Oversized Front Spindle Blocks for the ECX Boost, Circuit 2wd, Ruckus 2wd & Torment 2wd are designed to solve several issues reported to us by RPM customers. The first and most obvious is bearing life. The stock 5x10mm bearings simply can't handle the loads generated by these little monsters. RPM Oversized Front Spindle Blocks bump up the bearing size to 5x11mm bearings (not included). One millimeter in diameter may not seem like much but that little change nearly doubles all load and fatigue ratings over the stock bearing size! That means better performance, longer life and more time between maintenance periods.