For a BOV to work consistently and not leak, tolerances must be held to an extremely high standard. The EVP BOV body, cap and piston are precision CNC machined from billet aluminum with tolerances that approach within .0003". The sliding components are hard anodized and coated so the BOV will last the lifetime of the engine. For each vehicle / engine type we make BOV kits for, the spring rates are properly matched for optimum performance.