Piston weight and 2nd ring removal

[Brewmaster]

These are tough topics with strong opinions.
I am trying to get solid research on these two topics. (Not opinions)

• Unless it is --- "I tried that and the saw blew apart and took my eye out".
• Then of course opinions that "left marks" are appreciated.

Regarding "piston weight" --- The standard response from folks with great experience on saws is: "Removing any piston weight will increase vibration and throw the crankcase out of balance."

• I have been processing this and it seems to me that --- It is "One" Cylinder (not 2 or more) there is nothing to balance - the Crank keeps its balance with itself; a lighter piston simply reduces stress on the crank and bearing.

Regarding 2nd ring removal (which also reduces piston weight btw, race saw builders do this.) --- The standard response from folks with great experience on saws is: "It will kill the life of the saw and you might get piston slap via rocking; you will also lose too much compression".

• This one is a bit tougher; there is compression loss; but also a huge reduction in parasitic drag. (Meaning performance in the wood increases beyond the negligible compression loss.)
• That tiny compression leak won't offset the increase compression of a base gasket delete. (My thoughts after some research)
• The saw "will" have a shorter life; but such a small percentage that it is negligible. However it is the top ring that wears most; so replacing it as needed may keep its performance higher over its life cycle. (I will have a spare labeled for that 3-10 years later, depending on use.) I would prefer better performance in the wood sooner and don't care that in 10-20 years the saw needs a total rebuild.
• There seems to be a bit of divergence among the reputable builders here. (and disreputable)

Yes; the saw designers are smart;
Yes it is a saw, use it;
Yes folks can get all emotional about these things and do playground "armchair" bullying ...
It is a chainsaw; it is either logical proven or not; hence isn't personal but data points.

Logic is fine "IF" you don't mind sharing; I am trying to learn a few things before I go work on my saws.
Pro builders prove their craft.
I am not planning on being a pro builder; but do know what it takes to be an expert in numbers of other fields.
But successful folks ask questions and learn and develop skill from it.

Data, videos, fact sheets, charts please.

 

[Lightning Performance]

You're probably not going to get any responses because one you didn't specify is this all going to be a 3-second cookie cutter or does it actually need to run all day for months on end.

And the second thing is what kind of fuel do you plan to run like there's 9000 different kinds available and I'm just curious if you're interested in pushing out parts or not.

Did I miss anything?

 

[Brewmaster]

You're probably not going to get any responses because one you didn't specify is this all going to be a 3-second cookie cutter or does it actually need to run all day for months on end.

And the second thing is what kind of fuel do you plan to run like there's 9000 different kinds available and I'm just curious if you're interested in pushing out parts or not.

Did I miss anything?

Thank you for your response. Good points. I want long term reliability and the best performance I can get for the time and money spent. Normal fuel (Non ethanol high octane) ...

"IF" something is going to cost me $1000 and 50+ hours of my time to save $1.00 in fuel per day and 30 min per 8 hours of cutting; THEN I am not interested.

"IF" something is going to cost me $100 and 2 hours of my time to save 10% fuel per day and 1 hour of cut time; Then I will study to see worth.

I have to get all the saws out and work on them soon anyway, so I am looking to put some time into several of them. Some to sell, some to use for performance utility.

 

[sean donato]

I could go either way on lightening a piston. Yes it will throw the balance off. The crank has a counterweight to help reduce primary vibrations, but it accounts for the rod weight as well. Pretty easy to take too much from a little piston and have it explode on you. Did that a few times.
You're not going to gain much helpful removing the second ring. They aren't very high tension to begin with, so firction loss isn't great, the dynamic compression loss would be more of a loss then anything gained from removing the ring.

 

[Brewmaster]

I could go either way on lightening a piston. Yes it will throw the balance off. The crank has a counterweight to help reduce primary vibrations, but it accounts for the rod weight as well. Pretty easy to take too much from a little piston and have it explode on you. Did that a few times.
You're not going to gain much helpful removing the second ring. They aren't very high tension to begin with, so firction loss isn't great, the dynamic compression loss would be more of a loss then anything gained from removing the ring.

