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A chainsaw that cuts in a curve almost always has one of two faults: cutters sharpened unevenly, so one side removes more wood and steers the bar, or a worn bar rail that lets the chain lean out of vertical. Diagnose it in one step — fit a known-good chain. If the cut straightens, the old chain was the problem and needs its cutters matched. If it still curves, the bar is worn and needs dressing or replacing. A uniformly dull chain cuts slowly but straight, so slow cutting alone is not this problem.
How do you tell if uneven cutters are the cause?
Look at the chain in good light and compare the cutters on the left with those on the right. On a chain that steers, one set is visibly longer from the rivet to the cutting corner than the other, and the saw pulls toward the side with the shorter cutters because the longer ones are doing more work. Measure across several cutters with a caliper if you have one; by eye is often enough once you know to look.
The fix is to find the shortest cutter on the chain, then file every other cutter back to match it, using the same number of strokes on each and the factory top-plate angle shown by the witness marks on the cutter tops. That means removing more steel than feels necessary from the longer side, and it is the only way the chain will track straight again. The full method is in how to sharpen a chainsaw chain.
How do you tell if the bar is worn?
Take the chain off and inspect the rails. Three checks find nearly every worn bar. Lay a straightedge flat against the side of the bar spanning the rail — it should sit square with a clear gap below, and if it rocks, one rail is taller than the other. Run a fingertip along the rail edges for burrs and flaring, which develop from a chain running under too much tension or with poor lubrication. And check the groove width against the drive link thickness: a groove worn wider than the drive links lets the chain tip sideways under load, which is exactly what produces a curved cut.
Minor rail unevenness and burrs can be dressed with a flat mill file, taking both rails down to the same height and squaring the edges. A groove worn wide, a cracked rail, or a nose sprocket with visible play means the bar is finished — replace it, and clean the oiler hole in the new one before fitting.
What causes what?
| Symptom | Likely cause | Fix |
|---|---|---|
| Cut curves consistently to one side | Cutters uneven left to right | Match every cutter to the shortest one |
| Curves the same way with a brand new chain | Worn or uneven bar rails | Dress the rails, or replace the bar |
| Cut wanders unpredictably | Chain too loose, or groove worn wide | Retension; measure groove against drive links |
| Slow cutting, fine powder, but straight | Uniformly dull chain | Sharpen normally — not this problem |
| Cut curves and the saw is hot | Over-tight chain plus rail wear | Correct tension, dress rails, check the oiler |
| Curves after hitting a rock or nail | Cutters damaged on one side only | Match all cutters to the shortest damaged one |
| Chain leans visibly in the groove | Groove worn wider than the drive links | Replace the bar |
How do you stop it happening again?
Count your file strokes and keep them identical. The single biggest cause of uneven cutters is filing by feel — a few extra strokes on the cutters you can reach comfortably, fewer on the awkward ones. Do all the cutters facing one direction, then turn the saw around and do the others, using the same count throughout.
Flip the bar at every chain change. Rails wear unevenly because gravity and cutting pressure load one side more, and turning the bar over evens that out and roughly doubles its life. Clean the groove and the oiler hole while it is off.
Set tension correctly and check it often. Over-tightening is what grinds rails away fastest. Correct tension is snug against the underside of the bar with the drive links staying seated when the chain is pulled away — see how tight should a chainsaw chain be.
Keep the oiler working. A dry chain wears rails at speed. Check for a thrown line of oil onto a light surface at part throttle before every session; a saw running dry will also smoke, which is covered in why is my chainsaw smoking.
Keep the chain out of the dirt. Cut on a sawhorse or log stand rather than on the ground. One contact with soil or gravel dulls the cutters on whichever side hit — which is precisely how a chain becomes uneven.
Is a crooked-cutting chainsaw dangerous?
Yes, and not only because the cuts are untidy. A saw that steers is a saw fighting you, and the natural response is to twist the bar to correct it — which loads the chain sideways, increases the chance of the chain grabbing or the bar pinching, and puts the nose somewhere you did not intend. It also encourages pushing, and force is what turns a routine cut into an uncontrolled event.
Kickback is that uncontrolled event and it is the mechanism behind most serious chainsaw injuries: when the upper quadrant of the bar nose contacts wood or any solid object, the chain cannot pull through and instead drives the bar backward and upward toward the operator's head and shoulder in a fraction of a second, faster than any human reaction. A wandering cut makes unplanned nose contact far more likely. Never cut with the nose, keep the tip in view, wrap the thumb of your leading hand fully under the front handle, stand slightly to the side of the cutting line, and check the chain brake before every session — detail in what causes chainsaw kickback.
Fix the saw before the next job rather than working around it. And whatever you are cutting, OSHA lists a hard hat, safety glasses, hearing protection, heavy gloves, cut-resistant chaps and boots covering the ankle as standard chainsaw kit — see best chainsaw chaps. Never cut from a ladder or above shoulder height, and hand any leaning, split, hollow or power-line-adjacent tree to a certified arborist.
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