John noticed that his 11-second Camaro was losing top-end power. He checked fuel-pump pressure and initial timing, but the car was definitely lying down at high rpm. After several hours of searching for the cause of the problem, he finally decided to check the timing at high rpm. He discovered that the timing retarded 12 degrees between 4,000 and 6,000 rpm. At first he blamed the ignition box, then the distributor, but all were working fine. Finally, after much head scratching, he discovered the aluminum roller cam button had galled itself against the inside of the timing chain cover, which allowed the roller cam to walk forward 0.040 inch. This cam walk moved the distributor, which retarded the timing.
Roller camshafts are becoming increasingly popular with the high- performance set, but with these trick new parts can come headaches if you dont know how to properly set up a roller cam. Flat-tappet cams for Chevys V-8s incorporate a built-in feature that prevents cam walk, but roller cams do not. For both the small- and big-block Chevys, cam manufacturers grind a slight taper into each lobe across its face that loads the cam toward the rear of the engine. GM uses a 0.0025-inch taper while aftermarket cams use as much as 0.005 inch. This taper also spins the lifter in the bore, which prevents wear and break-in problems, especially with aggressive camshafts.
Roller cams do not have this tapered ramp design, which means that other external forces can cause camshaft walk. Generally, situations like lifter bore placement, position of the distributor relative to the cam, and other factors affect the fore or aft thrust of the cam. One of the most important considerations is the vertical orientation of the crank gear relative to the cam gear. So if the crank gear is slightly forward of the cam gear, this offset will pull the camshaft forward.
One danger that exists with significant cam walk is the real possibility that the roller lifter could crash into an adjoining lobe. If this happens, catastrophic engine damage is only seconds behind. At the very least, your first indication of a problem will be metal shavings throughout the engine.
Now that we know what the problem is, lets look at a few solutions. All the major cam and valvetrain companies offer several styles of cam buttons that limit cam walk. The spec is to keep camshaft movement to around 0.005-0.010 inch. The best way to do this is with a roller-bearing cam button. This style button uses an internal roller bearing that does not wear into the timing-chain cover. In some cases, the cam gear may need to be machined in order to fit this cam button spacer.
But just installing the button is not enough. Stock small-block Chevy stamped-tin covers are flimsy, and the forward thrust of the cam button can easily deflect these thin covers. It would also be wise to stay away from any thin, stamped aluminum cover. The best plan is to invest in a thick, cast-aluminum cover. Cast covers are far more durable, they dont deflect like stock tin covers, and they look trick. Several companies also offer two-piece covers in various styles that allow access to just the cam drive, or the entire face can be removed without having to drop the oil pan.
Another option to consider is that many aftermarket aluminum water pumps offer small adjustment bungs at the rear of the pump, which can be used to establish a limiter that rests up against the timing-chain cover to eliminate deflection. This isnt necessary if you are using a cast-aluminum cover.
Finally, there are also several different styles of cam buttons available. Most of the cam companies have discontinued aluminum buttons, but they are still out there. You should avoid them because the aluminum wears quickly. The next best style is the nylon button thats available from several companies, including Comp Cams. This piece wears well yet has no moving parts that could fail.
Before checking cam endplay, its wise to investigate the relationship between the crank gear and the cam gear. Often the crank gear ends up farther forward on the engine than the cam gear. This will pull the cam forward. Use a straightedge to check this gear alignment. If necessary, use a shim between the cam and gear to adjust the gear forward. You can also purchase a crank gear shim from BLP that will move the crank gear forward to prevent rearward thrust, which could cause excessive wear on the gear and the block.
Checking and adjusting roller-cam endplay is best done before the engine is assembled. This way, you can set a dial indicator right on the end of the camshaft before the bellhousing cam plug is installed in the engine. We have photos of a trick tool the guys at Reggie Jackson Performance made that makes checking this endplay quick and easy and requires only a feeler gauge. If the rear cam plug is already installed, the endplay is best checked with one of those trick two-piece timing-chain covers from Comp Cams or BLP. These covers have a plug in the center that allows the use of a dial indicator.
The easiest way to adjust the cam endplay is to use the roller cam button. It offers shims installed on the button to adjust its height. These roller buttons are also available in three different lengths depending upon the application. If you choose to use the nylon button instead, these units are designed to be slightly long, allowing you to sand or grind the button down to establish the proper clearance.
Setting roller-cam endplay isnt a big deal but it is critical. Like most things in life, its the little details that can make all the difference. The bottom line is, if youre not sure, check it. You might be surprised by what you find.