Thank you! Answers ... finally!!! ...

1. Have you noticed greater vibration?
2. Did the piston blow from the greater vibration or weakened areas?
3. I am trying to understand just why balance is thrown off by the 2-8 grams of piston weight change ... meaning I don't "get it" ... isn't the crank balanced to "itself" and the weight of the rod and piston of lesser concern?

Thank you for bringing light and experience into these questions and dialog.

 

[sean donato]

You need to account for the mass of the rod and piston moving up and down. Hence the large counterweight on the crankshaft. If you didn't have that it would vibrate very badly. Primary vibrations is what it's trying to take account for, unfortunately secondary vibrations are pretty much impossible to deal with in a single cylinder engine without a balance shaft. Adds weight and complexity thats not needed in a small 2 stroke.

No I haven't noticed greater vibrations, although it would be improbable I'd notice anyway. The only pistons I worry about making lighter arnt for a saw. Don't notice things like that in rc engines.
The ones that flew apart, I'd say was from being greedy taking too much material vs vibrations. Ultimately just making the piston lighter just isn't worth the effort put into it imo. You won't magically gain hundreds of rpm just from looking a few grams. Will throw balance off a little and run the risk of blowing a piston, cylinder and possibly crank/rod.

 

[guym3]

As an inexperienced builder I have a few thoughts to share....

You talk about piston weight which is important. But imho not for the change in weight which is negligible but for what you achieve with that weight reduction ie which bits you remove.
The windows in pistons are often tiny (and different sizes in different non-OEM replacements). They maybe rough cast as well. I find a few minutes enlarging the windows and smoothing them off causes no problem with vibration and has got to help mixture flow. To be fair though I've never lightened a piston and reassembled the saw without doing some flow work on the transfer ports, deleting base gasket and numerous other tweeks, so not a controlled experiment.

 

[sean donato]

I always clean up windows if needed, but not all cylinders have windowed pistons or need them for that matter. It's a negligible amount of material removed vs trying to make the piston as light as possible.

 

[NSEric]

My 660 clone saw has the windows opened up in the piston and only the top ring. It's a work saw for me but doesnt get a lot of run time as it's too big for my wood and smaller saws are better most of the time.
Removing the second ring made it a bit snappier, it revs a little quicker that's all. The downsides to having only one ring will take years to show up with my use.
I dont worry about changing piston weight much at all, just dont weaken it too much.
A lot of saws have the exact same crank/balance with multiple different sizes of top ends and piston weights. No saw crank is balanced all that good honestly.

 

[Brewmaster]

My 660 clone saw has the windows opened up in the piston and only the top ring. It's a work saw for me but doesnt get a lot of run time as it's too big for my wood and smaller saws are better most of the time.
Removing the second ring made it a bit snappier, it revs a little quicker that's all. The downsides to having only one ring will take years to show up with my use.
I dont worry about changing piston weight much at all, just dont weaken it too much.
A lot of saws have the exact same crank/balance with multiple different sizes of top ends and piston weights. No saw crank is balanced all that good honestly.

Very helpful! Thank you. The light bulb "went on" regarding many different sises of top ends. Man I should have reasoned that out on my own. Thanks for the proper logic and insight. Clarifies things a bunch.

 

[Lightning Performance]

Start thinking about port transfer and not piston weight when you remove a second compression ring.

What supports the first ring lower land area?

Do rings transfer out heat?

Should I be asking questions like where to loose weight in a rotating assembly not the reciprocating one

Will my piston crack if I break the surface tension in the casting?

What is three grams worth from the right places?

How long do you have to live?

And so forth

 

[Brewmaster]

Start thinking about port transfer and not piston weight when you remove a second compression ring.

What supports the first ring lower land area?

