Stihl MSA 120 C & MSA 140 C Chain: Size, Part Number and Fitting

Check your bar first: 10” or 12”

Stihl’s manual approves two bars for both saws, the 10” and the 12” Rollomatic E Mini, and they take different chain loops. The pitch and gauge are the same; only the drive-link count changes. A 64-link chain will not tension on the 10” bar, and a 56-link chain will not reach around the 12” one.

The quickest check is the old chain’s box or tag. On Stihl USA’s PM3 listing the part number ends in the drive-link count: 3670 005 0064 is the 64-link loop, 3670 005 0056 the 56-link one. If the box is gone, lay the chain flat and count the drive links, the teeth that ride inside the bar groove.

Changing the chain

These saws have no bar nuts to undo. The sprocket cover is held by a single wingnut, and chain tension is set by a tensioning gear on the bar (manual pp. 49–50):

1. Switch the saw off, engage the chain brake and take the battery out.
2. Lift the wingnut grip, turn it counterclockwise and take off the sprocket cover.
3. Turn the tensioning gear clockwise as far as it goes. The chain is now slack and lifts off the bar.
4. Fit the new chain in the bar groove with the arrows on the tie straps on top of the bar pointing in the direction of rotation, and seat the drive links on the sprocket.
5. Put the bar back so the tensioning gear faces you and the screw head sits in the slot in the bar.
6. Release the chain brake and turn the tensioning gear counterclockwise until the chain sits snug on the bar.
7. Refit the cover flush with the saw. If it won’t sit flush, turn the adjusting wheel slightly so its teeth mesh with the tensioning gear, then tighten the wingnut and fold the grip down.

The manual says the bar can go on either way round, even with the logo upside down. That is normal on these saws.

Setting tension

A new chain stretches during its first cuts, so check it often at first. With the battery out, loosen the wingnut two turns, hold the bar nose up and turn the tensioning screw until there is 1–2 mm of sag at the middle of the bar and you can still pull the chain along with two fingers. Tighten the wingnut while still holding the nose up (manual p. 50).

When a new chain doesn’t fix it

A worn sprocket or bar will wear out a new chain quickly. Stihl’s manual gives two limits:

• Sprocket: replace it if the wear marks are deeper than 0.5 mm (p. 52). These saws use a 6-tooth 1/4” P sprocket.
• Bar groove: replace the bar if the groove is shallower than 4 mm, or if it is pinched, splayed or damaged (pp. 52 and 61).

Sharpening

Stihl’s manual asks for a round file that matches the chain pitch, held at a right angle to the bar, at a 30° filing angle, filing from the inside of each cutter to the outside (p. 59). For 1/4” P that is a 3.2 mm file. Stihl also recommends having its chains sharpened by a servicing dealer.