Stihl Ring End Gap Wear Limits?

[SteveSr]

Hello,

Stihl, to my knowledge, doesn't publish wear limits when it comes to allowable ring wear limits. They just want you to buy a new top end! So I have a 4134 series FS-120 brushcutter that I suspect has low compression.

Here are the specs:

• Displacement: 31cc
• Bore: 35mm
• Stroke: 32mm

I pulled the top end off and stuffed the top ring into the cylinder to a point above the exhaust port. I then used a combination of feeler gauges to measure a ring end gap of .045" which seems kind of big. What do you think?

Thanks,
Steve

 

[traffic903]

45 thou is HUGE mate. Time for a new ring at least. What does the piston look like?

 

[SteveSr]

45 thou is HUGE mate. Time for a new ring at least. What does the piston look like?

Piston looks good and so does the jug. No scoring. This thing must have a LOT of hours on it. Surprisingly it still starts easily which I wouldn't have thought for an engine with low compression. The Elastostart makes it hard to tell compression by pulling on the rope.

I guess that I need to check the ring gap at the bottom of the cylinder to see if the jug is also worn.

Thanks,
Steve

 

[Steve Taylor]

That's bigger than the allowable limit on a big block chevy.
Think you have a problem.

 

[SteveSr]

That's bigger than the allowable limit on a big block chevy.
Think you have a problem.

I think you're right! So does the .004"/inch of bore "rule of thumb" that I have read on this site apply in this case?

 

[SteveSr]

I just checked the gap clearance at the bottom of the jug and got the same result so it would appear that the jug is not worn excessively. However since this is a quad transfer port jug the ring was only touching on the intake and exhaust side of the cylinder.

 

[traffic903]

I think you're right! So does the .004"/inch of bore "rule of thumb" that I have read on this site apply in this case?

I would think that for a 35mm bore, 5-7 thou would be an acceptable range for end gap. Can you post a piston pic? As close up as you can manage.

 

[Al Smith]

I'd just pop a new ring on it and be done with it .

 

[CR888]

I'd just pop a new ring on it and be done with it .

Correct a few bucks and a few minutes and problem solved.

 

[backhoelover]

Stihl ring gap limit is in the fault analysts manual. IIRC .10

 

[SteveSr]

I would think that for a 35mm bore, 5-7 thou would be an acceptable range for end gap. Can you post a piston pic? As close up as you can manage.

Here you go...

 

[SteveSr]

Stihl ring gap limit is in the fault analysts manual. IIRC .10

Thanks, I'll take a look.

 

[traffic903]

New rings and run it mate. If you want to go all out, change the piston too. Let us know how you get on.

 

[SteveSr]

Stihl ring gap limit is in the fault analysts manual. IIRC .10

Sorry, this information is not in my 26 page version of the analysis manual. Do you, perhaps have a newer version that has this information. My version is an old paper manual that was scanned and has no identifying revision/date on it.

Thanks,
Steve

 

[SteveSr]

New rings and run it mate. If you want to go all out, change the piston too. Let us know how you get on.

Rings and base gasket going in the next parts order!

Thanks,
Steve

 

[AlfA01]

Steve,

On your other post I had mentioned ghosting. At least that is what I call it. See your pic I reattached below. In the black box is wear on the piston skirt that I call ghosting. Whereas, in the blue areas you can still see the original machine marking from when the piston was manufactured.

I argue that due to high piston to cylinder clearance tolerances, the piston encounters a floating effect as it cycles in the cylinder, which tends to wear the skirt area of the piston significantly more than the upper portion.

At the top of the piston and the sides, this effect is reduced due to the rings holding the piston top correctly in the cylinder, and the fact that the piston doesn't experience torque force from the crankshaft side-to-side.

Due to this ghosting wear, you will most probably have more wear in the cylinder lower portion than the upper. You need to measure as far into the cylinder the ring gap, and measure with a new ring, not the old one. Also, mic the piston and the cylinder to see what the piston to cylinder clearance is. Remember also, the bottom of the cylinder is where the skirt will ride in the cylinder. This area is more insignificant of a measurement, as most likely the rings will never cycle in that portion of the cylinder. There is still a need to have a correct tolerance in that area, I'm just saying to get an accurate ring gap measurement, you should measure where the rings will ride in the cylinder.

I've also attached a pic of my saw piston. This pic is taken at BDC. That means, that is the lowest point in the cycle that the rings will be in the cylinder at any given time during operation. You need to make sure you measure your ring
gap somewhere at and above that point in the cylinder to be entirely accurate with how the machine will function with the piston, ring and cylinder installed. If you have a good measurement with a new ring in that region of the cylinder, then the cylinder is serviceable. If you don't, you'll need a new cylinder.

Hope this helps....Cheers, Dan

 

[SteveSr]

Dan,

Yes, the skirt will wear more than the crown. Unfortunately, my measurement tool arsenal is limited to a vernier dial caliper which is kind of crude for measuring pistons and cylinders.

I don't have a good number for piston skirt clearance limit either. I don't recall seeing it in the failure analysis manual but I'll check.

I'll check the new ring end gap when I get it somewhere above the exhaust port. Earlier measurements showed non detectable cylinder wear.

Thanks,
Steve

 

[SteveSr]

It has been a while. I bought a set of Caber rings from the Greek guy on Ebay for about $13. Measured the ring end gap this time at about .008" above the exhaust port. Put it all back together and measured the compression at about 120 PSI with my new Mityvac compression tester.

I have a second FS-120 with considerably fewer hours on it and it measures about 130-135 PSI. The compression on the first one will probably come up once it has been run a while.

BTW, I later found out that you can buy an OEM complete piston kit including rings and pin for about $22 through Midwest Stihl (Bryan).

Thanks,
Steve