It wasn’t long ago that a roller cam in a street engine was considered exotic. But technology has pushed the performance world into better days. Even the venerable small-block Chevy converted to hydraulic roller lifters in the mid-1980s, so there’s less of an excuse for not running a roller cam today. The biggest challenge for older, traditional engines has more to do with ensuring that retrofit cam is properly installed.
Back in the days of flat-tappet camshafts, the lobes were ground with a very slight angle. This accomplishes two things. First, this slight lobe taper induced the lifter to rotate, preventing undue wear. The lobe taper also counterbalances the normal forward cam thrust that will push the cam out the front of the block. Roller camshafts must use a zero-taper lobe, which subsequently allows the cam to move forward.
This forward movement of the cam in the block is to be avoided because once the cam begins to move, this also affects the relationship of the distributor gear to the cam gear. When the cam thrusts forward, it also counter-rotates the distributor gear, retarding the timing. Cam walk of more than 0.010 inch induces retarding of the ignition timing. We’ve never actually tested the negative results, but it is possible to see 20 degrees of retarded timing at higher engine speeds from a roller cam with no button or thrust plate.
All of this points to why, when early engines were converted to roller valvetrains, the factory equipped the block with a limiting plate. As an example, the ’86 and later small-block Chevrolet and the Gen-II LT1 engines that followed were equipped with a steel limiting plate that required the use of a stepped nose camshaft.
Even with this requirement, converting an older engine, like a big-block Chevy, to a roller cam configuration isn’t that difficult. Most cam companies like Comp, Isky, Lunati, and others sell cam buttons that will take up the space between the cam gear and the timing chain cover. It’s then up to the engine builder to install the button, measure the endplay and adjust the system to limit the cam movement to between 0.003 and 0.010-inch.
There are a multitude of different ways to accomplish this task and we’ll cover several variations. The most basic and least expensive approach is to invest in a nylon cam button. These buttons fit inside the center hole in the cam gear and press up against the inside of the timing cover.
Getting to the Thousandth of an Inch
Because of variations in the thickness of the timing cover gasket and position of the timing chain cover, you can expect that this will require some effort to custom match the buttons to each specific engine. For example, we’ve often had to make shims to fit behind the button in order to create the proper clearance. In one case, we needed at least 0.010-inch of shim thickness, and cut up an aluminum soda can to make three shims that measured a combined total of 0.012 inch. A nylon button that is too long is much easier to remedy since that requires only sanding or filing the backside of the button until the proper length is achieved.
Another issue that comes up is properly measuring cam endplay. Aftermarket aluminum timing covers usually come with an access hole that is covered with an Allen plug. The hole lines up with a portion of the cam gear. This makes measuring endplay really easy. Stock timing covers do not offer this kind of convenience so the engine builder is forced to be innovative.
In situations like this, we’ve set the dial indicator on the backside of a lobe with the indicator plunger as close to parallel to the cam as possible. This angle induces an error in reading, but since the accepted range of endplay is 0.003- to 0.010-inch, a slight error of perhaps 0.001 to 0.002-inch will not be an issue. Of course, you can also measure from the rear of the camshaft if the rear cam lug has not been installed. This almost always requires removing the engine from the engine stand.
Another trick we’ve seen used on engines with stock timing covers is to epoxy a steel reinforcement plate to the inside of the timing cover to take up room so that a button will create the proper clearance. The insert also helps prevent cover deflection. Comp sells a relatively inexpensive small-block stock type timing cover with an insert along with a roller button that’s attached to a cam bolt plate. This makes the conversion on an older small-block Chevy engine a little easier.
Several companies have come up with innovative ways to adjust cam movement. One of our favorites is the Cloyes Quick Button which is a two-piece aluminum timing cover that incorporates an eccentric in the center of the cover. Once the cover and gasket have been installed and the center adjuster is in place, you can turn the center adjuster counterclockwise until it bottoms out. Next, making sure the cam is fully retracted into the engine, you turn the adjuster clockwise in until it contacts the cam. Finally, back the adjuster out an eighth of a turn. This will create roughly a 0.003-inch clearance. Because the cover is aluminum, it will grow slightly when the engine warms up, which will create additional clearance.
While checking and creating the proper clearances for a retro-fit roller cam requires an investment of both money and time, it is also well worth the effort. Roller cams offer more power with more valve lift that pays off with superior power benefits. While there’s nothing wrong with flat tappet cams if you’re on a Ramen noodles budget — there are many good reasons why roller cams are the only way to go.