Piston weight and 2nd ring removal

[sean donato]

May not be practical in a saw. Weight loss and you mentioning wrist pins made me remember the old man's race boat. He ran a 2.5L black max mercury. When he first started he had a lot of issues breaking pistons. Would only last 2 or 3 races, then we'd be swapping them out. He went down the keys weight rabbit hole. Can't remember the pistons he ended up using, but he had light weight wrist pins made for it. Dropped quite a bit of weight with them too. I'm fairly sure they were made from tool steel and had an internal taper as well. It was a great combination. But he did have to get the rotating assembly re balanced. Vibrated something terrible on the first run. After it was all dialed in he could spin that engine a bit faster then 12k rpm. Didn't have pistons fail much after that either.

 

[Brewmaster]

May not be practical in a saw. Weight loss and you mentioning wrist pins made me remember the old man's race boat. He ran a 2.5L black max mercury. When he first started he had a lot of issues breaking pistons. Would only last 2 or 3 races, then we'd be swapping them out. He went down the keys weight rabbit hole. Can't remember the pistons he ended up using, but he had light weight wrist pins made for it. Dropped quite a bit of weight with them too. I'm fairly sure they were made from tool steel and had an internal taper as well. It was a great combination. But he did have to get the rotating assembly re balanced. Vibrated something terrible on the first run. After it was all dialed in he could spin that engine a bit faster then 12k rpm. Didn't have pistons fail much after that either.

Yeah my Dad raced cars ... Formula Ford and Formula B Atlantic and Formula C and numbers of other categories way back in the day. Always tweaking and tuning "balancing" ... my understanding is multi-cylinder it is 100% required. In standard production the tolerances are balanced but not to the required need for racing.

With single piston, and varying weight pistons often within "class", or folks dropping "big bores" on the same crank ... I am suspecting a few grams may well be worth it. Especially if a tapered wrist pin has the strength, and porting for better flow is strait forward and relatively easy. I am not convinced fully yet however of taking much action. I did buy a cheap Chinese spare J&P to perhaps play with.

Quality parts may just be the real issue, gasket delete, muffler mod etc.
https://dominantsaw.com/collections/performance/products/ceramic-hybrid-main-bearing-set

I am trying to clear the stable of other saws now.

 

[sean donato]

Cost vs reward on those bearings. Some of the guys i know that race 1/5 scale rc swear by them. Too expensive for me to play with.

 

[srcarr52]

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.

Don't let the naysayers dissuade you.

Most vertically oriented cylinder chainsaws are 40-60% balanced. 100% balance on a single cylinder engine (without a balance shaft) would result in terrible vibration opposed to the cylinder. Measuring the balance factor can be done pretty easily, there are all sorts of YT videos explaining the process.

I've taken as much as 20% off a piston for a work saw with no noticeable change in vibration nor did it sufferer a failure.

Removal of the 2nd ring can be beneficial in a race saw, although it does increase spit back through the carb with the ring land being open to the intake port.

 

[Brewmaster]

Don't let the naysayers dissuade you.

Most vertically oriented cylinder chainsaws are 40-60% balanced. 100% balance on a single cylinder engine (without a balance shaft) would result in terrible vibration opposed to the cylinder. Measuring the balance factor can be done pretty easily, there are all sorts of YT videos explaining the process.

I've taken as much as 20% off a piston for a work saw with no noticeable change in vibration nor did it sufferer a failure.

Removal of the 2nd ring can be beneficial in a race saw, although it does increase spit back through the carb with the ring land being open to the intake port.

Most outstanding! I was thinking of doing a very mild ring detensioning on the upper ring and more on the second ring. Best of both worlds.

 

[thompsoncustom]

Most outstanding! I was thinking of doing a very mild ring detensioning on the upper ring and more on the second ring. Best of both worlds.

Ring detensioning? As in your somehow going to take spring tension off of the ring? One that's a terrible idea and two even if you could remove 100% of the spring tension the combustion is what sticks it to the cylinder wall. Opening up the ring gap will do nothing but give you blow by and power loss. In race application they do the exact opposite by gas porting the pistons to increase ring pressure on the cylinder walls. Pretty sure theses days Total seal rings even sells a gas ported piston ring.

Running the top ring only in a chainsaw can be beneficial if you have lower the cylinder enough to get the exhaust port to open where you want it but your compression is higher than desired or if your worried about catching the second ring in the intake port. In that case removing the lower ring makes sense as it reduces drag and lowers compression and/or avoids hanging up in the intake port.

 

[Brewmaster]

Ring detensioning? As in your somehow going to take spring tension off of the ring? One that's a terrible idea and two even if you could remove 100% of the spring tension the combustion is what sticks it to the cylinder wall. Opening up the ring gap will do nothing but give you blow by and power loss. In race application they do the exact opposite by gas porting the pistons to increase ring pressure on the cylinder walls. Pretty sure theses days Total seal rings even sells a gas ported piston ring.

Running the top ring only in a chainsaw can be beneficial if you have lower the cylinder enough to get the exhaust port to open where you want it but your compression is higher than desired or if your worried about catching the second ring in the intake port. In that case removing the lower ring makes sense as it reduces drag and lowers compression and/or avoids hanging up in the intake port.

Yeah ... not sure on that second ring issue ... though am leaning that way. Detensioning is just removing a few thou off the inner diameter of the ring and using a Caliper to make sure to have no weak spots. Theoretically reducing parasitic drag. Same with removing the second ring. I will need more study there.

 

[MustangMike]

I am NOT a professional saw builder, but I have done a good amount of tinkering and have numerous professional built saws.

Here are my opinions:

Saws are NOT balanced (like a racing car engine). which is why an AV system is so important. Also, the steel parts are much heavier than the aluminum piston, so things like piston pin weight are often more important. That said, unless you do it to the extreme, reducing piston weight will often result in less vibrations and less stress on the saw. Many AM pistons (and piston pins) are heavier than OEM and I can tell you from experience will add vibrations to the saw.

This is especially true with Hybrids, like a 460/440 saw. The larger piston is often heavier than the crank was designed to work with. Some builders will use husky pistons for these applications because the pistons are lighter than 460 pistons and result in lower vibrations.

I have both single ring and double ring piston saws, and don't notice a big difference due to that factor. I personally would not remove a 2nd ring from a 2 ring saw.

That said, builders will often use Husky single ring pistons in Stihl saws because it allows them to make the exhaust port wider and produce more power. I believe this is common practice in Hybrids, MS460s and MS660s.

Hope this is helpful to you. I currently have 14 running saws from 50cc to 99cc. Most of them have been ported, all have been modified.

If your piston is heavier than stock, or your saw is modified to operate at higher RPMs, I believe it becomes very important to run a good oil at a good ratio.

I am not saying it is the best, but Amsoil Saber at 40:1 provides very good lubrication and burns clean.

 

[Homer6679]

I am still very much a novice at porting/building saws and this thread has made for some good reading.

Limited personal experience leads me believe that removing the 2nd ring is negligible in terms of performance unless you are really pushing the limits of a saw.

I was cleaning up an older saw and accidentally broke the 2nd ring trying to get the cylinder back on. So I just removed the base gasket to try and add some compression to compensate for losing that ring. The saw runs great but I’m not sure how it ran before. Seems to be nothing special.

Thinking about the AV systems on saws, I wouldn’t be worried about vibrations at all. In my mind the AV has a certain tolerance/threshold that it can handle and as long as the saw is under that tolerance/threshold then it will feel the same

 

[MustangMike]

A lot depends on the AV system, but especially with rubber mount AV you will notice a change in piston weight.