Beginner’s Guide: The Differences Between Shaft and Stud Rocker Arms

PRW PQx LSx Aluminum shaft mount rocker arm assembly (1.7 ratio) – One stand ties the whole rocker arm system, increasing torsional rigidity

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.

Comp Cams Gold Series Aluminum stud mount rocker arms – these are equipped with roller trunnions and roller tips

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

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.

PRW PQx LSx Aluminum shaft mount rocker arms – each rocker arm is paired by one shaft, and they ride on one stand that ties them all together

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.

PRW PQx big block Chevy shaft mount rocker arm assembly – These are individual pedestal mount shaft rocker arms. One rocker, one shaft, on one stand per cylinder

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.”

Material Choice

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.”

PRW stud mount rocker arms – this picture exemplifies the various materials and types of stud type rocker arms. The blue anodized are made from 6061 Aluminum and have roller tips/trunions. The rocker arms to the right, are made from durable 17-4ph Stainless Steel, featuring roller tips/trunions. The bottom rockers utilize a pivot ball, instead of a roller trunion, but feature a roller tip and are made from 4130 Chromoly

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.

About the author

Bryce Kirk

Bryce has many years of experience on and off the track circuit, working on both domestic and Import performance vehicles. With numerous years of experience in the automotive performance industry, he has worked behind the scenes as an enthusiast and professional. He is passionate for all makes, models, and styles in the automotive culture.
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