this is a 2 pinion set meant specifically for scx24 or crawlers with scx24 transmissions that use upgraded 130 or 180 brushed motors, or rocket 1410 brushless motors, or any other motor with a 2mm shaft. these are 20 and 24t and meant for high speed bashers, monster trucks, mud trucks, or any other fast scx24 speed conversion. these are 0.3m pitch and mesh with the stock scx24 spur. these pinions have a true inner diameter of 1.98mm and are tight fit for 2mm shafts, meaning a vice is recommended for installation.