I bought a new chainwheel/crank for my bike which uses hub gears. I removed the smallest of the three sprockets (the middle one is needed to keep the largest tight). The largest sprocket has a sort of metal cylinder (about 4mm diameter and 15mm length) in it, perpendicular to the plain of the sprocket, and pointing towards the crank arm. The cylinder is close to the edge of the sprocket and would therefore prevent me from having a chainguard over the sprocket. What is this cylinder (some might call it a pin)? It wont budge and is made from a different piece of metal than the rest of the sprocket.