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.
“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.
“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.”
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.”
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.
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.