Engine Rebuilding -Automotive Machine Shop
Vintage - Marine & Performance Engines
SETTING VALVE LASH FOR MECHANICAL TAPPET CAMSHAFTS
Turn the crankshaft by hand in the direction of normal engine rotation until the exhaust pushrod of the cylinder that you are adjusting begins to move upward, opening the valve.
You can now adjust the intake lash for that cylinder by tightening the intake rocker nut with the correct thickness feeler gauge is inserted between the valve stem and the tip of the rocker. Tighten the rocker nut until there is a slight drag when moving the feeler gauge.
To adjust the exhaust valve, rotate the crankshaft until the intake pushrod opens the valve all the way and then goes 1/2 to 2/3 of the way back down. Adjust the exhaust rocker nut (with the correct feeler gauge) using the same procedure as with the intake rocker above. Repeat for all cylinders.
After setting your valve lash with the engine cold, start it up and break-in following the appropriate procedures. After breaking in the camshaft, your engine will be at normal operating temperature. Due to thermal expansion, your valve lash will now be tighter than it was when the engine was cold. Repeat the entire adjustment process above to ensure that your valve lash matches that specified by your cam card at normal operating temperature.
SETTING VALVE LASH FOR HYDRAULIC TAPPET CAMSHAFTS
When installing a hydraulic camshaft or lifters, or new rocker arms, it is necessary to establish the correct lifter preload in order to ensure great performance and long engine life. Insufficient preload will cause excessive valve train noise and wear. Excessive preload will cause rough idling and low manifold vacuum, and can even lead to severe engine damage. With an adjustable valvetrain, proceed as follows:
Install the pushrods into the motor. Install the rocker arms, pivot balls and nuts onto the rocker studs. Be sure the pushrods are seated correctly in the lifter pushrod cups and rocker arm seats. Turn the engine over by hand in the direction of rotation until the exhaust pushrod just begins to move upward to open the valve. You are now ready to adjust the INTAKE rocker of the same cylinder.
Carefully tighten the nut on the intake rocker arm while spinning the pushrod with your fingertips. You will feel a slight resistance in the pushrod when you have taken up all of the clearance. This is referred to as “zero lash”. Now turn the adjusting nut to the specified preload –this will typically amount to ¼-3/4 of a turn, but will vary based on lifter model .
Turn the engine in its rotation direction until the intake pushrod comes all the way up and almost all the way back down. Now set the EXHAUST rocker to “zero lash” and add the specified preload. Repeat this process for all remaining cylinders.
VALVE LASH FOR HYDRAULIC TAPPET NON-ADJUSTABLE ROCKER ARMS CAMSHAFTS
When setting lifter preload with nonadjustable rocker arms, a different procedure must be followed than that used with adjustable rocker arms. After applying lube, install the pushrods and torque all rocker arm bolts to specification. Turn the motor by hand in the direction of engine rotation until both the exhaust and intake valves have opened and closed completely. Allow a couple of minutes for the lifter to bleed down.
Using the valve cover gasket surface on the head as a reference point, place a mark on the pushrod. It is advisable to use a permanent marker or pencil to mark the pushrod. The smaller and more defined the mark, the more accurate the measurement. Be sure the reference point you choose for the first mark is easily accessible and easy to duplicate. You will be marking the pushrod twice. It must be from the same reference point and angle for the measurement to be accurate Loosen the rocker or rocker shaft bolts. Leave the rockers on the head so that they will support the pushrods. Be sure the pushrods are standing free in the lifters and do not have any preload. Using the same reference point, place a 2nd mark on the pushrod. Make sure the angle and reference point are the same as the first mark.
You now have two marks on the pushrod; one with the assembly bolted into place as the motor will run, and the second mark with the lifter unloaded. The distance between these two points will represent the amount of lifter preload present.
If you find that the preload is not within +/- .010” (High Energy lifters) range of your specified preload, adjustability will be necessary. The simplest way to accomplish this is by using different length pushrods. COMP Cams carries a full line of pushrods for such instances. When measuring to find the correct length needed, be sure to include the amount of preload that the lifter requires in the measurement.
If your engine has pedestal style (bolt mounted) rockers, you can use shims under the pedestal to lessen the preload. This method also works for shaft mounted rocker systems. Longer pushrods will be needed for insufficient preload.
In most cases, you will only need to check one intake and one exhaust pushrod; however, if your valve stem heights are not equal, you will need to check preload on each valve. If this procedure is not followed, it will almost certainly result in a poor running engine and ultimately engine failure.