Do rings transfer out heat?

Should I be asking questions like where to loose weight in a rotating assembly not the reciprocating one

Will my piston crack if I break the surface tension in the casting?

What is three grams worth from the right places?

How long do you have to live?

And so forth

"Will my piston crack if I break the surface tension in the casting?"
Excellent! Can a piston be heat "hardened" ... and will that further improve not only surface tension but also the overall piston strength?

I ask questions ... a lot of them ... all the time about a lot of things. I have become an expert in numbers of areas because of it. Then I go to taking those ideas, and trial run them.

Numbers of saws have different top ends or pistons or different weight pistons because of size or big bore etc. So changing the reciprocating weight is not really an issue except for less stress on the saw and crank bearings. The 372 XP Xtorq I have coming has far heavier pistons than the XP model and have had issues with crank bearings because of it; so my line of questions is to see if it is worth the effort to do a little tuning for that performance. I still need to research and find folks who have put up graphs and "in wood" results.

Thanks again!

My saws aren't here yet and most all my gear will be here before the saws arrive. This is the time to pose the questions and learn. I really appreciate your questions back ... I appreciate logical folks who try and help. Sure beats the peanut throwers.

 

[sean donato]

The thought that many different top ends on the same rotating assembly is crap engineering. Is the rotating assembly balanced for the heavy piston, light piston, somewhere in the middle? What are the piston weights? It will induce more vibrations primary and secondary with a change of weight. Something thats been fought in reciprocating engines since they came out. Increasing rpm (which I assume you're after.) Will increase the effects of the crank being out of balance and increase piston stress. Nothing thats great for longevity.

 

[Lightning Performance]

"Will my piston crack if I break the surface tension in the casting?"
Excellent! Can a piston be heat "hardened" ... and will that further improve not only surface tension but also the overall piston strength?

I ask questions ... a lot of them ... all the time about a lot of things. I have become an expert in numbers of areas because of it. Then I go to taking those ideas, and trial run them.

Numbers of saws have different top ends or pistons or different weight pistons because of size or big bore etc. So changing the reciprocating weight is not really an issue except for less stress on the saw and crank bearings. The 372 XP Xtorq I have coming has far heavier pistons than the XP model and have had issues with crank bearings because of it; so my line of questions is to see if it is worth the effort to do a little tuning for that performance. I still need to research and find folks who have put up graphs and "in wood" results.

Thanks again!

My saws aren't here yet and most all my gear will be here before the saws arrive. This is the time to pose the questions and learn. I really appreciate your questions back ... I appreciate logical folks who try and help. Sure beats the peanut throwers.

No you can't heat treat pistons after the fact. That is done during manufacturing.

Everyone is an expert now
You must be under 35

 

[Brewmaster]

No you can't heat treat pistons after the fact. That is done during manufacturing.

Everyone is an expert now
You must be under 35

Actually you just showed your childish behavior. Nothing personal; if you can't handle adult conversations.

 

[Lightning Performance]

Actually you just showed your childish behavior. Nothing personal; if you can't handle adult conversations.

You are spot on.
Leave the conversation if you can't handle the truth.

 

[Brewmaster]

You are spot on.
Leave the conversation if you can't handle the truth.

Yes; because I ask questions to learn. Some have been helpful, some more, some less. Some speak in idiocy seeking response. Some act mature and provide discussion with detail and information that has value. Here is a quote from a Race Saw site. ...

"Reducing rotating weight is only one part of faster acceleration. Trimming down reciprocating weight is the other. An engine's reciprocating mass accelerates and comes to a stop twice per revolution. You don't want a saw spinning north of 13,500 RPM to do any extra work. That piston is coming to a halt 27,000 times a minute! Dropping a few grams from the equation adds up fast and has a direct impact on the performance of your saw.

