Replace piston rings, piston, or cylinder?

[Spruce_Goose]

My question is a general one concerning top end replacements and compression. If a saw has poor compression, could simply replacing the rings give it a boost, or is it usually in need of more work? When do you know if you just need a new piston and when will you need a new cylinder? My understanding (perhaps incorrect) is that the piston rings are what makes the tight tolerance seal with the cylinder, and so why does the piston itself ever become scored and need replacing?

I am new (as is probably obvious) to engine work and have only a basic understanding of chainsaw internal workings. Does anyone have a good resource (preferably on-line, but a book would be fine too) to gain understanding of chainsaw internal components. (I don't mean a cartoon animation of two-stroke engines... more detail)

I've been perusing various posts on this forum and can pick up stuff here and there, but some of it goes over my head and I would like to get a good base reference. I run a 357xp for timber work and have recently picked up a 394xp for milling and bigg stuff. They will more than likely need work and I don't want to just go tinkering and blow something expensive ya know 

[Ianab]

It's possible for the rings to wear, and loose tension, with the piston and bore showing little sign of wear. In that case putting in new rings will probably revitalise the engine. Symptoms of that would be low compression, lack of power, but the saw still running and the piston / bore being on good condition.

But more common, or at least total failures, usually involve damage to at least the piston, and often the bore An air leak or carb fault makes the saw run lean, it overheats, and the piston starts to physically melt. At that point it's toast, and maybe the cylinder too.

Another failure is foreign matter, dust etc getting past the air cleaner, and that's going to physically wear (score) the piston, again probably beyond what new rings can mask.

But I guess the real answer if what the internals look like once you have the engine apart. Then you can see what physical damage there is. If the piston and bore are good, then rings is probably all it needs. More damage or wear? Replace what you need to.

Ian

[Spruce_Goose]

Ok that makes sense. Still one thing i'm fuzzy on there: its not just the piston rings that makes contact with the bore walls then? I always pictures that the rings protrude from the piston and are what make the seal. Is this not quite true? If it is true, then why would the condition of the piston itself even matter?
thanks
tk

p.s. how does one add images to a post. It says not to do it as an attachment...?

[Ianab]

Well technically there should be no actual metal / metal contact. Things run on a microscopic film of oil, so ideally there is very little wear. But back in the real world you get bits of dust, flakes of carbon etc that are bigger than the oil film. So the engine gradually wears out over time. The rings are the "pressure point", so they do tend to rear first. The metal of the rings can also loose their "spring" over time, so they don't press on the cylinder walls and seal properly.

So the are situations where only the rings need replacing. Other times, the engines "cooked" or eaten a handful of grit, and there is more damage

p.s. how does one add images to a post. It says not to do it as an attachment...?

You need to set up a gallery and use that that for pictures. It makes certain that your pictures stay with the post, where off-site photos tend to "go away" over time

Have a read here for instructions
https://forestryforum.com/board/index.php/topic,61788.0.html

Ian

[ZeroJunk]

You can usually save the cylinder if the saw has been run lean and melted the piston to the point it seized the rings. The cylinder plating is much harder than the aluminum piston. Some 200 to 400 grit sand paper will take it off with the help of some muratic acid. The acid works better if you warm the cylinder up in the oven. No more than quality Meteor pistons cost and as much trouble as it is to tear the saw down I don't often just replace the rings. If it has worn the rings out chances are the piston skirts are worn as well and you can get some slap in there that you don't want. You can usually tell by what is left of the original tooling marks. If it is worn slick it might be a good idea to go ahead and replace the piston.

[markkelly]

There are some good videos on  YouTube about rebuilding chainsaws . I did it for the first time,  I would recommend finding someone who has small engine experience to help.

Good luck and have fun.  I learned a lot and would recommend it.

[Al_Smith]

FWIW I 've salvaged several saws that were scuffed from dust by just replacing the rings .On a well used cylinder it takes a little longer to seat  in the rings but they evenually get there .

[H 2 H]

I've been using one of these the last four chain saws I rebuilt

Honing the cylinder



No problems so far

[Al_Smith]

There' a lot of methods that work to remove tramp aluminum from a cylinder .One is easy off oven cleaner .

It's smelly though,best done in a well ventilated area .

Of course 3M Scotch brite with kerosine works as well as a hone .Use extreme caution with abrasives because it's easy to cut though the plating then the cylinder is junk .

[Spruce_Goose]

Thanks for all the suggestions. I learn a lot of new stuff every time I come on this forum. I have thought about taking a small engines class someday. I currently don't see myself being around anyone to work on the engines with, so I figured a little self teaching research would be sufficient to some extent.

Probably not the typical route for learning chainsaw re-building, but has anyone else tapped a book or comprehensive online resource for help? I want to be able to understand the principles before trying to tweak the saw in ways that could leave it inoperable...

[Crow99]

Tyler, there is a lot of info available but I don't think you'll find any single source for what you want to know.  If you want to learn from the net, it just takes a lot of reading over time and you will run into some links with very helpful video's.
There are a couple of other sites that also have chainsaw discussions but in any case a lot of the info is presented in a scattershot fashion. Without naming or criticizing other sites, I do think that you will find this one here to be the most courteous and helpful, as well as the one most likely to remain on topic with the least "drama" involved.  And thanks to Jeff for keeping it that way. Many posters here also post on other sites and there is a large knowledge base represented.  And remember that Google is your friend.
I might suggest if you haven't already done so to see about getting ahold of some basic tools more or less specific to saws.  Check the sponsors here to get started. And keep asking questions.  I'm certainly no guru by any means, but by doing as I've described over time I've really increased my comfort level and have been able to successfully tackle things that I would not have in the past.

[ZeroJunk]

See if you can run across a  broken saw cheap or free and take it apart. Most everything post reed valves works about the same. There really isn't much to them and it is pretty intuitive. Jump in.

[Spruce_Goose]

"there is a lot of info available but I don't think you'll find any single source for what you want to know."

That's kinda what I figured but I was hoping for a secret cache of sorts I guess 

I might just take that advice ZeroJunk and find and old saw to play with.
Someone on craigslist has some old saws, they thought maybe a homelite zip would be a good tinkerer...

[ZeroJunk]

That's pretty much an antique. PM me your shipping address and I'll mail you a Stihl 019T that needs a piston. If you can disassemble that and fix it you can fix any of them.

[Al_Smith]

FWIW very few people on very few sites will poo poo you for a valid question.Now some might be a little rowdy but it's  usually in good fun with people they know .

Only two sites that I can remember got down right mean .The first was because they let it get out of hand ,it's no longer running . The second theoretically is the largest on the internet and I feel they encourage such behavior .

Just keep in mind every web forum has a specific crowd and some you night not like .Just stay off it .Not everybody belongs every place .