I have this, a knurling tool for it and some old pistons..
Why does piston scoring matter?
- Thread starter julian1234
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Will it oscillate the piston back and forth? Round won't work because pistons aren't.
It might? If he don't chuck it right! Worn out pistons are worse........
You knurl the skirt.
no oscillation here except for me. I honestly know nothing about knurling pistons.
Got to admit, I may be one of the few that have actually seen one in action.
I think I'll chuck up a piston just to see how far out of round one is. it can't be a lot I wouldn't think. I was just googling "knurled pistons" and the roundness may not be a factor as they only do 2 sides.
https://mechanics.stackexchange.com/questions/24894/what-exactly-is-a-knurled-piston
https://mechanics.stackexchange.com/questions/24894/what-exactly-is-a-knurled-piston
that 2nd link is interesting. the lathe is turned by hand, not spinning as there is a brace inside the piston pressing up on the skirt for support and the knurling tool "floats" using compressed air to push it down.
Just an update on the 353 & 346 I put together scored.
I've been working them hard, at first the 346 did not run well, since I had put these saws on 350 cases I pulled the cyl. to make sure I had modded the case for 346 impulse, I had.
Next I pulled the carb & put a new diaphram in, that cured it.
Lastly I cut down a newly dead 20'' on the stump red oak using the 346 with a 20'' 325 bar and blocked it up getting 11 22'' blocks, then cut off & ripped in half the first crotch piece.
I cut up the rest of the tree with the 353, 16'' 325 bar. Today I will start ripping those blocks into 4 pieces with the 346 20'', (normaly use 70cc 24'' but this is a test)
I am way too old to be moving those blocks by hand. Will keep you posted.
thenne1713
ArboristSite Guru
The piston score is of little consequence as long as the rings/ ring grooves are undamaged, and rings seal to the piston. You CUD machine grooves into piston that would work just fine. However, transfer on cylinder wall that pushes the ring away from cylinder wall, or damages ring, or grooves in cylinder wall all allow gas bypass of the rings and reduced compression pressure. Below 120 many CS/ 2-cycle will not run or will have big power loss. Period.
If you had read the whole thread you would know we know all that already, please add something new. thanks!
If you don't have enough compression, the rings are either melted or arn't sealing against the cylinder due to transfer, then you need to do something.
If the cylinder is unscored, you can remove the just transfer and replace piston and rings. But you still run the risk of over-sanding or an aftermarket piston not fitting exactly, so most of the time I replace piston and cylinder together. Ive had good experience with meteor.
If the cylinder is unscored, you can remove the just transfer and replace piston and rings. But you still run the risk of over-sanding or an aftermarket piston not fitting exactly, so most of the time I replace piston and cylinder together. Ive had good experience with meteor.
Concerning kneeling, the old timey backyard meca'nic could mount the piston between opposing files in a bench vise and then tap on one of the files to cause rotation of the piston, thus creating a kneeled surface. It's amazing what the noggin memory won't let loose of.
As far as a scored piston and performance, it will depend on how much and where. The piston skirt is the intake valve on a two stroke saw with no reeds. A really worn piston will usually show more performance loss than one with a small amount of scoring on the skirt. That is why we look for the machine marks on the piston as an indicator of wear.
So do those machine marks do the same thing as knurling a piston? Yes. That is why reknurling was a thing on car engines in the past.
So do those machine marks do the same thing as knurling a piston? Yes. That is why reknurling was a thing on car engines in the past.
AlotL1keVegas
ArboristSite Operative
Someone educate me. What's the point of a knurled piston? Common sense tells me it's to help the piston hold extra oil for the cylinder?
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That and help take up clearance on a worn piston skirt
Primarily yes.
Secondarly, there is an oil wedge created as the piston reciprocates. This should improve clearance and stability. If stability is improved, ring wear should be less.
julian1234
Barm Foss
**Bump**
Well, I am happy to report that the 353 & 346 are still doing just fine, I used them for most of the cutting all summer.
I normally do not use a 20'' bar on a 50cc saw but I did on this 346 to make it work hard, it bucked up a lot of 18-20'' red oak then ripped the 22'' long pieces twice.
I was surprised at how strong this saw was, 12 or 15 tanks thru it now, compression is 158.
The 353 with a 16'' bar was used for the smaller top of the trees 15'' on down, probably the same amount of fuel thru it as the 346, not nearly the power of the 346, I could have turned it up some more but it was such a jewel to run, always one pull start when warm always idled well, always ready to work, it made me smile.
Both of these pistons had been scored at the ex. port about an inch wide, I cleaned up the ring grooves, put in new rings, smoothed the scoreing, cleaned alum. out of the cyl.. Looks like it worked out.
I normally do not use a 20'' bar on a 50cc saw but I did on this 346 to make it work hard, it bucked up a lot of 18-20'' red oak then ripped the 22'' long pieces twice.
I was surprised at how strong this saw was, 12 or 15 tanks thru it now, compression is 158.
The 353 with a 16'' bar was used for the smaller top of the trees 15'' on down, probably the same amount of fuel thru it as the 346, not nearly the power of the 346, I could have turned it up some more but it was such a jewel to run, always one pull start when warm always idled well, always ready to work, it made me smile.
Both of these pistons had been scored at the ex. port about an inch wide, I cleaned up the ring grooves, put in new rings, smoothed the scoreing, cleaned alum. out of the cyl.. Looks like it worked out.
