Milling down the piston skirt?

[yo2001]

So is it bad to mill down the bottom of the piston skirt to increase the intake duration? I wouldn't just cut the intake side but both intake and exhaust side of the skirt same amount. What'll be the con of doing so? Thanks in advance.

 

[Metals406]

So is it bad to mill down the bottom of the piston skirt to increase the intake duration? I wouldn't just cut the intake side but both intake and exhaust side of the skirt same amount. What'll be the con of doing so? Thanks in advance.

Uh oh... You just opened up a can-o-worms...

Don't touch the skirt... Porting is the way to go.

 

[drmiller100]

every saw i've done port/time calcs on shows the intake is pretty huge compared to transfers and exhuast.

which means you need to work on the other two more.

also, be careful the skirts still close the exhaust port once you get the exhaust ports to where they need to be. for me, skirts seem to be short, and meat on top of the exhaust window is thin, so i'm struggling there and wouldn't consider cutting skirts.


good luck

 

[yo2001]

Interesting. I'm going to measure port timing next time I port a saw. I thought the reason not to cut the skirt got more to do with the piston stability rather than port timing.

The exhaust side skirt closes the port in TDC correct? So talking about cutting the piston short causing the intake/transfer to free port(?)to exhaust? Thanks.

 

[Crofter]

You gain more port area per degree of duration by removing material from the arc of the port than by removing it from the straight across piston skirt. Unless you also do extensive work to exhaust and transfers you will not benefit from more intake duration anyways except perhaps no load Wide Open Throttle rpms.

 

[HolmenTree]

So is it bad to mill down the bottom of the piston skirt to increase the intake duration? I wouldn't just cut the intake side but both intake and exhaust side of the skirt same amount. What'll be the con of doing so? Thanks in advance.

Madsens introduced me to the intake piston skirt mod back in the early 1980s on my Stihl 038s. 2mm [80 thou.] filed off bottom of piston intake skirt, leave exhaust side alone , mill 25 thousand off cylinder base,polish exhaust port,round off edge of piston to match arc of exhaust port at bottom of stroke, then we added up to 4 finger ports along side tranfers match this all up to a larger carb and intake manifold ring and those 038s really opened up nicely. Did this to Jonsered 670, Stihl 064,090.....that was the thing to do 25 yrs ago. Never caused any problems and sure won alot of races.

 

[rickyrooster]

I think you gain lift not duration by moddifing the piston skirt. I beleave you widen the port to increase duration. I've heard of guys milling off the bottom of the skirt on the pistons intake side but never the exhaust. most here port the cylinder for there gain and dont mess with the skirts.

 

[HolmenTree]

I never put much thought into understanding the piston skirt mod , Ijust knew it worked just fine ,if you didn't add finger ports you cylinder was still untouched besides the milled base ,but an extra base gasket could bring it almost back factory. I put all my energy into the chain.

 

[romeo]

Its a lazy trick to gain some intake duration. Some saws benefit from more of it and some don't. I am usually worried about gaining too much intake duration after the work I do above the piston, thats where you find the power.

I don't think it really helps when you look at the whole picture, to see real gain needs a proper balance of all factors. I'm sure there is a saw out there that would need it during the build so the idea is always on the table, but I haven't seen it yet.

 

[HolmenTree]

Yes not all saws can accept the same mods. My experience has only been with the Mahles on the Stihl and Jonsered. Have done a few cutoff heads but its hard to convince the judge at the stock appearing competition when there is a spark plug boot sticking outside of the top cover. One trick I did after lowering the cylinder:With the piston at bottom stroke and taking a 45 off that protruding piston edge at the bottom arc of the exhaust port, I would do the same for the 2 transfer ports, but on those 2 piston edges you can make a swirl intake charge effect into the combustion chamber by putting opposite left and right curled up edges on the 1 end of the 45 degree edges.I also put a curve in the 4 finger ports to match up with the 2 transfers to get the intake charge[air/gas] spinning and mixing up nice. But I always believed to leave the intake transfers a little rough on their surfaces to make a little turbulence to get that charge mixed a little better. Just something I learned from the 1980s. Also if you don't bother with the finger ports it is cheaper to screw up a piston then a cylinder if your just learning.

