Tips for setting preload on hydraulic lifters
Hagerty member Chris Kuzma writes: I’m rebuilding a 390-cubic-inch V-8 for my AMC AMX. I have roller rockers and stud girdles I’d like to use with a hydraulic cam. How do I adjust them correctly, especially for start-up?
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Chris’s AMX, like most 50-year-old American cars, has the camshaft in the engine block, one of three commonly used valve-train configurations, the other two being overhead cam and double overhead cam. A cam-in-block, or pushrod, engine has long pushrods reaching from the cam up to the cylinder head(s). As the cam spins, its lobes move lifters, or tappets—little buckets that sit between the pushrods and cam lobes and spread out the mechanical load. The lifters, as their name implies, lift the pushrods, which move seesaw-like rocker arms in the head(s) to open and close the valves.
On any engine, play in the valvetrain must be kept to a minimum, but some clearance is necessary to ensure that the valves close completely. On a pushrod engine with solid lifters, a small gap must be maintained to allow for thermal expansion of the components. The size of this gap needs to be adjusted mechanically at the ends of the rocker arms, with threaded studs and lock nuts or with rotating eccentrics. However, during the 1970s and ’80s, hydraulic lifters were increasingly employed to eliminate manual valve adjustment. Hydraulic lifters are hollow cylinders with spring-loaded pistons inside them and one-way check valves that allow them to fill with oil when the engine is running. Because oil is incompressible, the lifters pump themselves up, essentially zeroing out the clearance. The “girdles” Chris mentions are devices that attach to the studs on which the rocker shafts pivot. They help eliminate valvetrain flex in high-torque, high-horsepower applications.
Chris is installing a different camshaft with hydraulic lifters, using different rockers held in place by a girdle, and it all requires proper adjustment. On the surface, this sounds odd because the whole point of hydraulic lifters is to eliminate manual valve adjustment. However, the height of the rocker arms might need to be adjusted to preload the lifters and help minimize valve clatter on start-up before the lifters pump up with oil.
If the car has its original factory rocker studs with integral shoulders that screw flat against the head, there is no preload adjustment. That may be sufficient, even with the new rockers and cam, provided the lift of the new cam isn’t more than 0.5 inch, that the roller rockers have the same geometry as the originals, and that the car has the original 7.767-inch pushrods. If any of those assumptions aren’t true, Group 19 studs or other aftermarket adjustable studs should be fitted to provide adjustment.
[Editor’s Note: We’ve updated the following section to explain the procedure more clearly.]
To preload the rockers, with the engine cold, rotate each piston to top dead center (TDC) of the compression stroke so the cylinder’s intake and exhaust valves are both closed, adjust each rocker stud until the pushrod is loose, then tighten the stud until resistance is felt while turning the pushrod. This is the zero-clearance point. It is recommended that the stud be turned one-half to one turn past this point to preload the rocker and then tighten down the locknut. I don’t think the presence of the girdle changes this, but I’d recommend you recheck to make sure the pushrods aren’t too loose or too tight to turn (again, rotating each piston to TDC) after the girdle is installed.