The hard numbers: If you drop 7g your saw does almost 450 lbs less work per minute in the cut. Your chainsaw's internals will be less stressed. "

I have not formed a conclusion yet; but have plenty of experience dealing with "so called experts" in many fields. They oft make "heady comments" of their knowledge and prowess; but then when challenged or some seek "real data" they resort to childhood playground tactics; immediately reducing their credibility as any source of useful knowledge. Sad too, because they may be right or have a legit opinion; but fail emotionally in stress or mature dialog.

 

[Lightning Performance]

Yes; because I ask questions to learn. Some have been helpful, some more, some less. Some speak in idiocy seeking response. Some act mature and provide discussion with detail and information that has value. Here is a quote from a Race Saw site. ...

"Reducing rotating weight is only one part of faster acceleration. Trimming down reciprocating weight is the other. An engine's reciprocating mass accelerates and comes to a stop twice per revolution. You don't want a saw spinning north of 13,500 RPM to do any extra work. That piston is coming to a halt 27,000 times a minute! Dropping a few grams from the equation adds up fast and has a direct impact on the performance of your saw.

The hard numbers: If you drop 7g your saw does almost 450 lbs less work per minute in the cut. Your chainsaw's internals will be less stressed. "

I have not formed a conclusion yet; but have plenty of experience dealing with "so called experts" in many fields. They oft make "heady comments" of their knowledge and prowess; but then when challenged or some seek "real data" they resort to childhood playground tactics; immediately reducing their credibility as any source of useful knowledge. Sad too, because they may be right or have a legit opinion; but fail emotionally in stress or mature dialog.

Oh good your serious about learning then. I'm done wasting time in pissing matches with YouTube know it alls that think they somehow magically defeat math.

The question you pose is a good one.

The moment of inertia is something most of us don't fully understand and I doubt most engineers do either or they wouldn't design some of the stuff they insist will survive. Few have decades of racing experience with the data sheets to back it up from countless hours of testing and simple math before anything is regarded as the rule. We make rules to make things easier in the real world. Simple ones like driveline or power losses really matter in reciprocating weight. Heavy or light pistons are not the end all be all but the weight they carry is an inherent problem to reverse direction. Stronger is always better when you need to go there. Stronger is paramount at the expense of weight and vibration when it lives there. High RPM means more piston weight. This adds the the overall assembly so the crank must be beefier. Balance is more important then weight to most high performance engine builders for endurance. 900 grams takes more abuse than 800 given the right proportions. Material becomes the savings in weight in complete packages. Staying near a given rule usually applies here. It can be reached with simple math on a known weight. Some engines are over or under balanced on the crankshaft. That is a whole nuther thing. Balance in reciprocating mode is never an acual balance but more of a weight factor used to balance the rotating assembly. One cylinder changes the game some. You have to pick your battles and move through bad spots you will not dwell in very long like an offshore race engine does. It lives and dies in the air by the trottle man never really coming back down or going to the moon. They vibrate bad at lower RPM in certain spots.

I'm far from any leading source on the balance of anything but boat props. I watched once and stayed at the Holliday Inn express that night

Best to consult someone directly like David Vizard. He is online. He won't be around forever. The guy has the experience to answer your question directly. He likely made the rule based on there's research and lots of testing.

To me your question is asking, "What is in a black hole?"
I'm not concerned with spending my life trying to find out. Others already likely know so they have a rule for that we stick to.

 

[Brewmaster]

Oh good your serious about learning then. I'm done wasting time in pissing matches with YouTube know it alls that think they somehow magically defeat math.

The question you pose is a good one.

