Chainsaws

Nancy, A chain coming loose could be one of four possibilities, you didn't mention which model of saw you had, and whether it has a special tensioner or EZ clutch cover with No bar Nuts. I am asuming you have just a standard chainsaw with the normal two bar nuts and a chain tensioner built into the clutch cover.

Here are the four possibilitiies: Chain problem, Bar problem, Drive Sprocket problem, Tensioner problem. You will need to inspect all four of these areas'.

Remove the chain, hold chain in your hand so that you can move each link, every link needs to bend and flex freely with no stiff or stuck links, and the chain should be thoroughly clean and heavily lubricated.

Inspect the bar, the bar needs to fit the drive links of the chain and be the correct gauge that matches the bar, like .325, .375, etc. It is possible the wrong gauge chain was used on the wrong bar, like a .325 gauge will fit in a .375 gauge bar but it will fit very sloppy with a lot of side to side movement. But Its unlikely that both chains were the wrong size unless you purchased a replacement with the same size as the original. The bar has to be in good condition with the bar being perfectly flat, the groove cleaned and the edges of the bar, no burrs or rollover on rails, If there are burrs on the rail edges it will need to be flat filed with a slight chamfer to remove the burr.. Also If you stand the bar on a smooth flat surface it should stand up like standing a quarter on its edge. If the rails are uneven it will fall over to one side, and the chain also would lean to one side too, then the bar will need to be repaired!

Next the drive sprocket, behind the clutch or on some chain saws in front of the clutch. The Spur gear should not have any significantly deep notiches in the gear, If there are deep notches you will need to replace the spur sprocket. If you do replace the sprocket, I would recommend a "Power Mate Rim and Drum sprocket system", its initially a little more expensive but in future repairs, you just replace the worn rim. "Oregonchain.com" has a great Web site that explains sprockets. Also the drive sprocket, chain, and bar must all match since they have to work together to move the chain.

Finally, The tensioner system, usually consisting of a screw, a retaining snap washer, and a tensioner pin/ nut, these are typically  located inside the clutch cover, occassionally the tensioner screw gets bent from someone over tightening the tension. Then the tensioner will not work correctly and will feel tight when its really Not, its just that the screw is bent. This can be checked with the clutch cover removed and no chain or bar, just turn the tensioner screw in the cover and it should move the pin freely the entire travel of the tensioner, If there is any binding it will need to be replaced.

Also tensioning, here is what I do put the bar and chain onto the chain saw, loosen the tensioning screw about a turn to a turn and a half CCW. Then carefully install the clutch cover making sure the tensioner pin fits the hole in the bar, and loosely snug up the bar nuts. Now with a glove on, or a rag in hand pull the chain in the normal direction of travel, it should move freely and stay in the bar channel. Next turn the tensioner screw CW until all of the slack is out of the chain, then while holding up on the nose of the bar give the tensioner screw another 1/4 turn CW, and tighten both bar nuts equally tight. Again with a glove on pull the chain by hand in the direction of travel a few revolutions, the chain should move freely but still be snug to the bar. Tensioning is complete.

PS. I forgot about the bar nose, some bars have a sprocket in the nose, make sure this is lubricated and turns easily, and the chain travels around the nose sprocket with no problems. I hope the above information helps!