Cox .049 model engine mixture screw has a small knurled plastic knob which often cracks and falls off after repeated use. It is possible to use the engine without the knob but harder to know exactly what setting you have. As you can see in the picture, the knob I designed is about twice the size of the stock knob but that is unimportant since there is lots of room to grab it. I found when I tried to print multiple copies on a printrbot simple the size of the hole changed. sketchup file is included if you want to tweak the hole size or make other modifications to suit your application.