19418 Premium (Aggressive) Yamaha GYTR MT9 ski SnowTracker.  This Aggressive (Premium) kit requires a 1/2" hole to be drilled at the front slope of the ski to provide stability for the front of the U-Blades and a 4th mounting bolt hole that you must drill for to provide the SnowTracker with more rear stability.  Detailed instructions are in the package with hole size and precise measurements.  Because this product provides a tremendous improvement in traction/handling, sled set up is required to obtain best results. 2 page detailed instructions are included in your order.