There is an amazing amount of engineering that goes into the valvetrain operation of an internal combustion engine. The myriad of parts involved are all individually engineered to do a job and work in concert with the other components. Rocker-to-retainer clearance is an often overlooked subject within valvetrain setup, and giving it some thought and ensuring you that have enough can prevent premature wear and/or valvetrain failure.
When you step up from a beehive-style valve spring and retainer like the Erson 3250 on the left to something like the Erson Cams 508 Titanium retainer on the right, rocker-to-retainer clearance becomes ever-so-important. The 508 retainers are manufactured to fit up to a 1.630-inch spring.
“In most cases, you can’t have too much clearance,” explains World Products Director of Operations Lance Stillwell. “What most people don’t understand is that in the upper RPM range, the top of the valve is dancing around as much as the spring is. Any time you start having interference, you send harmonics through the valvetrain.”
The harmonics alone can prevent the valvetrain from operating as intended, but Stillwell also notes that insufficient clearance can cause the rocker arms to hit the spring retainers, which can grind away at the body of the rocker arms. The contact can also cause the springs to grind into the retainers, especially titanium ones.
Rocker-to-retainer clearance is best checked before an engine is ever fired, and using “checker” springs such as the ones pictured here not only allow the engine builder to verify the valvetrain geometry and piston-to-valve clearance, but also ensure that there is adequate clearance between the rocker arm body and the valve spring and/or retainer.
“Some high-end engine builders have told me that while doing an oil analysis at a molecular level they have found titanium from the retainers embedded into the cylinder walls,” Stillwell says. “This is usually caused by the spring cutting into the retainer.”
Here you can see a variety of rocker arms and valve springs. On the far left you have a stock stamped steel small-block Chevy rocker arm and stock-style, single-coil valve spring. Second from the left is a cast steel, high-performance rocker arm and beefier single-coil spring. Second from the right is a PBM 803-16 stud-mount aluminum rocker arm that is manufactured with plenty of clearance for the stout dual valve spring it must actuate. And on the far right is a customer-supplied, aftermarket aluminum rocker arm that required additional machining to obtain adequate clearance for the dual spring.
Stillwell says that bigger-diameter valve springs are usually the culprit in creating rocker-to-retainer clearance issues.
“You have to start looking at the radius of the rocker arm by the trunnion for clearance issues,” Stillwell explains. “And having your geometry set up with the correct length pushrods eliminates most of the problems. Some guys will try to put a longer pushrod in to add clearance, but that’s the wrong way to gain clearance, as it can hurt performance.”
From a different angle you can see how each rocker is shaped and the clearance that each one offers. An aftermarket steel rocker, such as the one second from left, sometimes offers more clearance than an aluminum one due to the shapes allowed by its steel construction, and might be a better option in some applications. Lightening the valvetrain with aluminum rockers such as the PBM one (second from right) has additional benefits, however.
Stillwell added that the correct way to go about gaining clearance is to buy the correct rocker to match the spring, and/or use a smaller beehive-style spring if possible.
“Clearance needs to be addressed through the rocker and/or spring design. More racy types of springs will need more clearance, though,” Stilwell says.
Beehive springs such as the one pictured here have become very popular with engine builders and enthusiasts. They are progressive springs that offer reduced mass and improved rocker-to-retainer clearance as a side benefit of their design. “They are normally used in hydraulic roller applications that have other factors that limit the valve lift they can handle,” explains Stillwell. “As the cams get bigger, the springs have to get bigger.”
Some people create the necessary clearance by grinding the underside of the rocker arms. Stillwell notes that, “every time you cut on the rockers in a critical area, you weaken them.”
Companies like PBM and Erson Cams offer valve springs and rocker arms and can provide engine builders and enthusiasts with a combination of parts that fit together and provide the necessary clearance so no additional machining is required.
Taking a look at the same PBM stud-mount rocker with a conventional dual spring and the clearance tightens up due to the spring’s cylindrical shape. You’ll want to get with the rocker manufacturer to verify what springs it is designed to work with.
Stillwell assembles engines in his spare time, and a customer supplied a set of aluminum rocker arms that didn’t have enough rocker-to-retainer clearance. “I had to machine roughly .150-inch off to clear the springs,” Stillwell tells us. “This was a cheaper shaft system that the customer bought off of the internet. I had to machine all 16 to make it work — there is no savings in the end.”
Here you can see the conventional dual springs and the modified rocker arms that allow them to be used.
For comparison, we have the PBM stud-mount rocker arm on the left and a shaft-mounted T&D offset rocker assembly on the right. According to Stillwell, stud-mount rocker arms and beehive springs are more for street applications, whereas the shaft-mount and dual spring combination is more appropriate for racing applications. The shaft-mount style does have a higher initial cost, but offers a more stable foundation for the rockers, which reduces flex. That means the camshaft lobe’s lift is used to opens the valves and not to move parts. The end result is more power and longevity.
The shaft-mount rockers are not as bulky in shape as the stud-mount version, because the rocker shaft provides better support than a stud does and thus less material is needed in the rocker body to keep the rocker stable.
Despite being checked for clearance before running the engine, this T&D shaft rocker has indeed encountered some interference, most likely at high-RPM, and this engine regularly sees around 8,000 rpm during operation. “When you’re up in the RPM range, things are moving and flexing and sometimes it can nick a spot on the rockers,” Stillwell says. “That sends harmonics through the valvetrain — but you don’t want any interference. Ask any engine builder and they will tell you that valvetrain stability just about trumps everything else.
If you are set on using a stud-style rocker setup, Stillwell suggests using a stud girdle. “It helps reduce flex in the rocker studs, which will better maintain the rocker-to-retainer clearance. You’re not only gaining valvetrain stability with them, but can also keep the rockers out of the springs.”
