Is it time to replace my rings / piston?

[Derf]

Tore down a saw, noticed the wrist pin was pretty tight coming out. When I got it out it was galled with marks.

I've heard that you can turn them down in a lathe with 400, 600 and 1200 grit Emory paper, but I don't have a lathe. Looks like you can't buy a new wrist pin either, likely because they are matched to the piston. So you buy a whole piston assembly for a new wrist pin.

Here's my piston. No scoring, just some carbon build-up. The machine marks are gone from the piston skirt. I don't know how many hours are on this used saw, it looks like the original parts from a 6-year old saw.

Probably the next question is what's the ring gap? Well I don't have my feeler gauges. So how's the compression you ask? My gauge says 148psi on a stock 346xp.

So I ask, does anyone replace a ring with a used piston or do you do rings and Pistons together do to wear on the side skirt? Should I be worried about the tight/discolored wrist pin? Is it time for a new piston kit? Or do people only replace Pistons when they get scored? And if so, how are the meteor kits at $38 compared to OEM Mahle for $56?

 

[Rookie1]

Machine marks are to hold 2cycle oil to keep lube on cylinder wall. You should replace piston in my opinion.

 

[huskihl]

Some wrist pins come out easy. Some need to be pressed. They have a mind of their own. Slide a bare ring up above the exhaust port. End gap new should be about .007 to .012". .020 is probably the upper edge, but will still run. Notebook paper is around .004". If it's a lot looser than 2 pcs of paper I'd get some feeler gauges and be sure. Duke thieroff on here has pretty good prices on caber rings.
I'd clean up and reuse your piston if it were mine.

 

[Rx7man]

I'm running saws with 30-40 thou ring gap.. far from ideal but they actually run pretty dang good still... I had one with .080" end gap, it was so close to just snapping, I don't know how that saw ran long enough to wear it, it was VERY tired. Huskstihl is right though, I like them to have about .012" max..

 

[Chainsaw Jim]

Oem Pistons are on sale right now at Bailey's for $77.79

http://www.baileysonline.com/Chains...5-346XP-Jonsered-2145-Chainsaws-503907371.axd

 

[drf255]

If you already have it apart, you may as well do the piston.

I'd soak that wrist pin in paint thinner overnight and then clean it with steel wool.

I'll clean that pin for ya for free on my lathe, just send it down to Long Island. But, I'd prolly just chuck it up in a drill and use a band of emery paper on it if that's what you want.

 

[Marshy]

I'd use some scotchbrite and some brakleen to get the varnish off that pin. I wouldn't use emery cloth in fear of taking off material. I've had some tight pins an pistons. So tight that I put the pin on ice and the piston on the stove top (hot potato) to get them clearance to just slide together. Just my preference but I'd replace piston and ring together in this case.

 

[Derf]

Thanks guys, I've been reading a lot of forums and web pages, the jury seems split. Shop manuals for race bikes suggest rebuilds for any class under 85cc at 5hrs! Other people say to run it until there is a problem. Some people suggest measuring the piston skirt if you know the original dimensions, others suggest measuring skirt-cylinder clearance but caution that may indicate an out-of-round cylinder.

I found this reference
https://books.google.com/books?id=-...HbpvBc4#v=onepage&q=piston skirt wear&f=false

Being a published book I give it an ounce more cred than posts on forums, and it states normal wear causes a wear pattern that covers 20-40% of the skirt thrust face. When wear on the piston skirts exceeds 50% it is excessive. I'm at about 85% on the intake and 60% on the exhaust. So I'm going to replace the piston, rings and pin. Now I'm torn since meteor gets good reviews and is $37

http://www.baileysonline.com/Chains...w-Edition-Chainsaws-Replaces-525-47-01-02.axd

And Mahle is on sale for $55
http://www.baileysonline.com/Chains...for-346-XP-New-Edition-Chainsaw-525470102.axd

 

[Agrarian]

I'm running saws with 30-40 thou ring gap.. far from ideal but they actually run pretty dang good still... I had one with .080" end gap, it was so close to just snapping, I don't know how that saw ran long enough to wear it, it was VERY tired. Huskstihl is right though, I like them to have about .012" max..

The Husky service manual states the upper limit for ring gap on my saws is 1mm (0.040").

 

[Derf]

The Husky service manual states the upper limit for ring gap on my saws is 1mm (0.040").

