Is it time to replace my rings / piston?

[Derf]

Just wanted to say thanks for all the input so far.

I played with the pin, pushing it into and out of the piston hole a bunch and strangely it got easier. It now practically slides through. So I'm happy about that. I don't care so much about the markings, I just don't want the pin too tight in there. If I somehow loosened it up removing a surface glaze or burr I'm less worried now. If I'm happy enough with the pin, my next question to myself is am I happy with the ring?

I still can't find my set of feeler gauges. Got some 20lb Xerox paper which has an avg. thickness of about 0.004" (according to Google 0.0035 to 0.0045") and I stuffed 4 layers thick but with 5 it went in but started chewing up the edge of the paper as I slid it back and forth. So rough estimate I'm between 0.016 and 0.020 ring gap. Well, it could be as small as 0.014 to 0.0175 or as large as 0.018 to 0.0245. I know I should really measure it proper, but that's all the info I have at the moment.

Sooo, looks like I am either near my service limit (according to huskihl) or only about half way through the useful life of this ring (according to Agrarian's shop manual). I suppose some of you would continue using this ring and some of you might figure a new ring is cheap enough to replace...

Which brings me to am I happy with the piston? I'm sure there are those out there that don't consider pistons as wear items, only rings, and the only pistons that get replaced are damaged one. But I've read that some people suggest changing rings and pistons at the same time since usually the side skirts are worn enough by the time the ring wears out. Somewhere I read that if you change rings regularly you can do a piston on every other ring change. I'm not sure which is better standard operating procedure, but how thick the wallet feels probably has a lot to do with that decision.

Chainsaw Jim thinks he sees more knurling than smooth area, but I'm sure that is an artifact of the camera; the skirts and the thrust face are almost totally smooth. The question is how bad is that? Should I believe that over 50% smooth is time to replace? Or should I run it until I have low compression and find out that a new ring can't fix it or, worse, until catastrophic failure? I'm sure there are just as many people that would keep running it as there are those that would replace it.

I have the saw apart and yes I could obviously just de-carbon and slap it back together and it will continue to run. It was running before I tore it down. I guess it's just a question of how much preventative maintenance do you do?

Stage 1: no preventative maintenance, run it until a problem happens.

Stage 2: tearing down the P&C was already a great job, de-carboning them is enough until something goes wrong.

Stage 3: measure the ring gap and replace the ring at more than 20 thou.

Stage 4: that ring is good till 40 thou or else when the compression drops so much the saw is losing noticeable power.

Stage 5: the piston looks worn, replace it before something bad happens, you get a fresh ring anyway.

Stage 6: I got the tools and the money, so a new piston/ring and a deglaze of the cylinder with a ball hone.

Personally, I'm on the fence between 2 (if I'm poor), and 5 (if I'm flush).

 

[blsnelling]

I played with the pin, pushing it into and out of the piston hole a bunch and strangely it got easier. It now practically slides through. So I'm happy about that. I don't care so much about the markings, I just don't want the pin too tight in there.

It doesn't really matter how tight it is, as long as you can get it in and out. It's not intended to move in the piston. That's the function of the wrist pin bearing.

 

[Derf]

It doesn't really matter how tight it is, as long as you can get it in and out. It's not intended to move in the piston. That's the function of the wrist pin bearing.

I agree, but now I'm caught up on the life of the ring/piston more than the pin.

 

[Agrarian]

I freely admit I am picky about rebuilding my saws. All my saws get new rings when I rebuild them - if the end gap of the used one is less than 0.020, it goes in the used parts bin (in case my finances change in the future ). They also get their wrist pin cleaned with fine grit emory cloth. If the cleaned pin does not press into the piston with a reasonable hand force, the piston pin bores are also lightly touch up with emory cloth. I install a new OEM wrist pin bearing too. From what I can see in your piston photos, I would not replace it unless the skirt measured too much clearance in the bore or the ring grooves were worn badly.

Speaking of ring grooves, if you choose to re-use, ensure the bottom of the groove has all of the carbon removed. I use the broken end of an old ring to do this.

