Piston/Cylinder tolerances

[TheTone]

I've done a little searching in the archives and haven't found what I'm looking for. When doing automotive engine rebuilding it's easy to find tolerances - piston-cylinder clearance, out-of-round, piston ring gap, etc. Maybe it's because cylinders are so easily replaced, but nowhere have I seen any guidelines for when to put a piston and cylinder back together after cleanup for another "50,000 miles" or to toss them in the recycle bin. Say you've got an original piston and cylinder with a little aluminum transfer and scuffing and get them cleaned up. How do you know if they are reusable? What if you have a cylinder where you can see some faint lines in the right light but you can't feel them with your fingernail? Any references or experience than might be helpful?

 

[happysaws]

I've done a little searching in the archives and haven't found what I'm looking for. When doing automotive engine rebuilding it's easy to find tolerances - piston-cylinder clearance, out-of-round, piston ring gap, etc. Maybe it's because cylinders are so easily replaced, but nowhere have I seen any guidelines for when to put a piston and cylinder back together after cleanup for another "50,000 miles" or to toss them in the recycle bin. Say you've got an original piston and cylinder with a little aluminum transfer and scuffing and get them cleaned up. How do you know if they are reusable? What if you have a cylinder where you can see some faint lines in the right light but you can't feel them with your fingernail? Any references or experience than might be helpful?

You mean like this??

This saw runs perfect.
The power comes from the rings, the piston is mainly there to guide the rings.

 

[Mad Professor]

The lined cylinders don't wear much,they are very hard, they get gouged and chipped. The pistons wear mainly on the skirts and that is the source of the knocking noise, piston slap. If you have piston aluminum transfer, that is not normal wear, that is due to excess heat and/or lack of lubrication.

I've not seen any wear guidelines for OEM cast pistons but new ones I've measured (50-70cc saws) have been about 0.0025" P/C clearance at the piston skirt. They will run, and slap, for a long time a lot looser but you take a chance of the skirt breaking eventually.

 

[TheTone]

Happysaws - at first I thought you were joking. That saw really runs? It looks like the gouge stops just short of the top of the piston, which may explain it. Do you know what caused that?
Mad Professor - the .0025 clearance figure helps. Is that a direct measurement, or did you subtract piston diameter from cylinder diameter? It would be nice to find a wear tolerance limit, since the downside sounds like catastrophic failure.

 

[happysaws]

Happysaws - at first I thought you were joking. That saw really runs? It looks like the gouge stops just short of the top of the piston, which may explain it. Do you know what caused that?

Nope, not joking.
This saw really runs, and it runs strong.
It pumps somewhere around 180 PSI.
Here's a better pic of the top of the piston.

I believe something got in the engine and gouged the piston and cylinder, and the previous owner replaced the cylinder and rings but did not replace the piston.

(Wish I knew how to resize pics... Sorry)

 

[hotshot]

Yep, sometimes it's not a perfect chainsaw world.

This piston, shown with new rings, is still in another great runner.

 

[Mad Professor]

Happysaws - at first I thought you were joking. That saw really runs? It looks like the gouge stops just short of the top of the piston, which may explain it. Do you know what caused that?
Mad Professor - the .0025 clearance figure helps. Is that a direct measurement, or did you subtract piston diameter from cylinder diameter? It would be nice to find a wear tolerance limit, since the downside sounds like catastrophic failure.

You can use a bore gauge or snap gauge on the cylinder and a micrometer on the piston. For a crude measurement that works pretty good just use feeler gauges

 

[lecbr600]

As we are talking about P/C tolerance, I'm wondering what should be the gap between ends of piston ring when it is only fitted inside the cylinder.
What are the Min value (piston ring brand new) and Max value (max allowable value before rings replacement)?

Thanks

Inviato dal mio Moto G (4) utilizzando Tapatalk

 

[Little Al]

Over the last 40 years of working servicing saws for logging companys, the piston skirt clearance has been in the 20/25 thou region with a rebuild when wear exceeds 30/32thou the ring clearances have always been set to 4/6thou per 25.4/1 "of bore size,with 5 or more thou over that reading being replace time excess clearance on piston skirt is not terminal as the motor will rattle for a goodly time before any bits are niggled off the skirt, but it's not good practice, compression reading is also a good indicator Over that time I found that saws with a richer oil fuel mix ie 32or 40/1 ratio to wear slightly less than 50/1mix for comparable run time This is just my take though others may have different take on things

 

[67L36Driver]

This came out of a 'runner'.

I measured .070" skirt clearance. [emoji15]

Doubtful it would last much longer before the skirt disintegrated.

 

[Mad Professor]

Over the last 40 years of working servicing saws for logging companys, the piston skirt clearance has been in the 20/25 thou region with a rebuild when wear exceeds 30/32thou the ring clearances have always been set to 4/6thou per 25.4/1 "of bore size,with 5 or more thou over that reading being replace time excess clearance on piston skirt is not terminal as the motor will rattle for a goodly time before any bits are niggled off the skirt, but it's not good practice, compression reading is also a good indicator Over that time I found that saws with a richer oil fuel mix ie 32or 40/1 ratio to wear slightly less than 50/1mix for comparable run time This is just my take though others may have different take on things

Wow! That is 5 times the clearance I set up up small block chevys with forged pistons (0.004-0.005").

