The contact point between the wiper arm and wiper shaft is conical (tapered). There are serrations stamped onto the steel wiper shaft that bite into the softer aluminum material of which the arms are made. The original fasteners that hold the wiper arm onto the wiper shaft are nothing more than a 6mm wavy washer and a 6mm nut, both of which reside under the plastic cap. The nut requires very little torque for the system to work properly. This is due to the tapered fit, which requires very little axial force (torque on the nut) to generate huge radial force (pressure between the shaft and the arm)—the force that keeps the arm from slipping on the shaft. The system works great unless it is overstressed and comes loose—your wiper blades freezing to your windshield, for example. Then the wiper shaft turns inside the wiper arm. Obviously, that's when things start to go bad, and here's why:
What happens is that the cone shape inside the soft aluminum of the wiper arm is literally machined out to a larger size by the serrations on the harder wiper shaft. The good news? Since the wiper shaft is constructed of harder metal, it does not get damaged at all—which is awesome, considering that replacement wiper shafts are not available new for most models, and removing the dash to install the shaft is a BIG job. From this point forward, things can go from "bad" to "really bad" in a hurry if you simply try to tighten down the 6mm nuts too much.
What occurs is this: The nut and wavy washer bottom out on the shoulder of the wiper shaft. Once this happens—no matter how much tighter you make the nut—it does nothing to tighten the arm against the shaft. Oftentimes, the shaft snaps off... and that's ugly. Thus, the first rule when working with wiper arms is never to tighten the nut more than about 8 ft-lbs (or 96 inch-lbs). Never, never, never ever. Never.
If the wiper arms have been machined out so much that the nut and washer are bottoming out on the shaft, STOP. You have to do one of two things:
1. Replace the arms with new ones, or
2. Figure out a way to get the arm further onto the shaft.
In most cases, the arm does not have to be replaced. A trick that we employ here at GoWesty on every Vanagon or Bus on which we work is to add a 7mm flat washer under the wavy washer. The inside diameter (ID) of a flat 7mm washer is just large enough to slip over the shoulder of the shaft. The outside diameter (OD) is just small enough to allow the cap to still fit over. An 8mm washer is too big on the OD; a 6mm washer is too small on the ID. The added thickness of the 7mm flat washer allows you to shove the wiper arm that much further onto the tapered arm of the shaft before the 6mm washer and nut bottom out on the shoulder of the shaft. Just take off the arm, clean the aluminum out of the serrations, and bolt it back together with a 7mm washer added. This usually fixes the problem permanently. If that doesn't fix the issue, you can get some new arms at GoWesty!
In the event that you do break off the threaded portion of the wiper shaft, there is a fix that does not require removing the entire dash to replace the possibly-no-longer-available-new shaft. You can very carefully file off the end of the shaft so that it is flat and as close to perpendicular to the shaft as possible. Then you very, very carefully use a center punch to put a dimple in the exact center of the shaft. Then you very, very, very carefully drill a hole right down the center of the shaft about 1/2" deep. Then you very, very, very, very carefully tap the hole to 6mm x 1.0. Now you can use a bolt instead of a nut! And, while you've probably just had a ton of fun performing this magic, we don't recommend being obsessive-compulsive about this and adding one on the other side... just so they match!