 

[Derf]

Ok, I found in a Husqvarna workshop manual pertaining to many saws from 36 to 3120xp the spec of no greater than 1mm ring end gap. I'm really a little confused by that since the piston diameters vary from 38mm to 60mm, which means minimum ring gap varies from (0.004" x bore) .006 to .010, but maximum is consistently no more than 0.039. Whatever, I'll take their word for it. The ring gap is less than that so I assume my rings are still good. And I mentioned my wrist pin is no longer tight in the piston, and I have a new wrist pin bearing. That right there should prove 3 of my 4 issues are fine.

Since money is tight right now I'll just decarbon the ring lands and piston and run this piston, and assume that since the ring is still good the piston isn't worn out yet, even though I'd like to replace it since most of the machine marks on the thrust faces are worn off.

Thanks guys for listening and sharing the input. Hope this thread helps someone in the future.

 

[Agrarian]

Help me

the spec of no greater than 1mm ring end gap. I'm really a little confused by that since the piston diameters vary from 38mm to 60mm, which means minimum ring gap varies from (0.004" x bore) .006 to .010, but maximum is consistently no more than 0.039.

I was not aware there was a minimum ring end gap and I don't understand how you arrived at the variation based on piston diameter. Can you explain this?

 

[sunfish]

That thing still has a LOT of life left in it! My 15 year old 346 still has original piston and ring in it. And it's been ported 3 times, had a pop-up cut in the piston and the base cut.

 

[glock37]

Put wrist crank and wrist bearing and piston in mean green and place in ultrasonic cleaner for a half hr all crud will be removed rinse with hot water

Oil all surfaces and big end bearing and its ready for installation


Sent from my iPad using Tapatalk

 

[Derf]

Help me


I was not aware there was a minimum ring end gap and I don't understand how you arrived at the variation based on piston diameter. Can you explain this?

Sure. I've seen several references around the web about minimum end gap. A minimum ring end gap is necessary because the rings expand as they heat up. If there isn't enough gap they can butt up against each other and squeeze out of the piston causing excess friction, heat, thinning the oil layer they ride on, causing more friction, heat and damage. When it isn't specified, the acceptable minimum clearance is 0.004 times the bore diameter in inches. Some people have gotten away with 0.0035 times the bore diameter, while high-heat / race applications suggest 0.005 x bore diameter (in inches). Wiseco pistons also post the same information. Since the bore of the 346xp is 44.3mm you get 1.744" x 0.004 = 0.007" minimum ring end gap. You can make similar calculations for the other pistons.

 

[Agrarian]

Thanks, I learned something!

 

[Derf]

That thing still has a LOT of life left in it! My 15 year old 346 still has original piston and ring in it. And it's been ported 3 times, had a pop-up cut in the piston and the base cut.

Most of the references around the web give service intervals for replacing rings after a certain number of hours. I saw 15 hours for 125cc dirt bikes, 40 hours for 240cc bikes, and 7 hours for 50cc-85cc bikes. I'm not sure how many hours you run your saw per year, but next time you open it to port it (lol) measure the ring end gap and estimate the number of hours you've used it. I don't know how many hours are on my saw as I got it used.

 

[sunfish]

Most of the references around the web give service intervals for replacing rings after a certain number of hours. I saw 15 hours for 125cc dirt bikes, 40 hours for 240cc bikes, and 7 hours for 50cc-85cc bikes. I'm not sure how many hours you run your saw per year, but next time you open it to port it (lol) measure the ring end gap and estimate the number of hours you've used it. I don't know how many hours are on my saw as I got it used.

Chainsaws aren't the same as those old dirt bikes and don't need to be rebuilt as often. My old 346 is still blowing around 180 psi, but that's with some machine work.

 

[Derf]

Chainsaws aren't the same as those old dirt bikes and don't need to be rebuilt as often. My old 346 is still blowing around 180 psi, but that's with some machine work.

