Many Pontiac enthusiasts seeking more power look to the heads for the answer. Milling, longer valves, taller springs and porting may necessitate a cam change, higher ratio rockers or both in order to capitalize on the added flow and power potential. I am one of these power-hungry Pontiac guys.

For example, my 406 (+.030” 400) sports a Comp Cam 270H Magnum hydraulic cam. With .313” lift at the lobes, a stamped rocker arm provides a theoretical lift of .470”. (.313 x 1.5 = .4695”) With a 1.65 ratio rocker the lift approximates .516”. Considering the extensive porting work performed on my d-port heads, the higher ratio rockers are a good bet to help the engine develop it’s potential.

When a high ratio rocker is used, the pushrod, which transmits the open and closing events of the cam, moves closer to the rocker arm stud (intake side of the engine). Consequently, all pushrods holes (except heads #614 or #722 requires the pushrod holes to be elongated 1/3rd of the way down toward the intake side to avoid any possible interference between the rod and head. Every pushrod must move freely! Elongating the pushrod hole is performed off the engine. It is done with stones or high-speed carbide bits.

However, valve train geometry can be altered when changes are made to the heads. Valve train geometry must not be overlooked and one must never assume that it is okay. The object of proper valve train geometry is to get the most out of your heads and that includes durability. When the geometry is not correct, overloading may result shortening the life of the valve guides.

What is proper valve train geometry as it applies to our traditional Pontiac engines? If we look at the valve tip surface imagine it being divided in half. The ideal contact point is slightly before the centerline on the intake side (when the valve is closed) and slightly after on the exhaust side (when the valve is open). The contact point of the rocker arm moves away from the intake side to the exhaust side and back. Below are simple illustrations.

How do you go about checking valve train geometry? It’s not a difficult process. Below are the steps to follow:

Step 1:

With the heads bolted in place you’re ready to begin. Conduct your testing on #1 cylinder, on both the intake and exhaust valves.

On #1 cylinder, locate TDC. On the camshaft this position will be on the base circle—where there’s no lift. Both valves will be closed when preliminary lash adjustments are made.

Step 2:

Clean off the valve tips so there is no residual oil on them. Take a felt tip marker (black or blue is good) and color the valve tips on both valves. Coloring the valve tips will allow you to observe a contact pattern when the rocker is in place.

Step 3:

With #1 at TDC, and the valve tips colored, install the rocker arms on each valve on #1. Slip in a standard sized pushrods, the ones you hope to use, and adjust the lash on both valves.

Step 4:

Valve Adjustment revisited: Turn the pushrod in you fingers while tightening the poly lock nut. When resistance is felt, you’ve reached zero lash. An extra 1/2 turn typically seats the pushrod (pre-load) in the hydraulic lifter. Don’t bother with the set screws for testing.

Step 5:

With the #1 rockers in place and adjusted, turn the engine over by hand two full rotations returning it to TDC (use the damper for reference).

Step 6:

Remove the poly lock nuts and rocker arms and examine the top of the valve tips.

There should be a pattern worn into the colored markings. Look closely. Hopefully they will be near the centerline of the valve tip as illustrated above. When the pattern is not centrally located as the illustration below shows, a different length pushrod is required.

Determining the proper length pushrod can be by trial and error. However, adjustable pushrods can be used to simplify the determination process.

Step 7: Measure the length of the pushrod you used. Then add or subtract .050" to an adjustable pushrod you will use. Follow Steps 1- 6 again and determine the pattern. If the pattern is centrally located, measure the pushrod length. Comp Cams and other similar companies have pushrods of varying lengths in 5/16' diameter. Talk with a tech representative from the specific manufacturer for details on what dimensions they need so you can order the right size pushrods. Note: Comp Cams recommends ordering a few extras in case you bend one. That's pretty good advice, I think.

Copyright © 1997-2005 Bill Boyle. All rights reserved.
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Note: It's possible to fabricate your own adjustable pushrod from a stock pushrod. It requires cutting it and taping the inside for threads so that you can fasten the top part of the rod to the threaded stock. I've tried doing this and it took me hours to do and in the end was less than happy with my effort. A good adjustable checking pushrod, like the one from Comp Cams that allows one full turn on the thread is a choice tool and will be more accurate in my opinion. Save time and effor with a good (measuring) checking pushrod. --Bill
Tip: I recommend using a permanent marker, blue or black works pretty well.

To get the ink on the marker to stick to the top of the valve, you'll need to clean the flat surface of the valve tip. I've found that Brakeleen works best in removing oils.

Color the top of the intake and exhaust valve completely. With roller tip rockers or true roller rockers, the contac pattern will appear but could take more than two full crank rotations of it to do so. If in doubt rotate the crank again by hand. The fricition between the roller and the tip is present to remove or lighten the ink. In either case, you'll need to look closely. Stamped rockers will reveal a pattern, it may be appear more erratic looking and less defineable than with rollers.