1SlowVW
1SlowVW Dork
4/12/23 2:07 p.m.

I picked up a little husqvarna 51 chainsaw last week and while It did start with a little gas down the plug hole the compression was only 125psi so I figured I better do a preventative top end job. 

Long story short upon disassemble I found the cylinder and piston in great shape and a ring gap approximately the size of a og mini cooper. 

Received my replacement ring today in the mail and lo and behold its a ring for a husky 55 instead of a 51. Its for a 1mm larger diameter cylinder than I have. 

 

Being a cast iron ring and being fairly close is there any reason I shouldn't just take a file to the end of the ring (was going to gap it anyone) and save myself the aggravation of returning this ring? 

Here's a pic of the old ring for fun.

maschinenbau
maschinenbau UberDork
4/12/23 2:40 p.m.

Piston rings are made into a very specific shape that is neither circular nor oval so that it applies nearly constant face pressure around the circumference of the bore. If you cut a larger ring shorter and try to fit into a smaller bore, you will have high and low pressure spots around the bore. There will also be more static stress in the ring, which will make it more susceptible to breaking. 

That said, it's an old Husky chainsaw and will probably run no matter what. 

1SlowVW
1SlowVW Dork
4/12/23 2:51 p.m.

In reply to maschinenbau :

I hear what you're saying about ring pressure. Would cast iron (being a softer ring ) be less susceptible to this? 

Kind of leaning toward filing it and seeing how the tension is just going by feel. I know that's not a very quantifiable measure. But if it feels good I'm leaning towards trying it, and if not I'm out 20$

Like you said, this is a saw that will see 10-15 hours use this summer then sit around in my garage for occasional home use. 

I actually ordered a complete 55cc top end kit (saw is originally 51cc but it's a bolt on swap) for it but the quality of the piston had me a little apprehensive. 
 

APEowner
APEowner UltraDork
4/12/23 3:58 p.m.

That's one of those things that's so wrong I've never tried it.  Personally, I'd rather fix it correctly now rather than risk having to take the time to do it again but if you've got the time to try it and don't need the saw right away I'd love to know how it works out.

Toyman!
Toyman! MegaDork
4/12/23 5:22 p.m.

Keep in mind those rings should be pinned. Take that into account when you are filing. 

 

1SlowVW
1SlowVW Dork
4/12/23 5:30 p.m.

In reply to Toyman! :

Yes, I'll need to grind both sides then clearance for the pin. The pin in offset and only partially obstructs the ringland. So you only need to cut half the width of the ring on the top to allow space for the pin. 
I plan on breaking out my trusty ring file tonight and seeing where that takes me. 
I'll update this as things progress towards victory or failure. 

1SlowVW
1SlowVW Dork
4/12/23 8:12 p.m.

well that settles that 

WillG80
WillG80 Reader
4/12/23 8:22 p.m.

Is that still in the plastic?!

1SlowVW
1SlowVW Dork
4/13/23 7:35 a.m.

In reply to WillG80 :

No, I just put it there to take a better picture.

I think you could probably easily get away with a ten or fifteen thousands over but 1mm is close to 40 thousands and that was just too much to ask. Then I got impatient and results were as expected from there. 
 

maschinenbau
maschinenbau UberDork
4/13/23 11:09 a.m.

I was about to say, a steel ring might handle the flex but a cast one will be more brittle and likely snap if you bend too far...I guess you found out first hand.

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
4/13/23 1:29 p.m.

It is not uncommon to use rings for a slightly different bore if you are trying to juggle ring gaps or ring tension.

 

This is for "normal" rings, though, not rings with a built in weak spot for a locating dowel.

1SlowVW
1SlowVW Dork
4/13/23 2:15 p.m.

In reply to maschinenbau :

Sure did. 

Live and learn I guess. 

 

Pete

On my outboards that I race you can't get a factory ring and the ones that are available are all over sized. Everyone just files to fit but there's a lot less filing required. 

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