Modified Mustangs & Fords
Rocker Arms and Adjustable Valvetrain Geometry
Rocker Arm Reaction
Whether you use the stock ratio, or increase the ratio for more lift, it's imperative that the rocker arm fits the heads, clears your valve covers, and its geometry is properly set up to move the valve in the most efficient manner. If the rocker arm isn't checked and fit to the engine, you could induce several issues into your valvetrain, including excessive valve guide wear, rocker-to-valve retainer contact, piston-to-valve interference, damaged cam lobes, bent pushrods and more. While there is a lot to an engine's valvetrain to be considered, we're going to concentrate on the typical "bolt-on" setups, as in fully assembled aftermarket heads, or valvesprings being installed on existing heads that are specified/called out by the cam manufacturer's spec card or tech line. We simply don't have the room here to go through degreeing a camshaft, setting up valvesprings by opening and seat pressures, and so forth. If you're building a truly custom engine, your engine builder will deal with these measurements/adjustments, so for the gist of this story, we'll focus on the weekend wrench installing aftermarket heads or a new cam and spring kit and wanting to get their rocker arms and pushrods correct for their new bolt-on package.
There are two schools of thought when it comes to rocker arm geometry and pushrod length optimization. The easier, and more popular, method that Joe Wrench can accomplish at home during a cam swap, or even just upgrading rocker arms, is called the rollout method. The rollout of the rocker tip across the valve tip is measured and held to a minimum by changing the pushrod length. The second, more involved, method is called mid-lift method. It's also commonly referred to as half-lift centering. This method requires a dial indicator to read the valve's lift so that the midpoint of the valve's lift can be determined to allow measuring the geometry at the mid-lift point. Pushrod length is then determined to optimize this reading. Mid-lift measuring places the priority on centering the rocker tip over the valve at mid-lift. For all but the most extreme racing engines, or those who love to dabble in dial indicator measurements and check every single spec (twice) on their engine upgrades, mid-lift geometry checking is your bag. For the rest of us, you'll get perfectly acceptable results using the rollout method.
So, concentrating on rocker arms once again, why would you want to upgrade them in the first place? For starters, Ford, like many OE companies, used a basic stamped steel rocker arm on the majority of their engines. These stamped steel rockers do not have accurate rocker ratios due to the stamping process. Furthermore, the stamped rockers can flex under higher cam lift specs, effectively reducing the potential of that shiny new bumpstick you just installed. Lastly, the stock stamped steel rockers, often called sled or sliding rockers, have no bearings or roller parts to them to reduce friction, just a ball-type fulcrum to pivot on. The friction robs the engine of power and increases oil temperatures. If you're taking the time, effort, and money to install decent aftermarket cylinder heads, converting to a roller cam and lifter setup, or just want to bump up your stock cam lift a bit with increased rocker ratios, you'll be best served with a quality aftermarket roller rocker design. Most importantly, you need to set the rocker arm geometry and pushrod length accordingly.