Hi and welcome to the M1 Carbine Forum from New Mexico!
The tech manual for the carbine does not specify a torque for the piston nut. It only instructs the arsenal personnel to stake the nut after assembly. I would suggest tightening it to a little more than hand tight to ensure that it has seated fully against the internal shoulder of the gas cylinder. The assembly drawing of this part just says "Stake in the places provided". These are the small openings in the recessed areas of the nut. You will be staking the cylinder, not the nut. One stake should do the trick. Check out this link for more info.
Jim as usual is a 110% correct. If you crack the gas cylinder you cannot weld it back. It will not stay and will crack again. You have to replace the barrel. It's the only way. Make the nut snug but no need to be King Kong in tightening it. Now if the nut only moves a little then don't worry about it as it's most likely been staked. The nut moved a little bit as it was staked so now there is a bit of movement in it. No big thing.