Stud Mount vs. Shaft Mount Rockers
Up until the mid-1950s, overhead valve engines used shaft mounted rockers. When Chevrolet introduced their high revving small block V8s with stamped steel stud mounted rockers, it opened the eyes of engine designers to the possibilities of stud mounted rockers. Ford and others soon followed suit, and stud mounted rockers became the “hot” setup for the time.
As rockers continue to evolve, the ongoing trends toward lighter weight and stronger designs will continue.
Stud mounted rockers started to show their weaknesses as engine builders made modifications to increase engine speed and power. The press-fit rocker studs had a tendency to pull out if the engine was revved excessively or spring pressures were increased too much. Some performance engine builders started pinning the studs to keep them in place, while others replaced with press-fit studs with screw-in studs.
As valve spring pressured continued to increase, it became obvious that the rocker arm studs were flexing excessively at high rpms. The fix was to install longer studs and to clamp a bar (stud girdle) across the top of the cylinder head to tie all the studs together.
This, in turn, required taller valve covers to accommodate the stud girdle. It also made valve adjustments more difficult.
Aftermarket roller rocker arms were also introduced to replace the flimsy and rather wear stamped steel stock rockers. The performance rockers featured a roller bearing center fulcrum and a roller on the valve end of the arm to reduce friction. These were a huge improvement over the stock rockers and allowed higher rpms with more dependability and less friction.
As racers continued to push the envelope, it soon became apparent that some of these stud mounted aluminum rocker arms were not strong enough to handle the valve spring loads and rpms they were being asked to handle.
Aftermarket shaft-mounted rockers were introduced as a means of stiffening up the valvetrain, and steel rockers became an upgrade option for serious high-dollar racing.
According to some manufacturers, changing from stud mounted rockers to shaft mounted rockers (using the same lift ratio as before) will typically produce 10 to 15 more horsepower thanks to increased valvetrain stability.
The advantages of a shaft rocker setup are that the shaft holds the rockers in better alignment, eliminating the need for a separate guide plate for the pushrods. This reduces flex in the valvetrain at higher speeds for better valve control. The position of the shaft may also lower the pivot point of the rockers slightly with respect to the valves and pushrods to reduce friction between the tips of the arms and top of the valves. The shaft can also supply oil pressure directly to the rockers to improve lubrication and reduce friction.
A shaft mounted rocker arm system is overkill for most street performance applications because such an engine doesn’t really need that level of stiffness and strength. But for racing, a shaft-mounted system can provide increased rigidity and reliability.
Supporting the rockers on a rigid steel or aluminum shaft means the rockers can’t deviate from their fixed location due to stud flex or vertical motion on the rocker stud. The stiffness provided by the shaft holds all the rockers in perfect alignment and allows them to safely handle higher loads and rpms. Shaft-mounted rockers also don’t require a slot cutout on the underside of the rocker body to clear a stud, so shaft rockers are inherently stronger.
Shaft-mounted rocker systems are available for many aftermarket performance cylinder heads. In many instances, it’s a simple bolt-on installation that requires little or no modifications to the head. Pedestal mount rockers are also available for many engines with stud-mounted rockers.
A pedestal mount system can provide many of the same benefits as a shaft-mounted rocker system but at less cost. Many of these are simple bolt-in installations but won’t perform at the same level as a true shaft-mount system in an all-out racing application.