Matters of Control – Stud Mount vs. Shaft Mount Rocker Arms

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COMP Cams takes an in-depth look at two easy ways to improve valve train durability and raise your redline at the same time

Engines, by nature, are a bundle of compromises. Durability versus weight. Fuel economy versus power. You get the idea. It all comes down to making decisions on what you want versus what you’re willing to give up.

And nowhere are those decisions tougher than when it comes to the valve train. Or at least that’s what we’ve always been told. Engineers at COMP Cams have been working to eliminate weaknesses without giving up anything in return and have found ways to simply eliminate many of the compromises that engine builders are normally forced to make.

They can do this because COMP is involved in the valve train from the camshaft all the way through the valves. This includes everything from retainers to locks to valve springs to lifters and even lubricants to keep everything moving smoothly. And one area where they have found engine builders and horsepower freaks can greatly improve their engines’ valve trains without giving anything up in return is creating a stable platform for the rocker arms to do their job.

When it comes to pushrod engines, the most popular design for locating the rocker arms is the pedestal-mount rocker stud. On the low-end (think OEMs), they are 3/8-inch diameter rocker studs pressed into the cylinder head. The OEMs liked this system because it was cheap to produce, but in performance applications, raising the valve spring pressure can pull the studs right out of the heads.

Stud mount rockers

The next step up is a screw-in rocker stud. These are commonly available in either 3/8-inch or a larger 7/16-inch size diameter. Screw-in studs can handle much greater spring pressures, and while the larger, 7/16-inch diameter studs do add stiffness, the final result is probably not as much as you might think.

"Rocker stud flex doesn’t just take place in the upper RPM range,” explains engine builder Keith Dorton of Automotive Specialists, whose race engines regularly dominate in the USAR Pro Cup Series. “You can see it by doing a simple test. Pull the valve cover off your engine and place a dial indicator laterally on the end of the rocker stud, or the adjuster nut if necessary, on the opposite end of the valve stem. Then turn the crankshaft over by hand and watch to see how much movement you get on the dial indicator. If you aren’t using a stud girdle you will see some movement on practically every engine, and on some engines it will scare you to death.”









At first glance it isn’t obvious why a rocker stud will flex even when turning an engine’s rotating assembly by hand. After all, its only purpose is to provide a pivot point for the rocker arm, right?

Ideally, that is correct, but there’s a lot more going on in the real world. On almost all pushrod engines, the rocker stud is on an angle to the valve stem. More specifically, the many angles involved in getting a valve train to work as efficiently as possible force the top of the rocker stud and the top of the valve stem to be angled toward each other. So when the camshaft tries to open the valve, the spring pushes back through the rocker arm and causes the rocker stud to flex back, away from the spring.

When the rocker stud flexes, lots of things happen, and none of them are good. First, it puts unintended motion into the valve train. The rocker arm moves off the center of the valve stem tip and can cause early wear of the valve, the rocker tip and even the valve guide. There’s no telling how many engine builders have blamed a broken valve on the manufacturer or a too-aggressive camshaft when the real culprit was the rocker stud. Flex in the rocker stud can also break the rocker stud boss on the cylinder head.

A second effect—which may be even more important to racers—is that the flex in the rocker stud causes the valves to stay closed just a fraction of a second longer and not reach maximum valve lift. The effect is to take the cam that you spent so much effort finding the perfect specs for and make it act smaller. Everything from total duration to overlap changes can significantly reduce the amount of air and fuel you are getting into the combustion chambers.

Interestingly, Dorton says that the cylinder heads most prone to allow flex are the same ones most likely to be used by racers and performance enthusiasts. Aluminum cylinder heads are lighter than cast iron, but the material is also softer. Likewise, heads designed for performance usually also have any extra material carved away to shave off even more weight. But this also means the rocker stud bosses rarely have any more material than necessary. It’s not a design flaw—after all, reducing weight is important—but it does mean a smart engine builder should be aware of it.

