If you replace the input gear with an 18t mod1 pinion from Mike you do not need to drill the shaft, just file a flat area for the set screw to hold onto. I would use red thread lock on that set screw, as it is a pain if it loosens up. I had a writeup somewhere, might have gone with the temp forum. It was really easy to do with the gmaxx single speed kit. Just had to trim the end of a revo input shaft a few mm and make the flat spot on the shaft for the 18t pinion. I also used a spacer inside the tranny to keep the shaft from moving in or out. The 18t pinion is a bit thinner than the stock top gear.