Fits the Bachmann HO 3-truck Shay, Bachmann #81907 et al. Set of six steel lineshaft gears to replace the factory plastic lineshaft gears, which are prone to breakage. The kit includes 6 gears and step-by-step instructions for installation. This is an intermediate level project so if you're not comfortable with that level of work or just aren't sure, you might want to have a pro install these for you.