The moment of inertia is something most of us don't fully understand and I doubt most engineers do either or they wouldn't design some of the stuff they insist will survive. Few have decades of racing experience with the data sheets to back it up from countless hours of testing and simple math before anything is regarded as the rule. We make rules to make things easier in the real world. Simple ones like driveline or power losses really matter in reciprocating weight. Heavy or light pistons are not the end all be all but the weight they carry is an inherent problem to reverse direction. Stronger is always better when you need to go there. Stronger is paramount at the expense of weight and vibration when it lives there. High RPM means more piston weight. This adds the the overall assembly so the crank must be beefier. Balance is more important then weight to most high performance engine builders for endurance. 900 grams takes more abuse than 800 given the right proportions. Material becomes the savings in weight in complete packages. Staying near a given rule usually applies here. It can be reached with simple math on a known weight. Some engines are over or under balanced on the crankshaft. That is a whole nuther thing. Balance in reciprocating mode is never an acual balance but more of a weight factor used to balance the rotating assembly. One cylinder changes the game some. You have to pick your battles and move through bad spots you will not dwell in very long like an offshore race engine does. It lives and dies in the air by the trottle man never really coming back down or going to the moon. They vibrate bad at lower RPM in certain spots.

I'm far from any leading source on the balance of anything but boat props. I watched once and stayed at the Holliday Inn express that night

Best to consult someone directly like David Vizard. He is online. He won't be around forever. The guy has the experience to answer your question directly. He likely made the rule based on there's research and lots of testing.

To me your question is asking, "What is in a black hole?"
I'm not concerned with spending my life trying to find out. Others already likely know so they have a rule for that we stick to.

Thank you for taking the time to share wisdom and point me in the right direction! I "am" overthinking this on purpose and in actual practice will de-throttle well within wise limits when it comes to "metal" and real world function. Gaining knowledge outside of bounds and limits, I put in practice helps me know "why" I am doing things. Took way too much time to decide on muffler mods as the data points just weren't clear in many situations. Thus far most all I will be doing is making the gas driven "pump" (engine) breathe in easier and exhale easier reducing heat, increasing performance and longevity. I will caliper a few of the wrist pins and see if I can find maybe an old Stihl OEM that is lighter and fit it. Likely also open up the "gas driven pump" (engine/ports/etc) based on proven research. All measured by the diminishing return of time/value/performance.

 

[Brewmaster]

Oh good your serious about learning then. I'm done wasting time in pissing matches with YouTube know it alls that think they somehow magically defeat math.

The question you pose is a good one.

The moment of inertia is something most of us don't fully understand and I doubt most engineers do either or they wouldn't design some of the stuff they insist will survive. Few have decades of racing experience with the data sheets to back it up from countless hours of testing and simple math before anything is regarded as the rule. We make rules to make things easier in the real world. Simple ones like driveline or power losses really matter in reciprocating weight. Heavy or light pistons are not the end all be all but the weight they carry is an inherent problem to reverse direction. Stronger is always better when you need to go there. Stronger is paramount at the expense of weight and vibration when it lives there. High RPM means more piston weight. This adds the the overall assembly so the crank must be beefier. Balance is more important then weight to most high performance engine builders for endurance. 900 grams takes more abuse than 800 given the right proportions. Material becomes the savings in weight in complete packages. Staying near a given rule usually applies here. It can be reached with simple math on a known weight. Some engines are over or under balanced on the crankshaft. That is a whole nuther thing. Balance in reciprocating mode is never an acual balance but more of a weight factor used to balance the rotating assembly. One cylinder changes the game some. You have to pick your battles and move through bad spots you will not dwell in very long like an offshore race engine does. It lives and dies in the air by the trottle man never really coming back down or going to the moon. They vibrate bad at lower RPM in certain spots.

I'm far from any leading source on the balance of anything but boat props. I watched once and stayed at the Holliday Inn express that night

Best to consult someone directly like David Vizard. He is online. He won't be around forever. The guy has the experience to answer your question directly. He likely made the rule based on there's research and lots of testing.

To me your question is asking, "What is in a black hole?"
I'm not concerned with spending my life trying to find out. Others already likely know so they have a rule for that we stick to.

I gotta say I love this quote! "I'm done wasting time in pissing matches with YouTube know it alls that think they somehow magically defeat math." Brilliant!