Since the invention of the overhead valve internal combustion engine, auto manufacturers have been faced with the added complexity of the rocker arm becoming a necessary part of the valvetrain in the engine. Previous "flathead" engine designs incorporated a valve whose stem was most often located in the engine block, and pushed upward by the camshaft or tappet to open the valve at the appropriate time. In the more powerful overhead valve engine design, the upward motion of the cam lobe needs to be transferred to a downward motion of the valve itself, which is located in the cylinder head, and that transfer of motion is the function of the rocker arm.
Several rocker arm systems were incorporated by early automotive engineers, leading to two basic styles of rocker arms used in modern engines, pedestal (or stud) mounted rocker arms, and shaft mounted rocker arms. Pedestal mounted rocker arms mount individually and can be found in the Chrysler Magnum small-block engines. The vast majority of Mopar rocker arms, however, are shaft mounted rockers, which, as the name implies, ride on a shaft that bolts to the cylinder head. As Mopar owners, most of us are fortunate in that our small-blocks, big-blocks, and Hemis have shaft mounted rocker arms right from the factory, as this design has proven to be more stable and less prone to wear or failure than individually mounted rockers.
But while our factory shaft mounted rocker arms are arguably a very good system, there are limitations to the stock rocker arms found in most Mopar engines. The first, and biggest, drawback of factory shaft mounted rocker arms is that there is no real provision for adjustment of the valve lash, other than placing shims between the rocker shaft and cylinder head casting. So although factory stamped steel rocker arms can be adequate for small to moderate lift hydraulic camshafts, they really aren't practical at all for solid lifter cams. Additionally, the stamped steel factory rockers are prone to have a wide variance in fit, finish, and ratio, which is what we'll discuss next.
Rocker Arm Ratio
Rocker arm ratio is the length of the valve side of the rocker arm to the center (pivot point or fulcrum) of the rocker arm divided by the length of the cam or pushrod side of the rocker arm to the center of the rocker arm. Since the valve side of the rocker arm is typically longer than the pushrod side, the rocker arm will multiply its motion from the pushrod side to the valve side, as a function of the ratio. For example: If the cam lobe lifts the pushrod side of the rocker .350 inch, and the rocker ratio is 1.5, the valve lift will be 1.5 times the cam lobe lift which is .525 inch (.350 x 1.5 = .525). The same camshaft with a 1.6 ratio rocker arm would lift the valve .560 inch (.350 x 1.6 = .560), considerably more than the 1.5 rocker. Common rocker arm ratios are from 1.45 to 1.7 or even higher, but most Mopars utilize a 1.5 ratio rocker arm from the factory.
There are a couple of simple ways to figure rocker arm ratio, and the first is to measure each end of the rocker arm from the center of the pivot point to the contact area of the pushrod and valve, and divide the two numbers. For example, if the valve side of the rocker measures 1.5 inches and the pushrod side measures one inch, the ratio would be 1.5 (1.5/1 = 1.5). In basic terms, the longer the valve side of the rocker arm is compared to the pushrod side, the higher the ratio of the rocker arm.
Mopar Magnum engines utilize rocker arms that are mounted individually and called pedestal
Pedestal or stud-mounted rocker arms are prone to deflection, which can be compensated for
Most factory small-block and big-block Mopar engines were equipped with shaft mounted rock