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I hear it in the shop every day. "How tight should I tighten the chain on my chain saw? Or, "I make one cut and the chain gets loose, why?"
Had that problem?
When tightening the chain tension, you need to lift up on the bar tip while adjusting chain tension and tightenng the bar nut(s). Most saws have a bit of up and down play in the bar and when lifted, the chain loosens. Gravity pulls it down when the bar nut(s) are loose so many bars are tighted in the down position. Trouble is, the pressure of loading the chain into the cut pushes the bar up to it's upper position and you end up with a loose chain. Bear in mind, all chains will stretch a bit with use, cheaper chains a lot more. Most new chains will have a bit of break in stretch as well. In addition, a chain will stretch a bit when heated up from heavy cutting but will shrink back when cooled, so never adjust the tension when it's hot. Damage to the crank and crank bearings can occur when it shrinks back down.
Adjusting the tenson? What you want is to be able to lift the chain from the center top of the bar with light pressure and see about half of the drive tooth, but at the same time see no movement of the chain on the bottom of the bar. This is also a good time to hand spin the chain to see if it rotates freely. Wear a good pair of gloves while handling the chain for self protection. If the chain doesn't spin freely but hangs and sticks or if the tension tightens and slackens as spun, you need something replaced due to wear. Either the clutch sproket, bar nose sprocket or chain has wear, or damage, or a combination of any of them do. Continued use of these worn or damaged parts will cause premature failure of the crank, crank bearings or crank seals. Not a good thing.