Low Compression

[tapritchett]

I have a 026 Pro, that is down on compression. Cylinder and piston actually look good, will new rings fix that??

 

[redline4]

Carefully remove the rings and using the piston, center them up in the bore. Use feeler guages to measure the ring end gap. If its excessive, new rings might help.
Wouldnt hurt to measure out piston and jug if possible. Piston could have wear or out of round.

 

[SOS Ridgerider]

Moved this thread to Chainsaws, so you’ll see some more traffic.

 

[5155]

Same issue with my stihl weed eater, glad you asked.

 

[Wood Doctor]

You could try replacing the rings to see if that cures the problem. However, the total work required to do that is not much different from rebuilding the top end -- piston, rings, and cylinder. I usually just rebuild the top end after a loss in compression.

Beware also that some rings do not work well with existing pistons and cylinders. A complete piston and cylinder kit usually avoids that problem.

 

[a. palmer jr.]

I think while I had it apart I'd just replace the top end, same amount of work and not much more expensive if you shop around...

 

[cuinrearview]

A set of Caber rings on ebay is $8.89. Even if your originals prove OK by the time I pulled a top end I'd just throw a new set on Tom. That is, unless your piston is excessively worn. You said the exhaust side looks good. You can pull the carb and take a peek at the intake skirt. You can also see the intake side of the cylinder wall through the exhaust. I'm not sure what the advice on a new top end is about. Unless you have a lot of ingested crap or an air leak a modern plated cylinder should last through two or three pistons.

 

[BangBang77]

A set of Caber rings on ebay is $8.89. Even if your originals prove OK by the time I pulled a top end I'd just throw a new set on Tom. That is, unless your piston is excessively worn. You said the exhaust side looks good. You can pull the carb and take a peek at the intake skirt. You can also see the intake side of the cylinder wall through the exhaust. I'm not sure what the advice on a new top end is about. Unless you have a lot of ingested crap or an air leak a modern plated cylinder should last through two or three pistons.

Agreed. If the plating looks good, ring it and run it.

I would throw a new intake boot, fuel line & filter, and impulse line on while I had the top end off though.

 

[lehman live edge slab]

If your pulling it that far down I’d just put a new meteor piston set in and call it a day. Good fresh rings and no worry about the piston skirt being worn. As far as the earlier mention of a top end not being much more than a re-ring or piston change I don’t know what top ends he shopping but a quality oem kit is pricey. The 50-100$ Chinese top ends aren’t worth 50 cents to me.

 

[drf256]

How do you know it’s “down on compression” exactly?

Is is by feel, or by a gauge telling you it’s too low?

Generally, if the piston looks clean and the saw runs and starts well (and has an oem top end) it will still be within specs. A compression gauge can be used as a reference point, but the values can be all over the place when one goes from tester to tester.

Testers for small engines need to have a working shrader valance at the tip (And a tip that’s the size of the spark plug end) to eliminate the possibility that the volume of the line to the tester is being added to the measurement. The error will be magnified in smaller saws because the overall volume of the true chamber is smaller.

 

[SharpFrank]

Same issue with my stihl weed eater, glad you asked.

Sorry for up but which model do you use? I heard that some of Stihl's weed eaters got this problem as a chronic

 

[5155]

Mine is fs 80r
old, beat, and still whackin.
First glich last year.
Rings, carb work and fuel system if I remember correct.
and throttle cable.

 

[Vintage Engine Repairs]

As redline said above, check ring end gap. I wouldn’t be replacing anything without testing it. Test ring end gap at the highest point in the cylinder. If the top ring is more worn than the bottom, but both in spec, you can swap them over. Also test cylinder to skirt clearance. No sense in throwing away a good piston just because you’re in there..

 

[smokey7]

Measure and confirm what's out of spec. Piston to wall about .003 sae and ring gap on a used saw i would like to see less then .010 but would accept up to .015 all sae American measurements. When rings get up to .020 gap its obvious but if they have good tension it shouldn't be down that far on comp.

 

[Vintage Engine Repairs]

Measure and confirm what's out of spec. Piston to wall about .003 sae and ring gap on a used saw i would like to see less then .010 but would accept up to .015 all sae American measurements. When rings get up to .020 gap its obvious but if they have good tension it shouldn't be down that far on comp.

Very much in keeping with what I was told / taught and go by. I have run .020 and still got 150 psi though so I tend to stretch them out a little as most of my saws sit on a shelf - it’s just a hobby repairing them

 

[Cooper264]

I bought a pretty good looking 029 the other day. Piston was in pretty rough shape from what I saw through the exhaust side. Still pulled about 143psi. I was shocked. I will still probably end up throwing a 39 p/c on just for giggles

 

[Vintage Engine Repairs]

Piston ring end gap I go (note this is metric)

New / ideal = 0.1mm of ring end gap per 25mm of bore diameter
Replace at = 0.25mm of ring end gap per 25mm of bore diameter

 

[smokey7]

Its all setup and what the engine will do and for how long I expect it to do it. If you are going to be on the edge or do funny stuff tolerances open up.