Shaft-Mount Rocker Arms - Solid...And They Rock

Horse Sense: Although we're using our Project Real Street '89 LX coupe as the subject of this test, please understand that use of shaft-mount rocker arms is strictly prohibited in our 5.0 Mustang & Super Fords Real Street class. While it doesn't happen often, we always chuckle when Editor Turner shows us his wild side-which sometimes has him marching to the beat of an unconventional drum-and makes decisions such as using the class' flagship 'Stang for non-rules-compliant tech efforts. In all seriousness, the irony is a matter of convenience. March on, Turner, march on.

The 5.0 small-block has long been this magazine's engine of choice for street/strip/road-race Mustangs. Don't get us wrong-modular power is where it's at today, but there's something special about strong pushrod bullets.

There have been more than a few articles published that explain in detail the purpose and function of a four-stroke, internal-combustion engine's valvetrain components-from camshafts to pushrods in 5.0 and 351W engines, as well as lifters, springs, rocker arms, and such.

When we discuss a specific 5.0 valvetrain piece, our main intent is to give readers information about the installation, functionality, and overall influence of the part on the engine's performance. An example of this is Uncle Robin Lawrence's study of an NMRA Real Street engine's valvetrain as it was being evaluated under the high-rev influence of a SpinTron.

Robin's report ("Spin Control," Aug. '06, p. 150) focuses on using a SpinTron to track all the deep, internal goings-on in an engine's valvetrain during cam-lobe cycles. Based on data, changes are made to determine the best combination of pushrods and springs, which controls the havoc that breaks loose at high rpm.

A camshaft lobe triggers the up/down movement of the rockers by way of lifters and pushrods, and thus controls the opening and closing of valves. When a cam lobe raises the outside/butt of a rocker, the inside/tip is pressed against the valve stem, opening the valve. Conversely, when the cam's rotation lowers the butt side of the arm, a rocker's tip lifts and allows the valvespring to close the valve.

Rocker arms play a major role in camshaft/ pushrod/valvespring harmony. In the case of stud-mount roller rockers common to supercharged and nitrous-injected Real Street bullets and pushrod engines, the arms act on a needle-bearing-and-trunnion-style pivot in the center of their housings.

Our '89 Real Street LX notchback is motivated by a D.S.S.-built 306 short-block, stuffed with flat-top pistons and topped with Trick Flow's veritable Twisted Wedge cylinder heads. Per class rules, our bullet's camshaft is OEM stock, so D.S.S. took extra care to pay close attention to piston-to-valve clearance. We went with 1.7 rockers with 7/16 studs, as opposed to factory-spec 1.6 arms, in order to achieve as much total valve lift as possible. Note that rocker-arm ratio represents the number of times a rocker arm moves a valve in relation to the total advertised amount of cam-lobe lift. When multiplying the rocker ratio by cam-lift specs, we're able to determine the total amount an intake or exhaust valve moves from its seat during a cam cycle.

A rocker-ratio upgrade (1.6 to 1.7) is one of the oldest valvetrain-related tricks in the book of Mustang performance, as is installing stud-mount rocker arms as replacements for pedestal-mount pieces. The latter modification is done in the name of strength and stability. Rocker arms move at a blindingly quick rate when the engine's rpm is high; maintaining tip contact with pushrods and valve stems with minimal friction and deflection is critical to their efficiency. Deflection is the degree to which the rockers are displaced at high rpm.

Theoretically, because of their design, pivot-ball-style, stud-mount rockers aren't as efficient at reducing friction as rocker arms that utilize the needle-bearing-and-trunnion setup as their pivot source, mainly because the balls have a greater tendency to move around (deflect) as rpm increases. The needle-bearing-and-trunnion centers are dramatically better. The best method for knocking out excessive deflection is by installing shaft-mount rocker arms.

We selected the Pro-Series shaft mounts for this project. Its rocker bodies are made of 2024 billet aluminum. They're shot-peened for extra strength, and each stand is machined from billet steel or heat-treated aluminum stock for strength and durability. Crane's Tony Vigo tells us that for drag racing, the offset arms we're using will stand up to a SpinTron-tested minimum 800 pounds of open-spring pressure, which is light years beyond anything we'll experience with Real Street's engine. We're using 0.345 intake/ 0.265 exhaust offsets to ensure the rockers' tips will remain centered over the valve stems during opening/closing. We should be in great shape if we turn up the wick on our project car.