Over the past several issues, we’ve taken a close look at today’s shaft rocker arm systems. If you rewind, you’ll find we covered how they were first developed, how the use of a SpinTron nailed down the dynamics of a modern rocker and how weight reduction in the right places can not only improve the engine RPM potential, but also how it can actually increase reliability. There’s more to consider when it comes to reliable performance:
A huge benefit of today’s shaft rockers is the increased reliability they offer when compared to an original stud mount rocker arm. Consider a simple vintage Chevy stamped steel, stud mount rocker. The folks from Jesel remind us that the stud mount rocker slides up and down the stud and takes up the valve lash before it opens the valve. Jesel uses the fabled old Duntov cam as an example. Here, the called for lash is 0.030-inch on both the intake and the exhaust. In practice, that old Chevy has the rocker sliding up and down 0.030-inch on each stud, each time the valve opens. It wasn’t hard to run those old small blocks up to 7,000 RPM or so (higher if you were brave). The lash worked as a slide hammer. That meant that the original pressed-in studs were (often) pulled right out of the head. The initial fix here was to use a roll pin to capture the stud. That eventually morphed into a screw-in stud arrangement. This allowed more valve spring pressure and of course, more RPM. The next thing that happened was bending and broken studs, which eventually led to stud girdles, which is right where we started.
In contrast to all of this is the shaft mount rocker, which simply rotates to take up the lash. Jesel adds: “Because the longer pivot length in a shaft rocker doesn’t side-load the valve as much as a stud mount rocker, shaft rockers tend to be much easier on valve guides, valve seats and valve tips.”
T&D Machine points out that the way a rocker arm assembly is mounted to the cylinder head is critical. Essentially, the rocker arm must be secured to the cylinder head so that no movement is possible. In the case of a shaft rocker, the actual rocker arm along with the shaft must also be securely fastened to the mounting bar, again with no movement allowed.
As you might have guessed, shaft rockers offer all sorts of different ratio choices. The distance between the pushrod cup, the rocker shaft centerline and the roller tip determines the ratio of a rocker arm. In a shaft rocker setup, various pivot lengths can accept a big range of ratios from (approximately) 1.4:1 through to 2.25:1. That wide range or ratios simply isn’t practical (or possible in many cases) in a stud mount rocker. How important is increased rocker ratio? Very! It should be no surprise to astute engine builders that using a rocker arm to increase valve lift has it’s benefits. When you use the rocker arm to change lift figures, then you have access to an almost infinite means to fine-tune the valve train on a per-cylinder or per-valve basis. The bottom line here is, it’s a bunch easier to swap rockers than it is to swap complete camshafts.
When all is said and done, the biggest benefit of a shaft rocker system is the considerable improvement they offer when it comes to the geometry of the valve train. In order to effectively make a change in the geometry, you’re pretty much forced to move the rocker’s pivot point. That’s not exactly easy with a stud-mounted rocker. In a modern shaft rocker layout, the pivot can be moved further away from the valve. This allows for the longer rocker pivot length. With some shaft rocker arrangements it also possible for the manufacturer to lower the pivot point. This makes for a low pivot arc from half to full-valve lift. This is the point where valve spring pressures are the highest. The improvement in geometry eliminates much of the friction created by the rocker when it is dragged (scrubbed) across the valve tip. Needless to say, this also makes for gains in both power and of course, reliability.
Remember those days of seeing cast or stamped steel valve covers perched on top of a set of valve cover spacers? They’re gone for good (thankfully!). The modern shaft rocker is here to stay. For a closer look, check out the accompanying photos.