This is a little indicator that goes on the end of the motor shaft on the Filastruder so you can see how fast the shaft is spinning. It's helpful when it starts to get bogged down for some reason and you are in a noisy environment. I think the size of the hole for the shaft it actually bigger than the motor shaft on the Filastruder but (for me) when it gets printed it is too small and I had to drill it out. So, you'll probably need to drill out the hole so it'll fit on the shaft.