Why is it important to match chain pitch and gauge on a chainsaw?

You have to match both the pitch and the gauge exactly, no wiggle room. If either is wrong, the chain won’t sit properly in the bar, won’t get oiled right, and can come off or even snap. Check the bar stamp and chain package for both numbers, or bring the old chain to the store. Measure with a caliper to avoid mixing up similar-looking sizes like .325 pitch and 3/8" pitch.