Many DIYers make the mistake of replacing just the chain when their machine isn’t cutting right. But the drive sprocket, bar and chain work together as a system, and wear on any one of these three components affects the operation of the others. In fact, placing a new chain on a worn drive sprocket can cause rapid chain wear, as well as tensioning and binding problems. Since all chain movement starts at the drive sprocket, check it first.
A drive sprocket wears where it contacts the drive links. Over time the sprocket can develop slight grooves. Deep grooves, however, can be a sign of improper lubrication, excessive chain tension or pushing a chain saw beyond its limit. Remove the chain and bar so you can fully check all the components, starting with the drive sprocket (Photo 1). Next, check the drive links and the tie straps for wear (Photo 2). If you see any wear, replace the chain.
If the drive sprocket and links are good, check the length of the remaining blades (Photo 3).
Next, examine the bar. If the bar has a nose sprocket, rotate it with your finger. It should roll easily with no noise or binding. If not, replace the bar. Finally, check for flat spot wear (Photo 4). A good bar has an elliptical shape. If the curve is more pronounced on one side, replace the bar.
Breaking in a New Chain
Put the chain in a disposable pie pan, pour bar oil over it and let it soak for an hour or so to thoroughly lubricate the pivot points. Install the chain and tension it. Run the chain saw with no load at half throttle for several minutes until the bar and chain are warm. Shut off the motor, let the chain cool and then re-tension the chain.
Maintain a light load (only small limbs and branches) for the first half hour and make sure the chain has extra lube. After a half hour, let the chain cool and then re-tension it. Then you’re good to go.
Be sure the antikickback brake is released— check by moving the chain (with gloved hands). Then remove the drive sprocket cover and swap in the new sprocket (Photo 5). Wrap the chain around the bar with the cutting edges facing forward. Then engage the chain with the drive sprocket. Install the drive cover, lining up the adjusting screw mechanism with the notch in the bar. Snug the retaining nuts just enough to hold the bar but allow you to adjust chain tension. Then tension the chain (Photo 6).
Get the Right Chain
Buying the right chain is more complicated than you might think. Don’t assume any 18-in. chain will work on your 18-in. bar. This chain saw, for example, could have come from the factory with any of several 18-in. chain/bar combinations. To work properly, the new chain must have the same drive link thickness, chain pitch and number of drive links. And the chain pitch must exactly match the drive sprocket. If they’re mismatched, you’ll never get the proper tension.
The easiest way to get the right chain is to take your saw to a dealer. But you can figure out the correct chain specs yourself. You’ll need three numbers: the number of drive links, their thickness (gauge) and the “pitch” of the chain.
Measure the drive link thickness
The chain gauge may be stamped into the drive links. If not, measure a drive link gauge with a micrometer or vernier caliper as shown above. Common sizes are .043, .050, .058 and 063 in. This one measures .050 in. Then count the number of drive links. This one has 62.