Abstract: An apparatus for on head testing of a valve spring that biases a rocker arm assembly. The testing apparatus includes a trunnion body portion that is pivotally connected to a main body portion. A biasing member is movably mounted to the main body portion and abuts a first rocker arm of the rocker arm assembly during use. A latch member engageable with a second rocker arm of the rocker arm assembly is adjustably mounted to the trunnion body portion to permit different spacings with the biasing member to be furnished to accommodate different sized rocker arms. When a tester handle is used to drive the biasing member toward the first rocker arm during valve spring testing, the force applied by the biasing member to compress the valve spring is measured by the sensing of the movement of the biasing member relative to the main body portion, and is displayed on a gauge mounted on the main body portion.