Boob Job, as Gypo would say.

[Four Paws]

In reading various posts here, and on other sites, there seems to be a great debate about cutting the piston skirt. I am thinking of building a second worksaw. That said, I am going to experiement with some longer duration times, wider ports, tighter squish, and more modified lower transfers to come up with a 'hotter' saw. To get the longer intake duration times on this saw, I will almost have to cut the piston skirt. The stock intake duration is 122.5* - the piston skirt is the limiting factor as at TDC, it covers a bit of the intake port. When I turn the cylinder base down to get my squish down to .020" I will gain some duration, but (I think) I will run out of aluminum to remove on the intake port before I get the intake duration where I want it.

So, the questions are:

-Shave the intake and exhaust side of the piston skirt the same ammount?

-How much can you shave one side of the piston skirt before balance becomes an issue?

Intake:

Exhaust:

Pictures of the cylinder:

 

[Four Paws]

120 views and no responses

 

[Lakeside53]

here's one..

It is FRIDAY NIGHT

My initial reaction on the inlet masked by the piston was "huh"? Doesn't seem right at all, so I'll check my saws tomorrow..

 

[Crofter]

I saw that the exhaust duration was only 139 degrees. Was that a cutoff saw cyl and piston by any chance?

 

[drmiller100]

at tdc, if the piston skirt is covering the top part fo the intake port, it doesn't matter. what is important is when the intake opens.

what is even more important is when the exhaust port opens.

as far as ballance, it is a single cylinder engine. So what are you trying to ballance.

Oh wait. You aren't worried about the intake and exhaust side of the piston being ballanced, are you?????????

Ummm, it turns out the rod pushes sideways on the piston. so a few grams won't matter.

 

[drmiller100]

btw,what does the top of the case look like where the transfers start?????

 

[Lakeside53]

at tdc, if the piston skirt is covering the top part fo the intake port, it doesn't matter. what is important is when the intake opens.


.

Then.. why make the Inlet as "open" (tall) as it is??

 

[drmiller100]

Then.. why make the Inlet as "open" (tall) as it is??

why not?????

personally, i don't think he took the pic at tdc. even if he did, it is just free volume in the intake, and doesn't really matter.

again, the OPENING and the CLOSING is the most critical. at tdc not much is moving except becuase of inertia effects.

all the smart 2 stroke guys know this.

 

[Lakeside53]

I buy the opening and closing as most important, but... guess I haven't seen it quite like that pictured... so maybe it isn't at tdc.. Four paws?

 

[spacemule]

It would make no sense for the picture to not be at tdc, because

the piston skirt is the limiting factor as at TDC, it covers a bit of the intake port.

Why else would this picture be here if it was not to illustrate what he was talking about?

 

[Lakeside53]

It would make no sense for the picture to not be at tdc, because Why else would this picture be here if it was not to illustrate what he was talking about?

Hmmm... that occurred to me ... :monkey:

I'm just giving drmiler the benefit of the doubt. 4paw will tell all.

 

[Four Paws]

Andy, the saw is at TDC in the picture. The lighting makes it look like the piston covers a HUGE ammount of the port, but if you look closely, you can see 3 different colored 'frowns' in the intake throat. The top of the throat, where it turns from a light gray to a darker gray is the piston / cylinder interface. The actual portion of the piston that blocks the port is 5mm in height.

This cylinder is a Genuine Shindaiwa part, and to the best of my knowledge, is NOT a cut-off saw cylinder. Shindaiwa has 2 vertical cylinder cut-off saws that use the same engine. These cut-off saw cylinders have a plug-hole that is vertical, not entering from the side as this cylinder (and most saws) have.

The only thing I can think is that this cylinder is an old proto-type part or something. I got if from Big Dave Neiger - it has a 51mm bore just like a 757 chainsaw, but a different transfer set-up than all the 757 cylinders I have seen. It fits a 695 case perfectly - the skirt on the cylinder is too thin to fit a modern-day 757 case properly. The timing numbers I degreed with a 757 piston installed, as I am 99% sure that is what this cylinder is.

As far as 140*-ish exhaust timing, I have seen other machines with numbers in the 140-150 range. The 120* intake timing is what is strange! Once I cut the cylinder base down, and trim the piston, I should have good intake duration to start with before I port the intake.

