Fender's vibrato arm is a brilliantly simple and effective design -- as long as you set it up properly.
The screw thread on the arm is
not a machine screw that you "torque" until the threads distort: it's a helical bearing surface which is wound in until there is
just enough friction to hold the arm exactly where you leave it. Each full turn increases the contact surface area and therefore the friction.
NEVER "tighten-up" the arm: there should be no resistance to you turning it. It should move to where you put it and stay there until you move it again. If you can feel any resistance then you've over-tightened it and the threads will wear. Raising or lowering the arm to a comfortable position above the guitar is done by bending the arm, not by winding it in and out.
If you have distorted the threads -- or you have acquired an arm/block combination where the threads don't match or are worn -- then the friction adjustment
cannot work. Equally, if you use a hard steel arm in a soft alloy block then the alloy surface will wear out and the friction will decrease to the point where it cannot work. Basically, the whole thing is FUBAR and you have only two alternatives -- bodge it or replace it.
If you use a steel arm in a steel block with matched threads, then the wear is minimal. Even if there is some wear, the total friction is then kept constant by the thrust-spring, Fender part no.099-4931-000 (that was the one you threw away or lost
). The spring is intended to apply a force to the end of the arm and keep the faces of the threads in contact: it is
not a "helicoil".
If the threads don't match or are distorted/worn to the point where the arm moves freely and clunks, then you will have to bodge it and fill the gaps with a material to create some friction. Traditionally, this was "plumbers" or "gas-fitters" tape (adhesive-backed linen cloth) -- not PTFE, which is an ultra-low friction material! This tape will need replacing regularly because it is even softer than alloy.
The best solution is to buy a good quality steel arm and steel block with MATCHED threads and a thrust spring, which you can hold in place with a blob of BlueTac. Set it up correctly and it will give 50+ years of use, without any drooping or clunking.
DaveC.