line up. this topic doesnt pertain to me anyways so ill just leave it at that

everyone probly has their own views on this stuff. to me cranking compression numbers meen very little. without getting long winded ill just say ive found no usefullness in knowing what my cylinder pressure is , at cranking speeds. seems nearly everyone across the internet uses cranking compression to determine what fuel they need. IMO this is a seriously flawed method for deciding what fuel to use. corrected comp ratio isnt something i practice either. the tuners ive talked to dont use it either. maybe it has some value with these 2t engines but it just isnt a calculation i use. not saying its right or wrong just saying that i dont use it

being from the older generation im sure you grew up with the gordon jenning and graham bell books. ive glanced through them books as well. the problem is, some of what they wrote has proven to be inaccurate. its a shame there doesnt seem to be any very recent 2t books containing todays modern knowledge on these engines

heres a couple links someone might find usefull dealing with comp ratios and cylinder pressures

http://www.2strokeheads.com/tech2.htm

http://www.sacoriver.net/~red/uccr.html
 

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550, Thanks for the info; I just finished the links which you sent, very good scientific technical information. My only concern is a customer who is happy with his bore job.
For myself who only bores & hones cylinders 5 days a week, compression is my #1 consideration. I get this kind of work from all over the country because there are so few others who know what they’re doing. I don’t dare return one of my jobs which doesn’t have a round straight bore with excellent cranking compression. To me, it really matters.
 

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dANbOT, you asked about possible detonation. If that's happening, you should be able to see signs of it on the plug. Grey flecks or spashes ("death-ash") on the end of the ceramic insulator. Cement-boilout between the insulator and the center electrode sometimes. And distress signs on the piston, which is where the death-ash came from, if you had any. I don't see that in the photo. Anyway, I can't see detonation being a source of rapid ring wear. I do like your awareness of squish-height; getting that dim down reasonably tight (shall we have a show of hands, gents? I'll vote for about .038") could be a good move. But again, the ring wear has some other source, surely.
 
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