Stihl 026 - .015" Ring End-Gap!?

[SteveSr]

Hello,

I just received my Ebay P&C to replace the one that I screwed up trying to salvage. The cylinder is a Mahle 44ZN19-W6 if that means anything to the Stihl 1121 gurus. The cylinder has some wear marks but nothing that you can feel and I expect it to clean up nicely with some red Scotchbrite. I am assuming that this will be all that is needed to prepare it to seat some new rings.

The piston, on the other hand, was probably alright until the dismantler either tried to run the saw on a shot of gas or just measure the compression and managed to scratch mostly the intake skirt with debris in the intake. With dial calipers the cylinder measures 1.7325" while the piston skirt measures 1.7290" This indicates a clearance of .0035". If the skirt wasn't scratched would you reuse this piston?

The real eyeopener came when I measured the ring end-gap and would like to verify that I did it correctly. I inserted the ring into the bore to a point above the exhaust port where the ring was in contact with the whole cylinder. I was really surprised to measure .015" which seems pretty large. I know that Stihl doesn't publish any piston or ring specs so I am wondering what some acceptable numbers are for these measurements? I also just inserted the ring into the bottom of the bore near the base and measured a ring end gap of .014" This would indicate to me that the cylinder has very little wear on it. Is this correct?

Thanks,

Steve

 

[huskihl]

I'd say your piston to cylinder clearance is about right. When you stuck the ring in the bore, did you square it up with the piston? .015" ring gap is a bit big, but will be fine. Some manuals say .040" is the service limit.

Post up a pic of your piston. Chances are it's fine the way it is, or maybe 2 minutes with some sandpaper will fix it

 

[SteveSr]

When you stuck the ring in the bore, did you square it up with the piston?

Yes, I did.

Some manuals say .040" is the service limit.

That doesn't sound right for a small bore (44mm) engine. Are you talking Metric or English? .040" is larger than 1/32". Maybe 0.4mm which is .0156"? In this case these rings are shot!

Post up a pic of your piston.

I knew somebody was going to ask...



Thanks,

Steve

 

[huskihl]

No. Not metric. .040". It was in a husky manual. You could file or sand off the high spots on that and it would be fine. Or if it bothers you, buy a new piston

 

[SteveSr]

Anybody have any more thoughts or comments?

 

[SteveSr]

Well to update I got a new OEM piston and rings. I took the top ring off of the new piston and put it in the bottom of the cylinder and measured the ring end gap. I measured about .014 with a feeler gauge. So it appears that the original rings were not worn appreciably.

So rather than use the new piston I am going to use the one that came with the used cylinder since they were originally a "set". So what is the best way to clean up the old piston (pics above)?

What grit of sandpaper should I use? Should I use any kind of flat backing for the paper like a piece of glass? What direction is the preferred sanding direction in relation to the cylinder bore? Parallel, perpendicular, or cross hatch?

Finally, do I need to flush out the crankcase to get rid of any remaining aluminum debris? What should I use? Just some premix?

Thanks,

Steve

 

[DrewUth]

I generally set ring gaps to .011"...so .015 is a bit big but not a deal breaker by any means.

 

[jb-chainsaws]

For the piston don't sand it too much, you just want to knock off any high spots rather than trying to sand out all of the scoring, or you'll end up with too much clearance between the skirt and cylinder and get some nasty piston slap

 

[SteveSr]

I generally set ring gaps to .011"...so .015 is a bit big but not a deal breaker by any means.

Well, there is not much "setting" that I can do as the gap is already this wide with a NEW OEM piston & rings. The piston is marked AB and the cylinder is marked "B". This may explain some of it. Anybody know what a "B" selection cylinder is size wise?

 

[DrewUth]

Right- nothing for you to do here.

 

[67L36Driver]

Back in the olden days, with verticle 1/16" locator pins and square end gaps the gap was .070" (ish).

They ran fine at that.

So, count your blessings and run it.

 

[Marshy]

Weisco snowmobile pistons call for 0.004" per inch of bore minimum on the ring clearance. It might be larger for air cooled chainsaw engines but 0.014 doesn't seem unreasonable. I wonder what the max end gap is before Husky or Stihl considers the ring to need replacing... Anyone know?

 

[SteveSr]

Weisco snowmobile pistons call for 0.004" per inch of bore minimum on the ring clearance. It might be larger for air cooled chainsaw engines but 0.014 doesn't seem unreasonable

With the Weisco numbers the gap is huge in comparison.

I wonder what the max end gap is before Husky or Stihl considers the ring to need replacing... Anyone know?

That is what I was hoping to find out!

 

[huskihl]

Weisco snowmobile pistons call for 0.004" per inch of bore minimum on the ring clearance. It might be larger for air cooled chainsaw engines but 0.014 doesn't seem unreasonable. I wonder what the max end gap is before Husky or Stihl considers the ring to need replacing... Anyone know?

That is what I was hoping to find out!

See post #2. .015 is perfectly acceptable

 

[Marshy]

With the Weisco numbers the gap is huge in comparison.


That is what I was hoping to find out!

Tolerances for air cooled vs liquid machines are way different. Typically more clearance is needed for air cooled motors as they see more temperature variation than liquid cooled engines...

 

[SteveSr]

Tolerances for air cooled vs liquid machines are way different. Typically more clearance is needed for air cooled motors as they see more temperature variation than liquid cooled engines...

I sort of suspected sleds were liquid cooled. Don't see many of them in NC!

 

[Marshy]

I sort of suspected sleds were liquid cooled. Don't see many of them in NC!

There are air cooled ones also. The vast majority of them prior to the early 80's were strictly air cooled.

 

[SteveSr]

There are air cooled ones also. The vast majority of them prior to the early 80's were strictly air cooled.

Sort of like my Yamaha RD350!

 

[Marshy]

Sort of like my Yamaha RD350!

Those are nice bikes.

 

[fossil]

I assume the ring end gap calculation should always consider the locating pin dia in the ring land on butt cut rings. A lot of the older saws have very thin steel rings.

Do you guys take that into account when checking ring gap or am I off base?