Sachs Dolmar Cylinder Question

[sawjo]

Got a new piston for my SD120 - as you can see the old one (sitting on top of rewind starter) wasn't that bad. No scratches, but the finish
had worn. I could only get 125psi for compression. The second shows the cylinder which is pristine, maybe TOO pristine. Should I scuff it up
and what would you guys recommend, 600 black sand paper? Tried to buff it with the red scotch brite and that seemed to make smoother. I have
a small ball hone that would fit but I don't want to ruin it. Would a hand sand with ??? grit sand paper be OK?



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[discounthunter]

did the saw not run? why replace the piston? @ 125compresion your assuming your tester is 100% acurate. not busting your chops just wondering why.

 

[brian660]

dont touch the cylinder you`ll probably do more harm then good, if you absolutely have to do something go with the ball hone, smooth even strokes with the drill for a very short period of time, nothing ultra fine (500 finish or rougher)


I have no way or desire to verify if its the same in a chainsaw engine but in a racing engine that`ll spin upwards of 9800rpm`s we found that hone finishs higher then approx 500 were detrimental to ring seal and the loss was compounded the higher the rpm`s got, in a 500 inch engine the loss was around 8 horsepower, obviously in a chainsaw the loss would be almost minimal but the bigger concern would be oil blowing past the ring and glazing the cylinder over.

 

[brian660]

Also send me your old piston, i`d have no problem running that sucker as is. :msp_biggrin:

 

[sachsmo]

That cylinder is a buet.

Like said, what kind of tester are you using?

If it has a long hose, you will get erroneous readings with a small displacement engine.

I would ditch the base gasket, then check a few places around the squish band with some solder.

.018 or better should be fine on that old girl.

That original piston almost looks domed by the first pic????

 

[sachsmo]

By the way,

that looks to be a very clean old Dolmar.

120 or 120 Super?

 

[sachsmo]

Myself,

I would not touch that cylinder with anything but a clean cotton cloth.

 

[sawjo]

Thanks for the replies. The saw is a regular (47mm) 120. My compression tester is usually spot on and has the schrader valve, my other saws read 150 to 180 with this same tester. I was suprised at the low compressiom myself and thought maybe the rings had worn so for 30 bucks I am going to try this Mako piston set which looks as good as the original. Saw was difficult to start and would not stay running, first read was 110psi then 125 psi after it cooled.

 

[mopar1rules]

Your piston isn't what dictates the compression reading, but rather the conditions of the rings. Its the ring's job to create the seal. You could (and I would) simply reuse the original piston with a new set of rings.

 

[sachsmo]

Go figure?

As good as that P/C look, I'd guess a good bit more than that??

Sure would like to see the "after" number.

 

[rupedoggy]

Your piston isn't what dictates the compression reading, but rather the conditions of the rings. Its the ring's job to create the seal. You could (and I would) simply reuse the original piston with a new set of rings.

This is wrong information based on my experience. Piston fitment in TWO STROKES is critical to good compression. I don't care how new the rings are, if the piston is worn you will not get good compression!

 

[rupedoggy]

dont touch the cylinder you`ll probably do more harm then good, if you absolutely have to do something go with the ball hone, smooth even strokes with the drill for a very short period of time, nothing ultra fine (500 finish or rougher)


I have no way or desire to verify if its the same in a chainsaw engine but in a racing engine that`ll spin upwards of 9800rpm`s we found that hone finishs higher then approx 500 were detrimental to ring seal and the loss was compounded the higher the rpm`s got, in a 500 inch engine the loss was around 8 horsepower, obviously in a chainsaw the loss would be almost minimal but the bigger concern would be oil blowing past the ring and glazing the cylinder over.

Sounds like you are comparing two stroke to four stroke. "Oil blowing past the ring" WTF the oil is in the mix. That mix is already below the piston and all over the crankcase. Mix past the ring (below it) is how the two stroke lubricates the lower bearings.
Ever notice how dead smooth a new cylinder is finished? The builders sure don't want you to "scuff it up" before installation.