The chain adjuster does not actually hold the guide bar, that is done by the bar nut/nuts, the main reason for chains coming lose is inadiquate chain oil to the bar/chain, does the chain look dry? it can also be the result of what you are cutting, if it is very dirty timber or railway sleepers.
In my experience, low-profile chains longer than 16" don't stay on the bar reliably. This is because a low-profile chain has shorter drive links (less height) than regular chain. And if the chain isn't kept tighter than you'd normally want to keep it, the slack allows the drive links to come out of the slot in the bar. If any drive link is on top of or alongside the bar when the link reaches the bar nose, the chain comes off. With normal slack in a bar up to 16" there isn't enough slack for the short drive links to come far enough out of the slot in the bar for this to happen. However, with an 18" bar, the same amount of slack in the longer chain allows the drive links to come out of the slot, and if there's any side pressure on the chain at all, the chain comes off. I have a Craftsman 18" saw (made by Poulan) and had the same problem. Rather than just junk the saw, I decided to run it with the chain much tighter than I usually would run a saw. This almost completely eliminated the problem of the chain coming off. It will probably also result in shorter chain, bar, and sprocket life, but all things considered, I'm willing to put up with that for now. Eventually, I intend to fit this saw with a different sprocket, bar, and chain that is not low-profile. I keep the chain tension set on my Craftsman 18" saw so that there is no visible slack below the bar, but the chain can still be moved around the bar and sprocket without a great deal of force. There are a few other things to check out: make sure the bar isn't slightly bent, that the edges of the bar alongside the slot are the same height (check the top and bottom edges of the bar), that the sprocket cover is clean and undamaged where it fits against the bar, and that the bar mounting studs are straight and not damaged so that the bar is not prevented from being fully and accurately (no angle) mounted to the saw.In my experience, low-profile chains longer than 16" don't stay on the bar reliably. This is because a low-profile chain has shorter drive links (less height) than regular chain. And if the chain isn't kept tighter than you'd normally want to keep it, the slack allows the drive links to come out of the slot in the bar. If any drive link is on top of or alongside the bar when the link reaches the bar nose, the chain comes off. With normal slack in a bar up to 16" there isn't enough slack for the short drive links to come far enough out of the slot in the bar for this to happen. However, with an 18" bar, the same amount of slack in the longer chain allows the drive links to come out of the slot, and if there's any side pressure on the chain at all, the chain comes off. I have a Craftsman 18" saw (made by Poulan) and had the same problem. Rather than just junk the saw, I decided to run it with the chain much tighter than I usually would run a saw. This almost completely eliminated the problem of the chain coming off. It will probably also result in shorter chain, bar, and sprocket life, but all things considered, I'm willing to put up with that for now. Eventually, I intend to fit this saw with a different sprocket, bar, and chain that is not low-profile. I keep the chain tension set on my Craftsman 18" saw so that there is no visible slack below the bar, but the chain can still be moved around the bar and sprocket without a great deal of force. There are a few other things to check out: make sure the bar isn't slightly bent, that the edges of the bar alongside the slot are the same height (check the top and bottom edges of the bar), that the sprocket cover is clean and undamaged where it fits against the bar, and that the bar mounting studs are straight and not damaged so that the bar is not prevented from being fully and accurately (no angle) mounted to the saw.
Answers & Comments
The chain adjuster does not actually hold the guide bar, that is done by the bar nut/nuts, the main reason for chains coming lose is inadiquate chain oil to the bar/chain, does the chain look dry? it can also be the result of what you are cutting, if it is very dirty timber or railway sleepers.
In my experience, low-profile chains longer than 16" don't stay on the bar reliably. This is because a low-profile chain has shorter drive links (less height) than regular chain. And if the chain isn't kept tighter than you'd normally want to keep it, the slack allows the drive links to come out of the slot in the bar. If any drive link is on top of or alongside the bar when the link reaches the bar nose, the chain comes off. With normal slack in a bar up to 16" there isn't enough slack for the short drive links to come far enough out of the slot in the bar for this to happen. However, with an 18" bar, the same amount of slack in the longer chain allows the drive links to come out of the slot, and if there's any side pressure on the chain at all, the chain comes off. I have a Craftsman 18" saw (made by Poulan) and had the same problem. Rather than just junk the saw, I decided to run it with the chain much tighter than I usually would run a saw. This almost completely eliminated the problem of the chain coming off. It will probably also result in shorter chain, bar, and sprocket life, but all things considered, I'm willing to put up with that for now. Eventually, I intend to fit this saw with a different sprocket, bar, and chain that is not low-profile. I keep the chain tension set on my Craftsman 18" saw so that there is no visible slack below the bar, but the chain can still be moved around the bar and sprocket without a great deal of force. There are a few other things to check out: make sure the bar isn't slightly bent, that the edges of the bar alongside the slot are the same height (check the top and bottom edges of the bar), that the sprocket cover is clean and undamaged where it fits against the bar, and that the bar mounting studs are straight and not damaged so that the bar is not prevented from being fully and accurately (no angle) mounted to the saw.