These are square taper cranks/axles? Two thoughts, lowend bikes are more likely to have lowend crank fixing bolts. If the fixing bolts are really torqued they can stretch and not tighten adequately or worst case break. Check the end of the axle relative to the inside of the crank by torquing the bolt to suggested torque and then removing the bolt. The end of the axle should be a mm. or so short of the bearing surface inside the crank where the fixing bolt washer sits. If the axle is flush with the inside of the crank there is no way to tighten the crank. The taper square is off. Perhaps the end of the axle could be shortened 1-2mm to fix this. Replacement L cranks should be cheap and easy to get as another option. If the bolt won't stay tight Loktite or similar should be used. Steve