New project saw: 026 Pro - couple of questions.

[Mad Professor]

Well not all pistons have arrows on them and not all cylinders care which way the piston goes. The piston arrow is nothing more than a easy way to make sure people don't screw something up on the ones that do matter.

People build saws all the time with the pistons flipped around sometimes on purpose when porting sometimes cause they don't know what there doing. What matters is the location of the locator pins and on this model the pins are just outside of the piston skirt so you are no more likely to catch the ring pointing it either direction on a stock 026.

Now below are pictures of the pins on the piston and the cylinder port locations so you can see that it doesn't make any difference which way to flip it on a non-port one. I also added dots in the cylinder to where they approximately are as the intake port width is alot smaller then the skirt width.

View attachment 1079654
View attachment 1079655

So you are a fan of cheap chi-com pistons. Post above stated Meteor, they all have arrows.

Another reason for the arrows, is pistons have a thrust side due to rotation direction.

You ever build a V8 SBC/BBC? They have an orientation for the pistons also.

Not all pistons have arrows on them???
I would not be using those pistons. Pistons are not round. The rings are also oriented so as not to hang in ports.
But hey, what do manufacturers know.

Some people probably put on their pants/underwear on, back wards too......."it don't matter".......

 

[pioneerguy600]

In this case, thompsincustom, is correct, the piston in an 026 and MS260 can and often is run with the arrow pointing to the intake for a reason, that being the intake port has been widened enough that the ring ends do not have enough cylinder wall to ride against, the ring ends would pop out into the intake in that setup. I myself have ported more 0261s and MS260 saws than I have fingers and toes, they are all out there still running fine. Building saws and custom porting saws are two different things, a porter measures all aspects of his line of work where as a casual builder most often follows a manual and knows little or nothing to do with clearances, tolerances other than what is covered in a manual.

 

[Mad Professor]

In this case, thompsincustom, is correct, the piston in an 026 and MS260 can and often is run with the arrow pointing to the intake for a reason, that being the intake port has been widened enough that the ring ends do not have enough cylinder wall to ride against, the ring ends would pop out into the intake in that setup. I myself have ported more 0261s and MS260 saws than I have fingers and toes, they are all out there still running fine. Building saws and custom porting saws are two different things, a porter measures all aspects of his line of work where as a casual builder most often follows a manual and knows little or nothing to do with clearances, tolerances other than what is covered in a manual.

OK, I'll eat crow.

But what needs to be wider, intake or exhaust? And where are pins on a stock 026 with the piston correct vs flipped on ex and int sides vs port edges? I've got a NOS 026 P/C at my other place, so I'll check that, and report back here.

This is advice I received from You a while back:

"Widen exhaust and intake ports as much as possible. Some say leave 3/32” from the edge of the piston skirt, but I go as close as 1/8” with no problems so far."

No mentions of backwards pistons.......

 

[pioneerguy600]

OK, I'll eat crow.

But what needs to be wider, intake or exhaust? And where are pins on a stock 026 with the piston correct vs flipped on ex and int sides vs port edges? I've got a NOS 026 P/C at my other place, so I'll check that, and report back here.

This is advice I received from You a while back:

"Widen exhaust and intake ports as much as possible. Some say leave 3/32” from the edge of the piston skirt, but I go as close as 1/8” with no problems so far."

No mentions of backwards pistons.......

I will say this much, I and most of the other porters have agreed to not tell all when it comes to porting any chainsaw. Each porter likey uses his own judgement when it comes to what to make each component work the best for them. Just hogging ports out does not always work as there is much more in play to the movement of fuel and air into the combustion chamber, if a porter reverses the piston then there is a reason to do so to fit their porting style and what they have figured out works best for them, mostly by trial and error and some collusion between themselves and other porters. I will drop a little hint for everyone to think about, spent combustion gases are under much more pressure to escape than the incoming charge into the combustion chamber, so which port should flow more volume?

