Clydesdales/Athenas (200+ lb / 91+ kg) - couple questions

Hello im about 370 lbs , and just started riding a fixed gear mtb conversion i just got done putting together . Im having a problem with my left crank arm nut loosening while riding ,the arm has actually fallen completely off once .What would be the solution to htis problem ?And ,can you put a 26x2.0 tire on a 26x1.5 rim , or a 26x1.5 tire on a 26x2.0 rim? your help would be greatly appreciated . thank you

Hello im about 370 lbs , and just started riding a fixed gear mtb conversion i just got done putting together . Im having a problem with my left crank arm nut loosening while riding ,the arm has actually fallen completely off once .What would be the solution to htis problem ?And ,can you put a 26x2.0 tire on a 26x1.5 rim , or a 26x1.5 tire on a 26x2.0 rim? your help would be greatly appreciated . thank you

26X2 tire will go on a 26X1.5 rim nicely.

Crank nut: Proper torque and Loctite Blue on the threads should do it and you won't need a torch to remove either. Confirm this in the mechanics forum to be sure though.

It's possible the arm has been worn in the middle which is why it's unwinding.

Are these square taper or?
There are 2 standards used in sq. taper cranks, with the angle of the taper slightly different.
IF it's a used crank, it may have been "messed up" before you got it.
Typical torque, IIRC, is 30 FT-LB. You might just go ahead and exceed that some if 30 isn't doing the job. A 24x series loctite should do the job. That would let you know if the problem is the bolt/nut backing off, or if the taper/spline is damaged.
Fixed gear! Your knees are a lot better than mine!:)

Your crank arm is probably shot. Replace it and make sure you retorque the new one after the first few rides.

i feel like the nut isnt getting enough of the thread , i got the hammer out and tried to force the arm in as far as possible but it would still loosen after about 15 mins orf riding . The arm has got a sguare cut and it seems like the receiver tapers so im assuming thats what it is....

oh yeah and the arms are brand new bulletproof

After tightening, ride a couple blocks and retighten. Repeat at (hopefully) increasingly longer intervals, until they seem to NOT loosen anymore.
I was doing some crank swapping a little while back and just carried the wrench with me, while running errands. It took 3-4 "repeats" until they were tight enough to stay.