Why does piston scoring matter?

[julian1234]

If the rings of the piston hit the side of the cylinder, why does it matter that the piston is scored? It never hits the side of the cylinder...

If the rings are scored, cant you just replace them, and keep the scored piston?

 

[wildwes]

If the rings of the piston hit the side of the cylinder, why does it matter that the piston is scored? It never hits the side of the cylinder...

If the rings are scored, cant you just replace them, and keep the scored piston?

Rings have to seal against the side of the cylinder and the groove of the piston, so you need a pair of good surfaces. Not to mention, if a piston has scoring, it is almost 100% that the ring and ring grooves have taken damage too.

 

[Mad Professor]

The loss of the scored material is also going to effect P/C clearance.

 

[wildwes]

The loss of the scored material is also going to effect P/C clearance.

Which affects how straight the piston is in the bore, in turn affecting ring seal, correct?

 

[old guy]

Last year, just fer ta see what would happen, I put two saws back together with the scored piston. A 353 & a 346, both were scored at the ex. port, rings were stuck in the groove, cyl. had some alum.
I cleaned up the cyls., ran a flat file over the pistons so they slid in the cyl. freely.
Then I broke the rings loose, one had obviously lost it's tension when compared to a new ring. I used a broken ring to clean the ring grooves. I put new rings in making sure they turned freely. Put the saws back together and ran several tanks thru them both, compresion came up to normal. All seems well! YMMV

 

[Mad Professor]

Last year, just fer ta see what would happen, I put two saws back together with the scored piston. A 353 & a 346, both were scored at the ex. port, rings were stuck in the groove, cyl. had some alum.
I cleaned up the cyls., ran a flat file over the pistons so they slid in the cyl. freely.
Then I broke the rings loose, one had obviously lost it's tension when compared to a new ring. I used a broken ring to clean the ring grooves. I put new rings in making sure they turned freely. Put the saws back together and ran several tanks thru them both, compresion came up to normal. All seems well! YMMV

What was the P/C clearances? Did they make the slap slap piston wack?

@Vintage Engine Repairs posted a nice video/audio of that sound.

 

[cookies]

the piston holds oil against the cylinder under the rings, when scored its shape deforms and no longer holds the same oil layer in the scored area or properly center the ring face square to the bore at the same angle the whole stroke. You will get hot spots on the cylinder walls and on the piston and see rapid skirt and ring land wear as well as aluminum ingestion throughout the crank case. Yes it will run and cut, often without notice of performance decline until one day it is hard to start, shuts off at idle. You run the risk of damaging the whole powerhead instead of replacing the piston imo.

 

[old guy]

What was the P/C clearances? Did they make the slap slap piston wack?

@Vintage Engine Repairs posted a nice video/audio of that sound.

I can't hear piston slap, never have, did not check clearence.

 

[flatsfisher]

how about a vertical scratch in the cylinder wall on the exhaust side? I have a Stihl like this, it has good compression and runs and cuts fine. kind of amazes me.

 

[Mad Professor]

how about a vertical scratch in the cylinder wall on the exhaust side? I have a Stihl like this, it has good compression and runs and cuts fine. kind of amazes me.

How deep? and above or below ex port?

 

[flatsfisher]

can't remember, had it for quite a while, got it with several other saws. will have to pull the muffler and look again. it's pretty visible and can feel it with a dental pick.

 

[thenne1713]

If the rings of the piston hit the side of the cylinder, why does it matter that the piston is scored? It never hits the side of the cylinder...

If the rings are scored, cant you just replace them, and keep the scored piston?

Actually, piston w/ manufactured grooves would work fine, as long as cylinder smooth and rings seal properly. Problem comes from heat/ lack of lube scoring, =piston aluminum transfers to the cylinder wall, = compression loss and seizing.

 

[Mad Professor]

Actually, piston w/ manufactured grooves would work fine, as long as cylinder smooth and rings seal properly. Problem comes from heat/ lack of lube scoring, =piston aluminum transfers to the cylinder wall, = compression loss and seizing.

But the manufactured grooves, are horizontal to retain oil. Lot of new pistons have the horizontal grooves, not vertical.

There was a technique called "knurling", used on worn out, old time tractor/truck/car pistons. The worn piston was knurled in a lathe, and the raised portions of the knurl took up the extra P/C clearance space.

Never heard of anyone trying it with a saw/2-stroke piston? Who's got a machinist lathe here, and some worn out pistons to play with? Might be something to try on old saws , where pistons are NLA.

 

[thenne1713]

I have heard of knurling worn brass valve guides, never heard of same for any aluminum?

 

[wildwes]

Knurling pistons was generally pretty much a temporary bandaid wasn't it? Or did it actually hold up OK?

 

[Mad Professor]

Knurling pistons was generally pretty much a temporary bandaid wasn't it? Or did it actually hold up OK?

We have people here, on AS, that says just sand it off....

What might work better? I started on P/Cs in the early 1970s.......before I learned some..........

 

[Oletrapper]

The loss of the scored material is also going to effect P/C clearance.

Not to mention possible bearing damage. jmho

 

[old guy]

Not to mention possible bearing damage. jmho

I don't care about everybodys guesses, I will wait to see what happens to the ones I put together for that purpose. Actually 3 now as I just did a 365 also, also the ring on this one APPEARD to be ok so it was reused.
Sometime down the road we should have edjucation with evidence. I hope I live long enough.

 

[wildwes]

We have people here, on AS, that says just sand it off....

What might work better? I started on P/Cs in the early 1970s.......before I learned some..........

I was curious, I've never encountered knurled pistons firsthand, just heard of folks doing it, and I always heard that it was a very temporary solution. But you know how it goes believing everything that you hear...

I know in the antique engine community, where you have massive cast iron pistons that only see 400-1000rpm max, it isn't unheard of to bore an engine 20 thousandths or so over, braze a band of bronze filler around the skirt, turn it to the proper diameter, and install oversized rings filed to the proper end gap. Works like a champ on those slow turning engines, if done properly.
I also have heard of folks using jb weld to fill pits in the bores on those same engine. I don't care to even think about putting JB weld in the cylinder of an engine.

 

[stihltech]

It was a repair procedure on car engines went I was in automotive school in 75. Had an oscillating machine that did it. Was supposed to hold oil in on a 4 stroke. Would it do the same on a 2 stroke? Don't actually know and can't say it was recommended.