 

[NWCS]

if you shave the skirt of the piston, do not do the exhaust side. you do NOT want to see under the skirt at TDC! if you can see under the skirt at TDC with the saw assembled, your letting the fuel in the CC right out the exhaust, purpose has been defeated. only the top of the piston should open the exhaust port. the skirt is there to keep it "sealed"

 

[drmiller100]

dude, i'm just a poor chicken farmer fisherman, but it seems to me i would not worry about what happens at tdc on the intake port.

if you have some goofy prototype, it matters even less. the facts are by the time you hit tdc, 90 percent of the intake charge has gone down into the crankcase.

the facts are most of the charge goes down nto the intake while the piston is partially plugging the port on all piston port 2 strokes. air makes a great wall as it has little frictional loss.

 

[Four Paws]

Yeah, Ben, I know I don't want to see under the skirt on the exhaust side. I was asking about trimming both sides equally to maintain a balanced piston, with the same bearing length on the skirt. Didn't want it rocking back and forth with the intake side a few mm shorter than the exhaust. I will probably just cut the shape of the intake port off the piston skirt to allow some bearing surface on each side of the port window.

 

[Freakingstang]

Just a Kind Suggestion..

Are you sure that you have the right piston. Is the 757 piston, piston ported (windows)?

From the looks of the transfers, it may take a closed sided piston.


Also, be leary of the numbers....This may be a case like the Dolmar 5100 and 7900 with really low numbers out of the box.

I would get it running with your "stock" piston first, and then try your hand at it. I ran into problems with a dolmar 115 that I has the wrong piston in...

 

[Four Paws]

Hey Steve, no worries about making the suggestion about having the correct piston. The 757 is a piston ported saw - remember the "degreed my saw, now what?" thread? This cylinder has Shindaiwa cast into the side. It has a 51mm bore (same as 757). Shindaiwa made no other machine with 51mm bore besides their cut-off saws. The IPL's for the cut-off saws show specifically a vertical spark plug hole (like a Hemi). This cylinder has the slanted, side entry plug hole like almost every saw I have seen. To tell you the truth, I don't know what this cylinder is. I am guessing it is an early 757 cylinder. You have made a good suggestion about the piston - perhaps the 757 piston I have won't work. Good idea to assemble it first and see before I start cutting on it.

How conservative are the Dolmar 5100/7900 numbers - do you have any data?

 

[drmiller100]

Yeah, Ben, I know I don't want to see under the skirt on the exhaust side. I was asking about trimming both sides equally to maintain a balanced piston, with the same bearing length on the skirt. Didn't want it rocking back and forth with the intake side a few mm shorter than the exhaust. I will probably just cut the shape of the intake port off the piston skirt to allow some bearing surface on each side of the port window.

ummm, dude. the rod thingy pushes pretty hard sideways on that piston. It pushes a LOT harder sideways then a few stupid grams are every gonna matter.

As an example, think about 15 degrees after top dead after the big bang has occurred.

that ole rod is shoving down HARD, and that goofy ole crankshaft ain't hanging out directly under the crank throw.

the piston is perfectly capable of withstanding significant side thrusts.

But hey, feel free to grind away!

 

[Lakeside53]

Hey Steve, no worries about making the suggestion about having the correct piston. The 757 is a piston ported saw - remember the "degreed my saw, now what?" thread? This cylinder has Shindaiwa cast into the side. It has a 51mm bore (same as 757). Shindaiwa made no other machine with 51mm bore besides their cut-off saws. The IPL's for the cut-off saws show specifically a vertical spark plug hole (like a Hemi). This cylinder has the slanted, side entry plug hole like almost every saw I have seen. To tell you the truth, I don't know what this cylinder is. I am guessing it is an early 757 cylinder. You have made a good suggestion about the piston - perhaps the 757 piston I have won't work. Good idea to assemble it first and see before I start cutting on it.

How conservative are the Dolmar 5100/7900 numbers - do you have any data?

It makes sense that the piston isn't right... Even the exhaust side looks too high at BDC (Yes... DRMILLER, I know it's on the part that's least important!).

 

[drmiller100]

I (Yes... DRMILLER, I .

aaaahhhh.........


success!!!!!!!!!!!!!1