Chainsaws run at full throttle (9-10k) or idle. Motorcycles cover a whole range of rpm, but don't often sit at 9k for extended periods. Which one wears out faster I don't know, but a two-stroke engine is a two-stroke engine, be it chainsaw or motorcycle. Maybe nikasil cylinders in chainsaws wear slower than steel or chrome plated cylinders, but rings and cast aluminum Pistons should wear the same. These are just guidelines, and it'd be better to monitor compression, ring end gap, piston skirt thickness or piston cylinder clearance over the life of your engine to decide when it needs freshening up.

 

[drf255]

Whatever you do, whenever you do it, Don't use a ball hone on the cylinder when you redo the saw.

They don't need a heavy cross hatch like an iron cylinder. You will ruin the jug.

I use WD40 with a scotchbrite ball and more scotchbrite around it. Put it on your drill, spray the inside of the jug, and go into it back and forth. Make sure it fits kinda tight. You'll get a nice finish on the plating.

 

[Agrarian]

Whatever you do, whenever you do it, Don't use a ball hone on the cylinder when you redo the saw.

They don't need a heavy cross hatch like an iron cylinder. You will ruin the jug.

I use WD40 with a scotchbrite ball and more scotchbrite around it. Put it on your drill, spray the inside of the jug, and go into it back and forth. Make sure it fits kinda tight. You'll get a nice finish on the plating.

I use the same approach now but I do have ball hones. I gave them up because the scotchbrite approach will get to the very bottom (top?) of the cylinder, its messy with the oil spray from the balls and the ball hones only cover a narrow range of cylinder diameters which gets expensive fast.

There are models of the ball hones made especially for Nikasil plated cylinders. I believe they use aluminum oxide rather than silicon carbide for grit but you are right that the incorrect ball hone will destroy a cylinder.

 

[Agrarian]

I heat with wood and always have. I cut about 8 cords per year to get the 5 cords of hardwood I need. I bought a 262xp new in '94 and it is still going strong with no internal maintenance. One of these days I'll open it up to deep clean it and throw another ring and bearing on. It should be interesting to see what wear has occurred. I always use 40:1 fuel mixture (I have to be careful, I don't want to turn this into an oil thread ) which I feel helps.

 

[Marshy]

I believe "emory cloth" is a generic term and does not describe the actual fineness of the grit nor the type of abrasive. ScotchBrite is also fairly generic in that it does not define which type and there are several as you can see at http://academic.evergreen.edu/projects/biophysics/technotes/fabric/finish.pdf . Since ScotchBrite pads use silicon carbide or aluminim oxide, I don't see the difference between them and emory cloth which likely uses the same material. Am I not understanding this completely?

No doubt there are different grades of abrasive. I generally associate scotchbrite with less abrasive because I have maroon and grey, I don't usually have emery that fine. I also think it would be overkill to use 400 and 600 on it. I'd jump straight to 1000 IMO

 

[Derf]

So there is a minimum end gap for new rings, and a maximum end gap for used rings, but no mention of piston skirt thickness or piston-cylinder clearance spec that I can find anywhere. The only reference I've seen is to the wear pattern on piston skirt, and when it exceeds 50% the piston should be replaced.

I wonder how many thousand (million?) board feet a piston is good for? Do the manufacturers not consider them wear items? Why isn't there any mention of any way to check them?

I think for anyone who stumbles across this thread, if you're in a similar boat,
- clean your piston and cylinder of carbon, re-lube and reinstall and check compression.
-when compression falls enough that you tear into the saw again, measure ring end gap.
- if ring end gap exceeds 0.025", optionally replace ring(s). (0.025" is half way between the min of 0.010 and the max of 0.040). Note that single ring pistons will wear faster than double ring pistons.
- If non-original ring end gap exceeds 0.025" or original ring end gap exceeds 0.040" /1mm replace rings and piston.

 

[Philbert]

- clean your piston and cylinder of carbon . . .

Some threads have mentioned chemical cleaners for removing carbon, instead of just abrasion, but I don't remember what they were.
Any cleaners that folks use, which work well?

Thanks.

Philbert

 

[Derf]

I was using Easy Off oven cleaner and some aluminum safe wheel and tire cleaner I got at Walmart. I have a steel sink and you could just see the brown running off the cylinder and piston without even scrubbing. Direct application of SeaFoam didn't do too much.