Piston rings....

[mrcaptainbob]

Need to order a set of rings. Other than diameter and thickness...what about the shape?
This is for the elderly Olympik that had issues. Would it matter if there was/wasn't anything more than a square cross section?

[Tacotodd]

I don't know if you're going to have any luck with the "size only" type of deal. How do you account for wear? Most ring suppliers that I've seen in the past have not a clue as to how to do this by size. How do you account for: collapsed, wear, proper ID, max expansion (wore out), etc, etc? But, this is a GREAT place & question to ask. ME, no thought or ideas. But I'd like to know!

[ehp]

you need to know what surface in the bore the ring is to run on, if you got a chrome bore which alot of the older saws had and you put the wrong ring in it it will tear the chrome right off the cylinder wall

[mrcaptainbob]

Ehp, I have no idea what the coating on the cylinder bore is. Since this is a second hand (at least) saw, and pretty old, Info is close to none existent.
So, here's what happened: The saw, an Olympik 272i, runs a 32" bar. It runs VERY well. One day, doing some minor cutting, it instantly stopped with a clank - clank. Disassembly showed a chip in the piston top just above the compression ring. Part of tht chip was shiny, evidence of shear, and part of that chip was dull gray, like it was peeled away. The chip is not as deep as the ring groove. And very, very small maybe a bit more than 1/32". It was on a scavenge port side. And that scavenge port had a small chip from the bottom edge. Both of those chips were in the case bottom preventing a complete rotation. I suspect the crank/rod came around and hit it and bounced back for the second 'clank'. No idea why any of it chipped.
So, since parts are impossible to find, and due to some new found information on combustion gas flow, I decided to just replace the rings and reassemble.
The new found info regarding combustion flow is that the flow fans across the piston top, and finds the piston/cylinder gap, flows down that to the compression ring top. It then proceeds to the inner side of that ring and forces it further to the cylinder wall. Go kart racing and others often (as I just found out!!) drill holes in the piston top along the edge to facilitate that!! I am amazed! So...since there's this little ol' 'notch' that's hardly there, on top of that ring, and no other 'damage', expect for minor scratching (not scored!) on the cylinder in that area and on the piston in that area, I'm thinking new rings, a bit of minor cleanup on stuff and I'm hoping it's back to work for my helper.
I do have a question...what can I do to replace the jug gasket?
What is a typical jug bolt torque?
Thanks for the help from so very many people!

[Al_Smith]

A cylinder base gasket is easy to make .Try You Tube for a tutorial . As a suggestion measure the rings ,bore and thickness then refer to Caber that makes rings with a good reputation . Torque for metric bolts can be Googled . Some are M5 but most are M6 .
BTW in spite of what some might say you don't need to goober up the surfaces of the cylinder base with sealant .If everything is nice and clean it will seal .

[sawguy21]

You are right. So many use sealant to cover up sloppy assembly which includes lack of cleanliness. I used to hate it when an engine or transmission came in with silicone or form a gasket slobbered all over it. Sorry sir, no estimate. This is on straight time.

[ehp]

what color is the connecting rod where it joins the cranks , it should be a gold color . I'm just asking cause that piece came from somewhere and the connecting big end rod bearing will do just what your talking about when they break a piece off one of the bearing pins so check that out very care full, most times you can slide the conrod over a touch and you can look at the conrod bearing and look very close to make sure all the pins are perfect and the cage the pins sit in it together and not cracked . Its either that or something went threw the carb or whatever cause it came up threw the transfer port into the piston

[Al_Smith]

Speaking of which .My neighbor had some 50cc  ish  Makita model his buddy tore the bar stud loose, ruined the case .Must have used a 3/4" ratchet  with a cheater bar or something .I told him to get hold of Scott at Chainsawr who sent him a Dolmar case with the crankshaft .I told him he should check the crank and put a known good one in .He didn't and it tossed part of a  bearing .Got lucky and only dinged the piston .That's when he brought it over to me .A little work with a file ,sand paper and new rings and  seals  it runs yet another day .Pretty peppy little saw actually .