I used a stock knob on my raingear switch and even put one on the headlight switch shaft because I have a thicker dash trim and the stock light switch was too short. Hasn't come loose in 13 years. E-bay: 55 56 Chevy Billet Dash Bezel Set 1955 1956 Chevrolet New | eBay

 

This is the only thing I found when I was looking for one. You have to be careful to gently tighten the set screw or the plastic cracks. Really, the set screw does not do anything but keep it from pulling off. You don't pull on it to operate it. The shape fits tight over the shaft so the turning relies on the shape, not the screw. 1955 1956 1957 1958 1959 Chrome Wiper Switch Knob Chevrolet Chevy GMC Truck Dash | eBay

 

Somebody that has use of a lathe maybe able to make something up

 

To make the hex, a plunge EDM electric discharge machine or possibly a conventional broaching set up, both not the kind of machinery found in most home shops.

 

The shape of the hole, the 6 sided one, as seen in the pic I included, is what eventually strips out. Not the set screw.

 

There is no way the switch should be that hard to turn that it strips out the hex in the knob. The knob failure is a result of another problem you should be looking for.

 

Could you sleeve the knob so that rounding wont happen.

 

Can you buy a spare knob & keep it in the glovebox as a "just in case" thing?
Or maybe use a pair of plyers as temp fix in an emergency?

 

I think you need a new switch instead of a different knob.
What wiper system uses a hex shaft on their switch?

 

One thing I found is it's a reproduction one, if it takes a small screwdriver remove and toss the set screw. The one side of the slot broke off and I had to use heat to get it off. And it wasn't over tightened either. Replace it with a hex hole/head screw. Mine that broke came from Ol' 55.

 

had a stock knob on the newport switch in my 56 for a few years now, yes it does get used. I would say you need a new switch if it is that difficult to turn. mine turns like butter.

 

We have an aftermarket wiper switch which is a bit harder to turn when activating the wipers.

I'm kinda wondering why it is hard to turn...Sounds like the cable may need to come out and be lubed.

 

The

The wiper assembly is an aftermarket electric type setup. I don't know the brand, nor did we install it.. unfortunately.

 

Is it one of these? They sell replacement switches. New Port NE5556CP - 1955-56 Chevy Passenger Car Wiper Motor | eBay Does it use just an electric switch and no cable. If it has a round shaft with one flat side,I had to cut a small piece of metal to slide in the knob where the set screw goes in or the screw goes in too far and won't hold. Just a thought.

 

I know the motor mounts under the dash behind the gauge cluster, and doesn't use cables.

 

Is it one of these? They sell replacement switches. New Port NE5556CP - 1955-56 Chevy Passenger Car Wiper Motor | eBay

I know the motor mounts under the dash behind the gauge cluster, and doesn't use cables.

Sounds like a Raingear system. Same issue with the switch though. Raingear has the round shaft with the one flat spot.

 

With as many RainGear installations on this forum, seems like someone must have experienced this issue before and solved the problem.

Here is a previous discussion that I found: https://www.trifive.com/threads/rain-gear-wiper-and-original-switch.85034/

 

The two Raingear kits I have do have a flat spot on the shaft. Maybe a newer design. I didn't have a longer set screw, and the original screws in too far to hold properly. Their switch is pretty stiff to go from off to low. That's why I stuck a little shim in the knob. If you could find a longer set screw, I think that would take car of it.

 

I used the Newport Engineering Wiper set up and opted for the intermittent feature. The knob that came with it was black plastic and worked fine, just didn’t match. I’m pretty sure it was round shaft with a flat side on the switch. I took an original choke knob with the extended shaft and cut the shaft off and center drilled it to accept the shaft from the switch. I then drilled in fron the side and tapped that hole for a small set screw. Considering that I no longer have my old shop and did it all by hand with jig set ups it came out good. I do get caught in the rain occasionally so it does see use and remains rock solid! Just another solution, hope it helps.
George