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.
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.
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.
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 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.
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.
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.
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.
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.