Stud girdles

Fortunately, there are two easy ways to increase the stability of the rocker arm. If you have already invested in a stud-mounted rocker system, a rocker stud girdle adds very little weight and ties all the rocker studs on a cylinder head together to greatly increase rigidity. A rocker stud girdle is essentially a large clamp that locks down on all of the adjuster nuts on top of the studs. Once it is in place, it takes the force of one valve spring trying to push back on a single rocker stud and transfers it to the entire network. Since the valves are actuated at different times, this is an efficient way to spread out the load without creating any extra stress on the overall valve train setup. It’s a relatively simple device and since it can be constructed from aluminum, it adds very little to the overall weight of any engine. The only changes required will be extended rocker adjusters (which are usually included in any girdle kit) and possibly taller valve covers.

“A girdle is practically always going to be beneficial versus not having one,” explains Sandy Wilkins, who manages the drag racing engine program for Roush Yates Engines. “Anything you can do to stabilize the rocker system is a good idea. But, at least in racing, any time you are going from a flat tappet camshaft to a roller cam, I’d consider at least a rocker girdle to be mandatory if it is allowed in the rules. And that’s because of the higher spring pressures you can run with a roller cam. On a flat tappet camshaft, you might max out with a spring that has 400 pounds of open pressure, but on a roller you can get away with as much as maybe 700 pounds of open pressure. That extra spring pressure will really stress those rocker studs so strengthening that area can only help. I’ve seen gains exceed 20 horsepower on some engines by getting rid of deflection around the rocker arms.”

Shaft mount rocker system

A second solution for increasing the stability of the rocker arms is to swap out the rocker stud system for a shaft-mounted rocker system. Shaft-mount rockers, instead of bolting up individually on vertical rocker studs, pivot on horizontal shafts that are in turn bolted to stands which are securely mounted to the top of the cylinder head. This setup in much more stable than a traditional rocker stud system and virtually eliminates flex--which is why you will find shaft rockers in practically every engine in every top-level racing series.

As recently as a few years ago, shaft-mount rocker systems were expensive and tricky to set up on a new engine, which made them the domain of big-money race teams and high-end applications. But COMP has developed a line of sportsman level shaft rocker systems for many different applications that reduces the cost of adding a shaft rocker system to an engine without giving up any of the benefits. Most bolt directly to the cylinder head and include shims to help you get the correct valve train geometry for excellent valve control and maximum component life. Now, instead of a single 7/16-inch stud holding the rocker in place, you have a rocker pivoting on a shaft, which in turn is bolted to a substantial pedestal which leaves little opportunity for flex in any direction.

While researching this topic, we heard stories of engine builders adding a rocker stud girdle to their existing valve train or switching to a shaft-mount rocker system and being surprised to find horsepower drop. It turns out that it wasn’t a fault of the stiffer valve train, but over time the engine builder had constantly tested new camshaft grinds and wound up with a lobe design that compensated for the flex in the system. By going to a smaller camshaft, the engine not only regained the lost power—and then some—but it also eliminated the strain on the valve train from an over-aggressive camshaft, and the more stable system pushed the redline higher in the rpm range.

So which system is better for you? “It all depends,” is the answer Wilkins gave us.

“I would always promote using a shaft-mount system if you can,” he says, “but that’s not always possible. Maybe your rulebook won’t allow it or you have an oddball engine that nobody makes one for.

Then there’s also a financial consideration. If you already have your studs and rockers, then you will come out cheaper by investing in a good rocker stud girdle. But if you are building your engine from scratch, that’s a different story. When you consider that for a stud-mount system that you’ve got to buy the screw-in studs, guideplates, rocker arms, the stud girdle and the adjuster nuts to go along with it, that all adds up. In the end, you may be able to get away with only paying about a hundred bucks or so more for a good sportsman level shaft-mount rocker system. And once they are set up on your cylinder heads properly, they are pretty bulletproof.”