Variable Duration Chevrolet 348/409 Hydraulic Flat Tappet Lifters All new design. Specifically made for race applications. Features high grade tappet shell and close tolerance valving. Great for all endurance applications including street. Maximum low end torque without sacrifice of mid or top end power. More vacuum (up to 3 in.). Effectively lowers cam duration by approximately 10 degrees, up to 3000 rpm. Note: These lifters produce a mild ticking sound (similar to mech. lifters). Preload settings for best performance .020”-.040”. Made in the USA!