How to Check 0-21 Stihl Chainsaw Compression

The Stihl 0-21 chainsaw requires consistent compression for all of the other parts in the engine to continue working. Compression occurs from the ignited fuel and the moving crankshaft, which in turn helps the diaphragm in the fuel pump suck up more fuel for the next stage of combustion. A loss in compression will be quite noticeable, as power will drop sharply, the flywheel may not spin and the engine may lock up entirely. If the compression reading drops during a test, the seals and gaskets around the crankshaft and carburetor will need inspecting.

Combination bar tool
Compression gauge
Work gloves

Instructions

- 1
  Set the Stihl chainsaw on a level surface to check compression. Lift off the rubber boot hooked on to the end of the spark plug. Unscrew the spark plug using the large wrench side of the combination bar tool. Take the spark plug out of the engine.
- 2
  Slide the choke lever into the closed position. Pull on the starter rope three or four times to clear any fuel in the system. Screw the compression gauge into the cylinder hole where the spark plug was.
- 3
  Squeeze the throttle to hold it open. Pull on the starter rope again until the compression gauge meter reaches its peak. Stop pulling and watch the gauge. Pressure readings should maintain a consistent reading of 100 to 150 psi when cold and not drop off rapidly.

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