 

[Crofter]

Yes not all saws can accept the same mods. My experience has only been with the Mahles on the Stihl and Jonsered. Have done a few cutoff heads but its hard to convince the judge at the stock appearing competition when there is a spark plug boot sticking outside of the top cover. One trick I did after lowering the cylinder:With the piston at bottom stroke and taking a 45 off that protruding piston edge at the bottom arc of the exhaust port, I would do the same for the 2 transfer ports, but on those 2 piston edges you can make a swirl intake charge effect into the combustion chamber by putting opposite left and right curled up edges on the 1 end of the 45 degree edges.I also put a curve in the 4 finger ports to match up with the 2 transfers to get the intake charge[air/gas] spinning and mixing up nice. But I always believed to leave the intake transfers a little rough on their surfaces to make a little turbulence to get that charge mixed a little better. Just something I learned from the 1980s. Also if you don't bother with the finger ports it is cheaper to screw up a piston then a cylinder if your just learning.

I am sure you can make some gains over stock by the piston whittling method but its value is mostly that it doesnt permanently alter the cylinder; it does have some serious limitations in potential gains.

With todays saws and bore to stroke ratio being a lot higher, direction of incoming charge wants to be much more cleanly straight across the piston to meet the opposing charge from the opposite transfers, then up; if the initial charge is directed upward at all I believe too much exhaust gets mixed in and dilutes the incoming charge. I wonder if the angled surface at the transfers and the exhaust ports also might interfere with the squish turbulence pattern near TDC. I dont know what kind of compression you were getting in the old girls so that might not have been so important then.

I have just done a port mapping of one of Bailey's 372 BB jugs and the transfer tops are only 30 mm down from the top of the bore which is 52 mm across; the piston is only 24 mm from the top when it closes the exhaust port so it is very much a pancake volume you are trying to scavenge and charge rather than a tall cylinder.

I wish the combustion chamber were smaller as it stands somewhere over 6 cc's in volume; it will only come up with about 130 psi compression even with perfect ring seal. Got to do a bit of meditating to see what I have to move to boost that up a bit. They start and run real nice out of the box but we gotta find a bit more than that!

 

[HolmenTree]

Frank you need to get some more intake charge on top of that piston to get that compression PSI up, some old fashioned finger porting [4] and better squish will do it nicely.

 

[HolmenTree]

Seriously back in the day[1980s] saws were no different in basic design then they are today, besides the new strato charge saws. Now the 181 Husky came out in 1981 and besides not having air injection I would still call it a modern saw. The biggest increases in power in the day was sticking a circle crank into those guys along with some good porting, big carb and this still stands today in the stock appearing competition class.. Now I wouldn't recommend a circle crank in a woods saw .LOL.
In 1988 with a little hairpulling i got my 1986 Stihl 064 fitted with a circle crank and still retained the factory cylinder without a cutoff head. With the 084 carb, deep finger porting ,indexed plug for starters,she still in 2008 puts a big grin on my face.

 

[Crofter]

Seriously back in the day[1980s] saws were no different in basic design then they are today, besides the new strato charge saws. Now the 181 Husky came out in 1981 and besides not having air injection I would still call it a modern saw. The biggest increases in power in the day was sticking a circle crank into those guys along with some good porting, big carb and this still stands today in the stock appearing competition class.. Now I wouldn't recommend a circle crank in a woods saw .LOL.
In 1988 with a little hairpulling i got my 1986 Stihl 064 fitted with a circle crank and still retained the factory cylinder without a cutoff head. With the 084 carb, deep finger porting ,indexed plug for starters,she still in 2008 puts a big grin on my face.

Lol, that sure doesnt sound like cutting the piston skirt and carving notches in the piston crown!
My comment was that that method left behind a lot of performance potential!

 

[HolmenTree]

Lol, that sure doesnt sound like cutting the piston skirt and carving notches in the piston crown!
My comment was that that method left behind a lot of performance potential!

I agree, The piston whittling thing was how we did a woods port mod on a new saw in the day, which made an increase in power enough to justify not going up one size larger saw. Madsens sold alot of saws to the timberfaller market this way along with some finger porting, making their famous 056 mag a much sought after saw. Now when I got into the timber sports world [whole different ballgame from the woods mods] only a few guys were putting circle cranks in their stock appearing saws. Something only the factories were backing at first. My mods I found out cost me a big pile of cash,countless hrs on the phone,in the shop, in the local machine shop and a divorce ,I guess a few guys here can relate to that, LOL .