 

[Mad Professor]

I will say this much, I and most of the other porters have agreed to not tell all when it comes to porting any chainsaw. Each porter likey uses his own judgement when it comes to what to make each component work the best for them. Just hogging ports out does not always work as there is much more in play to the movement of fuel and air into the combustion chamber, if a porter reverses the piston then there is a reason to do so to fit their porting style and what they have figured out works best for them, mostly by trial and error and some collusion between themselves and other porters. I will drop a little hint for everyone to think about, spent combustion gases are under much more pressure to escape than the incoming charge into the combustion chamber, so which port should flow more volume?

Conversely, once the charge is ignited, it's volume greatly increases. So does more pressure overcome a much greater volume needed to be flowed?

I'll measure things up and report back.

 

[thompsoncustom]

So you are a fan of cheap chi-com pistons. Post above stated Meteor, they all have arrows.

I was actually only referring to OEM stuff cause you don't know what your gonna get with AM. There was a thread not to long ago with someone that had a Meteor or hyways piston with the arrow on the wrong side so it's always better to check the locator pins.

 

[Patrick1903]

Nice.

Saw is back together with the 0.5mm gasket and 44.7mm meteor piston. Ended out with 0.022" squish. The compression now feels 'normal' in terms of resistance on the starter cord.

Did a muffler mod (added an additional 2 x 10mm drill holes behind the spark screen), ran a tank of 40:1 through the saw, got a 2nd tank of 40:1 in there, then tuned to 13,500 rpm, sharpened the chain and .... ahhh - lovely. Now *THIS* is what I remember my MS260 cutting like!

Congrats on getting the saw back to good power. How did the original cylinder look? Did you clean that up and put it with your spare parts?

 

[kiwiguy]

How did the original cylinder look? Did you clean that up and put it with your spare parts?

Looked in good condition, to be honest...but by then I'd already bought the 44.7mm meteor jug & piston. So - the OEM is now in my spare parts pile...if only the OEM piston wasn't $230NZD! When / if this new topend burns out I might grab an aftermarket 44mm piston and throw the original jug back on just for fun.

Anyway - I pulled the saw apart again this morning. Turns out one of the circlips was the correct orientation anyway (opening facing at 12 noon), while the other one wasn't far off (opening around 4 o'clock). The clip was really tight & wouldn't rotate in the groove, so probably would have been fine, however I did pull it & reseat it so it opens at 6'o'clock.

...and the piston is facing the right way...or at least the way the arrow says it should (/ducks).

 

[Vintage Engine Repairs]

I have come across pistons that have arrows facing the intake and the locator pins are exhaust side, as well as no arrow on piston and from factory their exhaust side as said by thompsincustom. As he said, the key is that they clear the exhaust.

 

[kevin j]

I replaced the top end with the OEM but it was the 44.7. I've been told the 44 mm cylinders had better porting. Is there any truth to that and what are the differences? At my next rebuild I was gonna port and put the 44 back in. I did some work on a 44 mm on my sons saw and it turned out really strong

 

[kevin j]

I replaced the top end with the OEM but it was the 44.7. I've been told the 44 mm cylinders had better porting. Is there any truth to that and what are the differences? At my next rebuild I was gonna port and put the 44 back in. I did some work on a 44 mm on my sons saw and it turned out really strong

 

[berrnard]

sorry for the tardiness
check with hatch about the badge. saw a .5mm gasket on there too

https://www.ebay.com/str/seesawsrun...run|folenttp:1&_trksid=p3542580.m47492.l74602

 

[bwalker]

I will say this much, I and most of the other porters have agreed to not tell all when it comes to porting any chainsaw. Each porter likey uses his own judgement when it comes to what to make each component work the best for them. Just hogging ports out does not always work as there is much more in play to the movement of fuel and air into the combustion chamber, if a porter reverses the piston then there is a reason to do so to fit their porting style and what they have figured out works best for them, mostly by trial and error and some collusion between themselves and other porters. I will drop a little hint for everyone to think about, spent combustion gases are under much more pressure to escape than the incoming charge into the combustion chamber, so which port should flow more volume?

Hogging out ports seldom works well and on this saw in particular enlarging the intake is not a good idea.
The problem is most guys think in terms of bulk flow in a steady state. The problem is flow through a two cycle is unsteady. As such port timing is much more important in order to maintain flow through the ports.