If the LS1/LS2/LS6 engine family has a shortcoming, it has to be the rocker arm mounting, which doesn't allow for adjustment. Non-adjustable rockers have become standard in virtually all late-model pushrod engines, largely because they reduce the amount of labor, skilled or otherwise, required to assemble engines. But what is an advantage for an engine manufacturer is usually a decided disadvantage for vehicle owners and performance specialists when they modify an engine for increased power output.
Various types of adjustable rocker arms have been developed for LSx engines, but the most unique is the patented shaft rocker assembly designed by Bill Curtis of Curtisbilt Corporation. The Curtisbilt rockers are as easy to install and adjust as a typical stud-mounted rocker arm, but they offer the stability of shaft mounting. Curtisbilt actually uses Comp Cams Pro-Magnum rocker arm bodies, (both 1.75:1 and 1.85:1 ratios are available), which are machined to fit on individual pedestal-mounted shafts. A tongue on the end of one shaft allows it to interlock with a groove machined into the shaft of the mating rocker for each cylinder. When a pair of adjacent rockers are adjusted and locked, the assembly becomes a single-shaft assembly for each cylinder. With the tongue-and-groove interlock positioned inline with rocker motion, the pedestal beneath each shaft can be rotated to adjust lifter preload or lash (if mechanical lifters are installed).
Although the actual adjustment procedure is fairly simple, it takes a bit of "playing" to become proficient. This is largely because of the precise machining of the tongue and groove on the shaft ends. Each tongue fits in its mating groove with virtually no clearance. This extremely precise fit completely eliminates play between mating shafts, which consequently have the same rigidity as a single shaft with two rockers mounted on it. But when adjusting the rockers, it's easy to miss the fact that proper preload can seemingly be established without the pedestal firmly positioned against the cylinder head (because the tongue-and-groove fit is so precise, lifter preload can be established even if the rocker pedestal isn't fully seated).
After a bit of experimentation, we found the best way to adjust the Curtisbilt rockers is to rotate the adjuster and chase it with the lock bolt. Once you get the feel of the procedure, it's a simple matter to adjust all 16 rockers. In fact, the most challenging part of LSx rocker adjustment may be developing a means of determining when the lifter corresponding to each rocker is on the cam's base circle since there is no pointer or markings on the vibration damper to indicate top dead center or any other crankshaft position.
The easiest way to do this is to follow the practice used by some race-engine builders. Not usually ones to take the obvious at face value, many racers don't subscribe to the belief that both valves must be closed when a piston is at top dead center. Their method of adjusting lifter preload or valve lash is to rotate the crankshaft until an exhaust valve just begins to open, then adjust the intake rocker for that cylinder. Then they rotate the crankshaft again until the intake valve opens and starts to close. Just before the intake valve is fully closed, they adjust the exhaust rocker. This method ensures that each lifter is on the cam's base circle when the rocker is adjusted. It also makes it relatively easy (although somewhat tedious) to adjust all rockers because you don't have to be concerned with following a particular cylinder-by-cylinder sequence-just rotate the crankshaft until a particular exhaust valve begins to open and adjust the mating intake rocker. Then rotate the crank as required and move on to adjusting another rocker.
A variety of other procedures exist, and you may find one of these more to your liking. The means by which you work through the cylinders is strictly a matter of personal preference, and so is the amount of lifter preload. Personal preferences aside, for most engines, one-half to one additional turn after all clearance is eliminated is ideal.
With the Curtisbilt rockers, you should begin adjustment by ensuring that the pedestal is firmly positioned against the cylinder head surface and measurable clearance exists between the rocker and pushrod on one end and between the rocker and valve stem on the other. Then begin rotating the adjuster while chasing it with the lock bolt. After the lock bolt is snug, back it off a bit and rotate the adjuster again. Continue the process until all clearance is eliminated, then proceed to set preload or lash.
We installed 1.75:1 ratio rockers on the engine shown in the accompanying photos because we plan to install a performance camshaft (with about .600-inch lift) in the near future, and a 1.85:1 ratio would be a bit much. However, the higher ratio rockers represent a relatively easy means of increasing performance without taking the skinned-knuckle adventure known as a cam swap. With either type of rocker, the Curtisbilt shaft arrangement provides a significant increase in valvetrain rigidity and precision, and that's something any LSx engine can use.