Howards Cams Pushrod Tools Calculate Reliable Valvetrain Geometry

The inherent problem with an overhead valve engine, especially those pushed to performance limits, is the intricate group of components known as the valvetrain. The assembly of rapidly “up and down” moving pieces demands the utmost attention to detail to survive significant compression, power adders, and shocking RPM demands.

The goal of proper pushrod length is to center the rocker arm over the valve stem. As the rocker travels in an arched motion, it will be slightly outward on the valve face at the maximum lift and inward when the cam is at the base circle. The key is to have these two extremes centered over your valve face.

Let’s say that just before you shift, you’re spinning your engine at 7,000 rpm, a conservative number in many cases. That engine RPM calculates to all valvetrain pieces cycling up and downward approximately 54 times per second. We pulled our Howards Cams selection of tools from our toolbox to get our geometry exact on our current big-block Chevy engine.

If you view the motion of your rocker arms as the engine rotates, it travels in an arc with the fulcrum as its pivot point. Since our big-block is now equipped with new 24-degree aftermarket aluminum cylinder heads, plus a new cam and roller lifters, this accumulation of parts from different manufacturers will typically demand a new pushrod length.

We pulled our Howards Cams degree wheel, threaded pushrods, and lightweight valvesprings out of our toolbox. We sealed the bottom end of our block after degreeing our cam and put our degree wheel back in place. We can use the wheel to find the maximum lift point with each cam lobe using our cam card.

Getting the exact geometry for the valvetrain relies heavily on the pushrod length. If the pushrod is too long, the rocker will travel across the valve stem too far outward towards the exhaust side of your cylinder head. Too short, and the contact pattern will ride more towards the intake side of the valve.

 Option One: Using A Rocker Arm Fixture

The most simplistic option for small-block and big-block Chevy heads is a molded plastic fixture from Howards Cams that replaces the rocker arm on most Chevrolet applications with factory valvetrain geometry.

This easy-to-use tool slides onto the rocker arm stud and rests on top of the valve stem. We then adjust our Howards length-checking pushrods to rest against the inner side of the fixture. Since big-block Chevy heads use different length pushrods between the intake and exhaust, you simply invert the fixture by the arrow indicators.

Our new heads are in place with the proper head gaskets installed. It is critical to have your engine's final dimensions set before measuring. We compared our old pushrods with the Howards rocker arm fixture, indicating they were too short for our new heads, cam, and roller lifter combination.

Since we are using new heads with unique valve angles compared to stock, and different roller cam components, we also used a more involved procedure to confirm our pushrod lengths. 

Cams (especially racing cams) can have differing base circles, greatly affecting your necessary pushrod length. Additional variables such as block deck height, cylinder head deck height, overall lifter dimensions, and the valve stem height can cumulatively alter the overall geometry.

Option Two: Engine Rotation and Valve Stem Marking

The most straightforward way to measure pushrod length is to view the contact surface between your rocker arm and the valve tip. With compressed air filling our number one cylinder, we first replaced our actual valve springs with a lightweight version that hold our valve, valve spring, and retainers in place. 

These light springs allow you to rotate your engine by hand, thus opening and closing the valves and simulating your valvetrain motion. Still, they will not apply excessive pressure against the threaded portion of your adjustable pushrod tools, which will easily cause damage to the simulated pushrod’s adjustable threads. 

The full-size valve springs are replaced with lightweight checking springs. They hold the valves and retainers tight while not having too much pressure to damage the threaded pushrod length tools. With a black marker, we heavily color the face of our valvesprings.

The threaded pushrod tools from Howards Cams are offered in five length ranges that depend on your engine configuration. Every type of engine make and rocker arm configuration changes how you determine your pushrod lengths, so do your specific homework.

Since the big-block Chevrolet uses two different lengths between the intake and exhaust pushrods, we used two adjustable thread pushrods that span the 8.500- to 9.800-inches for the intake side and 7.500- to 8.700-inch tools for the exhaust. If you build a wide range of engines, kits are readily available with multiple pushrod checkers that span from short (6.500-inch) to your longest needs (11.500-inch).

In our current engine, we are using solid roller lifters. Therefore, all of our measurements and markings will be performed at zero-lash. This lash adjustment means that there will be no gap between the rockers and the valve-stem face. 

Rotating the engine 4-5 times, we wear a pattern into our valve face. Our first try indicates the rocker was riding towards the outboard part of the valve face. We shortened our pushrod tool slightly and repeated the process until a centered scribe was accomplished.

The next step is to mark the face of your intake and exhaust valve with your choice of a machinist dye, or a heavy coating from a black felt tip marker would also get the job done. With your adjustable pushrods and rocker arms in place, rotate the engine by hand three to four times to establish a wear pattern on the marked valve stem face.

After removing the rocker arms, you will see a wear pattern where the rocker arm tip traveled across the valve. If the wear pattern is located inward or outward of the valve face, you must adjust the pushrod length and repeat the process until your markings are centered.

Some engine designs and competition rocker arm assemblies use a ball tip at the rocker with a cupped end at the top of the pushrod. Different pushrod length tools can measure these configurations properly. These pushrod checking tools are available, especially from competition rocker arm companies that use this design.

Place the threaded portion of the pushrod tool near the lifter to avoid damaging it at the guide plate when rotating. A long (expensive) dial caliper is not necessary to measure pushrods; we machined a small piece of tubing to an exact length, measured the remaining pushrod, and added those two dimensions together.

Wanting to be prudent at this point, since we have entirely new cylinder heads, we used this process on the cylinders on each corner to ensure our overall valvetrain is square with the cam centerline. If you see any difference between the corners, you have a dimensional problem, typically with the deck surfaces of either the block or heads.

Pushrod Lengths Related To Hydraulic Lifters

When deriving pushrod lengths with hydraulic lifters, you need to factor in additional pushrod length to account for lifter preload. By following the zero-lash procedure above, you then refer to the amount of “preload” distance the lifter manufacturer wants the piston to be pushed down within the lifter and add that to your necessary pushrod length.

This preload is a bigger issue with popular LS-based performance and racing engines. More manufacturers offer a larger selection of big-lift cams that use hydraulic roller lifters. The LS engine also uses a solidly mounted shaft rocker arm with no lash adjustments.

Many engine designs have different configurations, but these theories still apply. An LS valvetrain has non-adjustable rocker arms, but establishing that the pushrod length preloads into the lifters to their proper depth is just as critical.

For the sake of example, many performance lifter manufacturers specify a .050-inch “preload” of the lifter piston down into the lifter body. Once you measure your pushrod length, in this, case you would add that .050-inches to your pushrod length.

Putting The Train In Valvetrain

A locomotive train is the cumulative connection of multiple rail cars. If you’ve ever witnessed a long train begin to move from a standstill, you will hear countless “bangs” as each connection between each car pulls tight. Your valvetrain is also an accumulation of components that cannot survive inaccurate tolerances.

Working out our valvetrain geometry is probably the final intricate step before buttoning up our big-block and heading for the track.

Additionally, a refined rocker arm motion results in less distance the rocker travels across the valve stem. As mentioned earlier, that is good for both the rockers and your valves at high RPM levels. 

This handful of reasonably priced tools from Howards Cams defines the difference between a racing or performance engine built by a craftsman, or just a “close enough” engine assembly. A valvetrain failure typically damages much more than just those specific components. So, unless you want destroyed pistons or a set of cylinder heads winding up in the recycling bin, be a craftsman.

Article Sources

About the author

Todd Silvey

Todd has been a hardcore drag racing journalist since 1987. He is constantly on both sides of the guardwall from racing photography and editorship to drag racing cars of every shape and class.
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