Must be used with adjustable pushrods. JIMS ''Hydrosolid'' tappets have broken the limitations of all previous hydraulic tappet designs and is the best JIMS tappet that is offered. This tappet works like a hydraulic tappet to about 5500 RPM; and then turns into a solid al the way up to where your rev. limiter takes control. At the time it turns into a solid the JIMS Hydrosolid will add 3-6 horsepower. Even when the tappet rollers start lifting of the back of your cam you can be sure the JIMS hydrosolid will never pump-up. Upgraded custom and oversized replacement for OEM 18538-99C Standard and 17900022A High Capacity; but OEM pushrods must be replaced an adjustable pushrods installed. Note: Must be used with adjustable pushrods. Note: Standard OD is .8425".