Chains only come off for a few reasons.
1. The chain is too loose. Make sure that when you are tightening the chain that you hold up on the bar the entire time you are tightening the chain and are tightening the bar nuts. If you do not, the chain will come loose very quickly.
2. It is the wrong chain, Many times when a person goes and buys a chain....especially if they go to a discount store, they buy a new chain of the same length as what they need but it is the wrong pitch. The drive links are not at the right spacing and do not mesh with the sprocket of the the bar and the saw.
3. The chain has come off before and damaged the drive links. Once a chain has come off of a bar, it usually damages the drive links and it will NEVER stay on the bar again. Look at the drive links of the chain and see if ther is ANY damage to them. If there is, replace the chain.
4. The sprockets may be worn. Check the sprockets at the end of the bar and also on the saw itself. Too much wear on either of these will cause this problem. The sprocket on the shaw should have NO wear grooves in it. Most people do not know what a new sprocket looks like. There are NO grooves in them at all. The sprocket at the end of the bar should turn freely and the chain should fit perfectly in the sprocket.
5. The chainsaw may not be oiling properly or the wrong oil is being used. IF the saw is not oiling , It creates too much heat and friction and can cause the chain to start to wear prematurely.
6. The chain may be worn out. Most customers do not know that the chains wear out more at the rivets than they do on the cutters. IF the chain has worn to the point that customers think the chain has stretched, the spacing of the drive links will have increased and will not fit properly into the sprockets. With the chain off, grab a hold of the chain about 4 inches apart and see if it will move back and forth a little bit. If it does, it is too worn to use.
I was just making sure that you got the message that I had sent you ans answer to your question. Let me know. Thanks.
The chain saw that you should buy is an ECHO BRAND CS-530 20" bar.
This will sell for about $399.00. Make sure that you get it at a reputable dealership that will put it together, start it and adjust it and show you how to use it.
Make sure that you use ONLY Echo brand oil and you MUST use premium Gasoline.
Not only is this the Best saw you can get, but it will have an extremely high resale value if you decide to sell it after you are finished with your projects.
I have been both an Echo and Stihl dealer for more than 17 years. The main reason that I have now stopped selling Stihl is that they now have started to sell caws and trimmers that are not of the good quality that they used to sell. They now have TWO different qualities of units....the PROFESSIONAL saws and now the HOMEOWNER models. You do not ever want to buy the less expensive saws that they have. There professional saws are still good, but they are much more expensive than you are going to need and they are NOT better than the ECHO saws that you can get for less money.
Stay away from ANY brand that is the Discount store variety. Brands like Poulan, Homelite, McCulloch, Craftsman or just about any other brand that you can get there.
Husqvarna is a better brand than the discount store saws, but you have to be very careful because they have THREE different Qualities of saws. ONLY the PROFESSIONAL models are worth buying. AND they are just as expensive as the Stihl saws. The biggest problem is that a customer never knows which quality you are getting. In addition parts availability is much harder to get because there are not nearly as many parts dealers as there are other brands.