ST Racing Concepts Traxxas TRX-4M Brass Front Caster Blocks. These optional caster blocks are intended for the Traxxas TRX-4M. Made from brass, each caster block is CNC machined for perfect fit and finish. Increase the durability, lower CG and move the weight forward in your TRX-4M with the ST Racing Concepts Brass Caster Blocks. Package includes two front caster blocks.