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).
Curtisbilt rocker arms for...
Curtisbilt rocker arms for LS1, LS2, LS6 engines provide increased strength, precision, and are easy to install.
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.
As might be expected, the...
As might be expected, the first step in removing the original rockers is to pop off the coil covers. If you're new to C5s, take note that while this operation isn't particularly difficult, it must be done carefully to avoid damaging the driver-side cover at the end of the fuel line slot. Removal of the passenger-side cover is a bit less challenging; it requires only the removal of the oil fill cap.
After the covers are removed,...
After the covers are removed, the next step is to remove the coil assembly. The coil bracket on each side is attached to the valve cover beneath it by six studs. After unsnapping the coil connector and pulling the plug wires out of the coils, the studs and bracket, with coils attached, can be removed. This engine has obviously been treated to a set of MSD multi-fire coils, which provides a nice improvement in combustion efficiency.
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.
| DIFFICULTY INDEX ::: ||NNN |
|ANYONE'S PROJECT | no tools required || N |
|BEGINNER | basic tools || NN |
|EXPERIENCED | special tools || NNN |
|ACCOMPLISHED | special tools and outside help || NNNN |
|PROFESSIONALS ONLY | send this work out || NNNNN |
An internal snap lock holds...
An internal snap lock holds the line in place. To unlatch it, you need a special tool or a LOT of patience. You put the tool around the fuel line, push it toward the connector, and the fuel line pops off.
After the fuel line is out...
After the fuel line is out of the way, the valve cover bolts and the valve cover are removed to expose the rocker arms.
You've undoubtedly noticed...
You've undoubtedly noticed this isn't the same cylinder as is shown in the previous photos. It's posing in this photo because it's not mounted on an engine and is therefore better suited for showing close-up details. Of note is the pedestal between the rocker and head and the Allen-head retaining bolt. The pedestal seats against a rocker mounting boss (one is visible behind the valve spring on the right) and retaining bolt holds everything in place.
A 7mm wrench is used to remove...
A 7mm wrench is used to remove the bolts that hold the rockers in place. After all the bolts are completely loosened, all eight rockers can be removed by lifting theong aluminum bracket that runs beneath the rockers.
Note the gap between the pedestal...
Note the gap between the pedestal and the rocker mounting boss. Until you get the feel for the adjusting procedure, it's easy to wind up with a gap like this when you think a rocker is properly adjusted.
After all the rockers are...
After all the rockers are installed and adjusted, stand back and admire them; then double-check adjustment before re-installing the coil assemblies and covers.
Now that you have all the...
Now that you have all the rockers installed, you're ready to adjust. Just slip the supplied adjusting socket over the adjusting nut and turn it with a 15mm box wrench. Use an Allen wrench to turn the lock screw and chase the adjusting nut to make sure the pedestal is firmly seated against the mounting boss. After all clearance is removed between the rocker and pushrod, rotate the adjuster another one-half to one turn and tighten the lock screw.