I'll agree good rings will help steady a rocky piston.

 

[Sepia]

Re: resizing pics...

-Right click your picture on the computer. A menu will pop up - select "open with"...
-Another menu will pop up. Select "Paint"
-That huge pic will immediately open in Paint - wow! That's big!
-Up at the top of the page, you will see lots of boxes/menus with tools that can change just about anything on your pic.
-Select "resize". A little box will open where you will see "horizontal" and "vertical". Both will say "100" next to them.
-Click inside the little "100" box and change it to 50. Do this for both "100" boxes. Now your pic is a manageable size.
-You can make it bigger or smaller by a number other than 50 - say 10 for a tiny pic, or 70 for something smaller than original, but still pretty big.
-You can mess around with the numbers, and if you don't like the result, you can "undo" by clicking the blue arrow the curves counterclockwise. It is at the very top/left of the page.

 

[Mad Professor]

This came out of a 'runner'.

I measured .070" skirt clearance. [emoji15]

Doubtful it would last much longer before the skirt disintegrated.

Wow! That intake skirt is just an eggshell. Got a picture of the whole skirt surface?

What model saw was it?

 

[hotshot]

Wow! That is 5 times the clearance I set up up small block chevys with forged pistons (0.004-0.005").

I'll agree good rings will help steady a rocky piston.

He meant 20/25 ten thousandths....or 0.002" to 0.0025" new cylinder clearance

Ring gap is normally 0.004" to 5 per inch diameter for a normally aspirated engine. I file them open to 0.008" to 0.010' new,
if you ever have to open them up at all...

 

[Little Al]

Wow! That is 5 times the clearance I set up up small block chevys with forged pistons (0.004-0.005").

I'll agree good rings will help steady a rocky piston.

"Ah" but your small block Chevey'sIs liquid cooled not air, & are lubed by pressure fed oil system not the hit & miss spray around fuel with 2% if you follow modern thinking of oil per litre/gallon of fuel & buzzing at 2 or more times the Max Revs You can reduce the clearances, to much though can cause a sudden stop but as said each to his own & with those clearances the saws working 5/6days a week 10months a year still have within tolerance compression & performance after 2years which is when we trade for new

 

[ironman_gq]

I guess my rule of thumb was always .001 per inch of bore diameter on a new motor. Not really a concern for a chainsaw as they don't rebore cylinders or usually offer oversize pistons.

 

[Mad Professor]

While we are on the subject, Stihl sized cylinders and pistons A or B. I forget which size was a little bigger? You were not supposed to mix and match. Replacement piston kits were mostly marked A/B which were a compromise designed for A or B cylinders. I don't have any newer Stihl saws, are they stihl making P/C assys this way? On the really older Stihls were there any other sizes ("C")?

I see Stihl Meteor brand pistons for sale on the internet, some are clearly marked A or B. Do people bother to check clearance? Are their cylinders marked A or B? Can you order a replacement specifying A or B?

I have an Italian NOS 038M Precision Tooling P/C kit which is stamped "D" on both cylinder and piston (piston also stamped ex. with arrow) so other manufacturers also sized P/C kits.

A little too much clearance will just slap around but too little and you are asking for a problem when it heats up. At least check things with feeler gauges when checking wear or before putting together. Maybe people who don't do this, is the reason why so many "rebuilt" saws, fail early on.

 

[67L36Driver]

Wow! That intake skirt is just an eggshell. Got a picture of the whole skirt surface?

What model saw was it?

Jonsered 510 or thereabouts.

Fellow who sold it to me claimed it was good to go.[emoji33]

 

[hotshot]

While we are on the subject, Stihl sized cylinders and pistons A or B. I forget which size was a little bigger? You were not supposed to mix and match. Replacement piston kits were mostly marked A/B which were a compromise designed for A or B cylinders. I don't have any newer Stihl saws, are they stihl making P/C assys this way? On the really older Stihls were there any other sizes ("C")?

I see Stihl Meteor brand pistons for sale on the internet, some are clearly marked A or B. Do people bother to check clearance? Are their cylinders marked A or B? Can you order a replacement specifying A or B?

Stihl used to grade them all the way up to E, and the A is the smallest . Get ahold of a Stihl 041 service manual & it explains it all.
I do not see the grade marks now in the newest saws, must be that the modern CNC machines are turning them out at spec.

You can go to Meteor's website to see the different A & B part numbers for popular Husky & Stihl piston offerings, but most folks are
just glad to find either sized one for their wasted saw...

 

[TheTone]

Special thanks to Little Al and Hotshot. Your years of experience have been very helpful to me and I'm sure others too.