RPM Caster Blocks are redesigned to slim down unnecessary bulk while improving strength and durability in this heavily impacted area. These are stock replacement items. No additional parts or modifications are necessary to mount a set to a vehicle. RPM Caster Blocks are molded in our trademark blend of ultra durable and super strong nylons and backed by the best breakage warranty in the R/C industry. Each package contains one left side and one right side caster block.