dual ring pistons vs single ring piston

[jokers]

OK bw, now is probably a good time for me to admit that there is the ever so remote possibilty that you are more expert on fuel systems than I am, LOL. But that wasn`t the original question anyway. So one mans sloppy tolerances could be another man`s necessity, couldn`t they, especially when we are talking about components that are expected to expand in normal operation? I see where you are coming up with your discussion of hydrocarbon linking, that was you wasn`t it? Don`t mean to attribute anything to the wrong person. I still hold my belief that Marty actually saw a repeatable reaction which would thus rule out instrument error for the sake of this discussion, and I would guess that you would concede that when you consider the variety of additives in gasoline, the chemistry is changed, and there may be more or less realized btu content in a given blend as a result, and obviously these additives are meant to have an effect on combustion. So anyway you see where I`m going and why I think Marty`s points are worthy of consideration and I`m sorry that I am sometimes( it is only sometimes, isn`t it?) an obnoxius jerk. Russ

 

[bwalker]

But that wasn`t the original question anyway. So one mans sloppy tolerances could be another man`s necessity, couldn`t they, especially when we are talking about components that are

The thing that worries me about the reason Dagger presented for the increased tolerances was due to variations in mfg. tolerances. This is not exceptable in my book period. The QC in the plants that make the pistons with that much variance should be strung up. On the other hand if they are keeping tolerances on the loose side to protect against seizure due to clogged up epa mufflers and lean carb settings than it is understandable. Dagger did not present it that way however.

 

[stihltech]

2 rings

Is it possible that all it amounts to is a durability issue. No self respecting manufacturer will put in 2 rings when one will do the job just. Not quoting anything scientific, just what i see come across my repair bench.

 

[Dennis]

Tolerances and QC

I personally dont believe that these saws are that well checked over...yes i know every saw that leaves the Husqvarna factory had been started and run...been there...but riddle me this..why can there be a degree difference in exhaust ports on the same model saw...ken and I ran into this with a pair of 346's....I suppose that is a bit different than a QC check but still...makes a guy wonder...same as all the screws on these saws...I have gotten loose ones from the factory...all these people on the assy line have air-regulated drills for a certain torque spec....but..we all know...s**t happens...

I remember when I was just a kid in the shop, my dad was selling Homelites and one actually showed up from the factory without a carb...lol..QC wasnt so good then...

Back to the issue at hand..2 rings vs 1....anything I can find with one ring I keep and try to save for race engines...Ken proved to me that the least amount of resistance works for racing...I have 5 2171/372 single ring pistons...and that is what I put in my racers....they just cut faster...my Super 930 has dual rings...but are thin...and there is next to no compression...the saw falls to the ground when you hold the starter handle...but does it ever cut....i guess once I get my dyno built...we can test these ideas alot further...not that it hasnt already been done...just need to see it for myself....ok..I am going to quit rambling on now and go back to work...

 

[bwalker]

When you think about what Dagger says it makes sense. As the piston wears and the tolerances open up A two ring piston will stabalise the piston in the bore better than a one ringer. This allows the piston to last longer because it is not slapping around the bore as much as a single ring would. So ultimatly a two ringer will last longer. Dagger am I close to correct?

 

[lostone]

Re: vaccumm "pulse"

Originally posted by don
how does a chainsaw create a vaccumm in the crank?

Don here is a link that made it easier for me to understand how it works.
2 cycle engine

 

[Dagger]

Well Bwalker, yes the scenario you describe would be correct. But it nevertheless has little to do with a design teams initial decision on weather or not to equip a new design with 1 or 2 rings.

And Motorhead, thanks for the kind words. As far as the 250 honda and weisco and such. Yes you too are correct that a larger bore usually has a need for a larger skirt/wall clearance. But it is because of the greater expansion of a bigger piston. And weisco is one of the best at creating pistons of lower mass so the expansion is smaller, and clearance can be less, so rpms can be higher. several advantages to their stuff. But alos keep in mind, that they can produce pistons that will be "hand fit" to each bore that they will be installed in. and a manufacturer cannot.

Bwalker, you called it a QC issue, well kind of, but 15 to 20 years ago it was very hard for us to get in Jugs from italy, and pistons from mahle, and hold a less than .oo5 tolerance. In sweden we decided to make our own pistons to each run of jugs we recieved from Italy for the then 2101xp saws to hold a closer tolerance and get better life at the Demanded rpm level that the market required. Consider this the old way, you might say. As manufacturing capability became better from vendors, this need was precluded. Im pretty sure other companies were trying the same methods.
Aslo you know that as a closer tolerance is required from a vendor, the price usually goes up, if they have the capability at all.
But as time and volume go up, these prices decrease. today you have a very large range of choices based on the need for tolerances from vendors, with the lowest quality and largest tolerence, but lower prices coming from guess where???...yup..the good ole USA
Ever wonder why the cheap saws in anyones lines are made in america?...and the good ones come from abroad?

Now, also I must address the engineered life of a design, it is ALWAYS one of the 1st criteria when a design team sets to work on a new product. this will give us parameters to hold as to the quality and tolerance levels or each component designed.
There are many Saws in the market today, and the Lifetime duty goes from an low of 30 hours, to a high of 2500 hours. These are industry standards men. and they all equate to predicted use by the owner to the warranty period issued by the manufacturer.
to say it another way..A consumer saw with a 2 year warranty directed at a market where the users operate the saw an average of 15 hours a year!...
that saw runs about 20 to 30 minutes on a tank of gas, and the owner will not run more than 30 to 40 tANKS FULL A YEAR.
This saw has 2 rings and a .006 to .008 new skirt clearance, and the 2 rings afre there to keep the short skirted , hogh rpm devil from pistonslappin itself to death for about 30 hours,,,then...we dont care!

so in summary, the original question had to do with determining the quality of a design by how many rings it has. I only hope I have been able to explain that that will not be an accurate measure. And that if anything, a single ring will usually represent a higher quality design made to a closer tolerance......Usually!