this will most likely sound like a dumb reply to all you sawnuts & gearheads but, after all these years, this still happens to me on occasion. when you disassemble the side cover and remove the bar you will always have to adjust/loosen the tensioner before reassembly. then you have to be sure the tensioner pin engages the bar when you put it back on. if you don't get it in there, and go torquing the bar nuts down, then you will most likely damage the cover and or tensioner. these are plastic covers. usually theyll spring back if not broken, but it sounds like you need to do a close inspection of your clutch cover, tensioner assembly, and bar. check to see if the tensioner pin is there, or sheared off. it should protrude up from the mounting surface on the cover. rotate the lead screw to see if it's bent. wiggle the pin on the tension screw threads & see if it's too loose. grasp the pin with needle nose and turn the screw at the same time to see if it's stripped. when you reassemble it, and you got it right, snug up the bar nuts finger tight, lift up on the tip of the bar, and tighten the screw. you should easily be able to see the chain tighten up, if it aint, somethings not right. so don't go reefin down the bar nuts till you figure out what. i never run a saw with a loose chain. this aint safe anytime. a chain flying off at high speed will mess you up. trust me. been there, done that. read the manual on chain tension.
good luck.