Which model saw? That seems excessive to me. I wonder what compression is when you have a 1mm hole letting air past the piston? The decomp hole is about that big. Imagine running the saw without the valve in place.

 

[Agrarian]

I'd use some scotchbrite and some brakleen to get the varnish off that pin. I wouldn't use emery cloth in fear of taking off material. I've had some tight pins an pistons. So tight that I put the pin on ice and the piston on the stove top (hot potato) to get them clearance to just slide together. Just my preference but I'd replace piston and ring together in this case.

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?

 

[Agrarian]

Which model saw? That seems excessive to me. I wonder what compression is when you have a 1mm hole letting air past the piston? The decomp hole is about that big. Imagine running the saw without the valve in place.

That is from the Husky service manual which specifies 0.040 for all saws from the 242xp to the 3120xp. I feel that is excessive too but for me it is irrelevant since I always replace with a new Caber ring when restoring my saws.

The comparison with the decomp hole is apples and oranges. The cross-sectional area of a circle is much greater than the area of a rectangular slot defined by the piston clearance on one side and the ring gap. The area of a 0.040 circular hole is 0.00125 si. The area of the 0.040" ring gap (assuming 0.002 piston gap) is 0.00008 si so the decomp hole would be over 15 times greater.

 

[Duane(Pa)]

That is from the Husky service manual which specifies 0.040 for all saws from the 242xp to the 3120xp. I feel that is excessive too but for me it is irrelevant since I always replace with a new Caber ring when restoring my saws.

The comparison with the decomp hole is apples and oranges. The cross-sectional area of a circle is much greater than the area of a rectangular slot defined by the piston clearance on one side and the ring gap. The area of a 0.040 circular hole is 0.00125 si. The area of the 0.040" ring gap (assuming 0.002 piston gap) is 0.00008 si so the decomp hole would be over 15 times greater.

I LIKE math! Especially when someone else does it

 

[Rx7man]

I don't care much for the compression numbers.. they can be all over the map when you change the squish... what I go by is the feel when you put a steady pull on the starter cord, that will tell you how much it's leaking down.. if it rolls over and stops you dead, the rings are good, if it's easy to get it to roll up to TDC with a couple pounds of pull on the rope, they're done.

 

[Rx7man]

Oh, and for the wrist pin, scotch brite does work well.. it's darned hard material, and unless you really rub on it after you take the varnish off, you aren't going to be removing any metal

 

[huskihl]

Which model saw? That seems excessive to me. I wonder what compression is when you have a 1mm hole letting air past the piston? The decomp hole is about that big. Imagine running the saw without the valve in place.

About 120. Still runs. But .007 might give you 160. It's just horsepower. They make it every day lol.

 

[blsnelling]

The wrist pin was snug coming out due to a small burr on the bore from removing the wrist pin. No way would I take sandpaper to it. That's a polished bearing race!

 

[Chainsaw Jim]

I just looked at your piston closer and I see more knurlings than smooth area. Throw it back together and run the piss out of it. And that pin doesn't need to sparkle silver in order to work.
It was a good idea to pull the jug to decarbon everything like you are doing, it will increase the saws life. Don't forget to get the carbon off the inside of the ring too.

 

[Philbert]

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 . . .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?

'Emery cloth' describes the type of abrasive bonded to the backing, although, it might be used in a generic way to describe any cloth-backed abrasive product intended for metalworking. But lots of different minerals and grit sizes are available.
https://en.wikipedia.org/wiki/Emery_cloth.

'ScotchBrite' is a 3M brand name for non-woven pads with abrasive particles bonded to them. The abrasive particles could be aluminum oxide, silicon carbide, ceramic, etc., with many different grades of each, and in a range of grit sizes / coarseness. For example, 3M makes fine grade ScotchBrite with very fine abrasive, sold as make up remover on skin ('Buf Puf'), and extra coarse grades used for floor buffing machines. Auto parts stores and metal finishing vendor will sell it in different grades, just like different grades of sandpaper.

Most of use are familiar with the ScotchBrite dishwashing pads, which work on a lot of other things, but you need to be more careful and specific when discussing the industrial grades.

Philbert

 

[huskihl]

The wrist pin was snug coming out due to a small burr on the bore from removing the wrist pin. No way would I take sandpaper to it. That's a polished bearing race!

Lots of them are difficult to remove even without any burrs on them. You get a saw that's been run ragged for 1000 hours and those pins don't always slide right out. About half of the ones I've done have to be pressed out