When speaking about roller rocker systems, there are typically two main categories that they fall into; shaft mount and stud mount rockers. It seems there is a large misunderstanding of when a shaft mount rocker system is truly necessary. One of the largest differences that most enthusiasts notice between the two types boils down to cost, shaft mounted rockers are primarily more expensive because of the cost involved to manufacture and the complexity of the systems.
Within the two groups of rocker arms, there are several variations within each group. Depending on the level of the application; the rocker arms can have roller tips, or roller trunnions, and even both roller tips and trunnions. The latter is the most efficient and durable solution, but also the most costly option out of the bunch. Most factory applications, like the LS factory rockers, have a roller trunnion and are cast or they are production stamped steel. Theoretically, having either a roller tip or a roller trunnion, or both, reduces the engines’ frictional forces and free up horsepower congruently.
Stud mount rockers are very effective at what they do, and can be improved upon with additional components. Items such as stud girdles, and upgraded studs, will reduce the amount of flex seen at the studs that the rocker arms pivot on. Shaun Snow of PRW says, “Most street application are just fine with stud mount rockers, especially when combined with a high quality 7/16-inch stud. The 7/16-inch studs resists flexing under load better than a 3/8-inch stud in the same application. The addition of a stud girdle will add a lot of the stability that a shaft system offers, albeit at the expense of ease of valve adjustments.”
Variables that would warrant the upgrade stud mounted rocker setup could be opening spring pressure or when solutions are needed for high RPM operations.
Shaft mount rockers require a more involved installation, but the reward is advantageous for most high horsepower applications. There are four basic types of shaft mounted rocker arm systems. The first utilizes a pair of trunnion rocker arms that pivot on their own stand, with a set per each cylinder. With the paired trunnion rockers, it’s possible to have one complete stand per bank of cylinders. Occasionally paired trunnion rockers will bolt directly to stock pedestals, these kits generally cost less. The third type of shaft rocker utilizes an individual mounting stand for a single rocker. The fourth type employs a shaft for an entire bank of cylinders, or in some applications two shafts per bank.
Shaft mounted rockers have multiple advantages over the stud mount setups. Since the rockers are mounted on a stationary shaft they do not twist and the trunnion does not move up, or down like when lash is experienced on a stud mounted rocker. Shaft mounted setups eliminate erratic valve opening motions because the trunnion always remains stationary. Another benefit is that the pushrod and roller tip can be offset to accommodate larger intake ports. Having all the rockers tied to a single stand that stretches the entire bank will increase torsional rigidity as well.
Shaft mounted rockers are not without some disadvantages. Typically when installing these systems the cylinder head must be machined to accommodate the rocker stands. Snow mentioned, “Shaft rockers can be more involved when setting up than a stud mount. Unlike a stud mount rocker where pushrod length can be changed to put the rocker where you need it for correct geometry, a shaft mount has to have the shaft raised or lowered to accomplish the same thing. This means at times there will be machining required to the cylinder head and/or stands to attain proper fitment. Other times you may have to replace the stands with taller ones or add shims to raise the shaft up.”
There are two options encountered when selecting stud or shaft mounted rocker systems, and this comes down to material. Primarily the choices lie with either aluminum or steel; various grades of each alloy determine strength and cost. Snow went on to say ,“Stainless steel is stronger than aluminum which allows the stainless rocker to be made with less material so the weight difference tends to be less than people think. Aluminum rockers are bulkier than a comparable stainless rocker which can also affect springs clearances in some cases. The body of an aluminum rocker will have a finite life span compared to one made of stainless which does come up in some racing applications, but in most street and mild racing applications the difference between aluminum and stainless is less a matter of strength or weight and more about budget and personal preference.”
So when it comes to choosing a rocker assembly it’s not always best to just upgrade to a shaft mounted setup, because it may be overkilll. A stud mount system may save money, but in the long run may not produce the best results in a racing application. Determining the correct material and type of rocker system for the application, will save money or future headaches.