Does anyone know where I can find a replacement starter button/kill switch for a 1985 K100RS? The top of my button is broken off. I just have the shaft through the middle of the kill switch. It works, but it doesn't look great and I'm a little concerned about accidentally activating the starter since I don't have a spring to retract the button.

   I've found plenty of sources for a full replacement switchgear, but I can't find just the starter button/kill switch anywhere.

   Any ideas?

They are difficult to find and usually you need to buy the whole switch block. Don't be too concerned about touching the button when the bike is running because the starter relay cant operate if the engine revs are over 711rpm (inbuilt safety don't ya just love BMWs).
Sidecar Paul made a button from aluminium if you do a search or look at his recent posts you will find it.

Have a look at this post:-

https://www.k100-forum.com/t11308-red-kill-switch-and-green-starter-buttons

Paul.

I learnt from an email sent to me that Itead Studio has a new 3D printing service.  They have a 72 hour turn-around time and printing starts at US$1.90.  I'm not sure what that gets you.  However, if you can get the 3D model of the button, you should be able to get one printed in high-strength white resin at 100 micron accuracy.

I followed a thread a while back posted by fellow member, Hollister. He used the black rubber plunger off a 10 ml syringe. It fits perfectly over the red kill switch ring. I tried it on my broken starter button and it's been great for nearly a year now. Search "syringe" and "starter button" on the forum for a picture. My sister in law works in a medical lab so she brought me a handful so I have spares. I did one more mod and that was to cut a small hole that the stub of the green starter button sits in. There is a slight conical shape to the plunger that pulls the stub up in rest position

Good luck.

Sunny

While the starter won't engage with the engine at or above 711 rpm, I believe depressing that button (or having gravity depress the button) can activate the Shed Relay which will create all sorts of other intermittent problems.

The load shed relay will only de-activate if the starter gets 12 volts from the starter relay.  Since the starter relay will not activate above 711 rpm that's not a concern.  A concern that I can think of is that when the starter button is pushed you enrich the air/flue mixture if I am correct.