DIAGNOSIS Yeah, this one’s common when the chain is new. It’s not the nuts backing off; it’s the chain itself stretching and seating into the bar groove as you use it for the first few tanks of oil. It’s called “initial stretch” and it’s normal, but annoying.
FIX You need to check and re-tension the chain frequently during the first hour or two of use. The proper method: loosen the two bar nuts with the combination wrench, then use the built-in tensioning screw (located just forward of the nuts) to lift the bar tip up until the drive links are just seated in the bar groove. Tighten the bar nuts back down while holding the bar tip up. The chain should pull smoothly around the bar by hand with no sag on the bottom.
IF THAT DIDN'T WORK If the chain is constantly loose even after the break-in period, check the bar itself. Look for uneven wear or a heavily burred bar groove, especially near the tip. A worn or damaged bar won’t hold tension well. Also, make sure you’re using the correct Oregon 91VXL chain or Milwaukee equivalent; a mismatched chain pitch will never tension right.
HEADS UP Never tighten a hot chain. Let the saw cool down first. A hot chain tightens more, and when it cools and contracts, it can put massive stress on the drive sprocket, bar, and even crack the saw’s housing. Always do your final tension check on a cold chain.