Calculating the correct pushrod length is a critical step in building or rebuilding an engine. An incorrectly sized pushrod can lead to a host of problems, from excessive valvetrain wear and noise to significant performance loss and even catastrophic engine failure. This guide will walk you through the process, ensuring your valvetrain operates smoothly and efficiently.
Pushrod Length Calculator
Why Proper Pushrod Length Matters
The pushrod is a seemingly simple component, but its length profoundly impacts your engine's valvetrain geometry. Correct length ensures:
- Optimal Rocker Arm Geometry: The rocker arm should ideally sweep across the valve stem tip in a minimal, centered arc. This minimizes side loading on the valve stem and guide, reducing wear and friction.
- Correct Lifter Preload: For hydraulic lifters, the pushrod's length determines the amount of preload applied to the lifter's internal plunger. Too much or too little preload can cause lifter collapse, pump-up, valvetrain noise, and performance issues.
- Reduced Valvetrain Wear: Proper geometry and preload prevent excessive stress on all valvetrain components, including lifters, pushrods, rocker arms, valve tips, and valve guides.
- Consistent Performance: A stable valvetrain translates to consistent valve timing and lift, maximizing engine performance and reliability.
Tools You'll Need
Before you begin, gather the following essential tools:
- Adjustable Pushrod: This is crucial. It allows you to mock up different lengths to find the ideal one.
- Checking Spring (Light Spring): Replaces the heavy valve spring during measurement, making it easy to rotate the engine and move the valvetrain without damaging the lifter.
- Solid or Hydraulic Lifter (The exact one you'll use): Ensure it's the same type you'll be running.
- Rocker Arm (The exact one you'll use): Install it with its proper stud/pedestal and torque it to spec.
- Dummy Head Gasket: To simulate the installed deck height.
- Prussian Blue or Felt-Tip Marker: To mark the valve stem tip for checking wipe pattern.
- Torque Wrench: For correctly torquing rocker arm studs/pedestals.
- Dial Indicator (Optional but Recommended): For precise measurement of lifter travel and valve lift.
The Step-by-Step Adjustable Pushrod Method
This is the most accurate and commonly used method to determine pushrod length.
1. Prepare the Cylinder Head and Block
- Place a dummy head gasket on the block.
- Install the cylinder head (at least one) you'll be using and torque it to spec.
- Install the cam and lifters (the actual ones you'll use).
2. Install Checking Spring and Rocker Arm
- Remove the valve spring from a chosen cylinder (typically cylinder #1 intake or exhaust).
- Install a light checking spring and retainer on that valve. This allows you to easily move the valve without compressing a heavy spring.
- Install the actual rocker arm you will be using for that cylinder onto its stud or pedestal and torque it to its final specification.
3. Find Optimal Rocker Arm Geometry (Valve Stem Wipe)
The goal here is to ensure the rocker arm's roller or tip sweeps across the center of the valve stem tip. This minimizes side loading on the valve guide.
- Mark the tip of the valve stem with Prussian Blue or a felt-tip marker.
- Install your adjustable pushrod. Adjust its length until there is zero lash (no play) between the lifter, pushrod, and rocker arm, and the valve is just barely closed.
- Carefully rotate the engine through one full revolution of the camshaft (two revolutions of the crankshaft). This will open and close the valve.
- Remove the rocker arm and inspect the mark on the valve stem tip. You should see a narrow, centered wear pattern (wipe mark).
- Adjust and Re-check:
- If the wipe pattern is too far towards the intake manifold side (away from the rocker arm pivot), the pushrod is too short. Lengthen the adjustable pushrod.
- If the wipe pattern is too far towards the exhaust manifold side (closer to the rocker arm pivot), the pushrod is too long. Shorten the adjustable pushrod.
- Repeat this process until you achieve the narrowest, most centered wipe pattern possible. This establishes your "zero lash, optimal geometry" length.
4. Measure the Adjustable Pushrod
Once you have achieved the ideal wipe pattern, carefully remove the adjustable pushrod without changing its length. Measure its exact length using a micrometer or a specialized pushrod length checking tool. This is your base length for calculation.
5. Determine Final Pushrod Length Based on Lifter Type
Solid Lifters:
For solid lifters, you typically run a specific valve lash (e.g., 0.010" to 0.020"). The pushrod length you measured at zero lash (optimal geometry) is your target length. You will then set the valve lash using the rocker arm's adjusting nut or screw. Therefore, the measured adjustable pushrod length is essentially your final required pushrod length.
Hydraulic Lifters (Flat Tappet or Roller):
Hydraulic lifters require a specific amount of "preload" to function correctly. This preload compresses the lifter's internal plunger slightly, taking up any slack in the valvetrain and allowing the lifter to pump up with oil. A common preload range is 0.020" to 0.060", with 0.030" to 0.040" being a widely accepted sweet spot for many applications.
To calculate for hydraulic lifters:
Final Pushrod Length = Measured Adjustable Pushrod Length (Zero Lash, optimal geometry) + Desired Hydraulic Lifter Preload
Use the calculator above to easily add your desired preload to the measured length.
Understanding Lifter Preload
Hydraulic lifters are designed to maintain zero lash in the valvetrain by using engine oil pressure to keep their internal plunger extended. However, they need a small amount of initial compression, or "preload," to function correctly. This ensures the plunger is always engaged and doesn't "rattle" at zero lift.
- Too Little Preload: Can cause valvetrain noise (ticking), potential lifter pump-up at high RPM, and inconsistent valve timing.
- Too Much Preload: Can cause the lifter to "bottom out" (plunger fully compressed), holding the valve open slightly, leading to loss of compression, misfires, and potential piston-to-valve contact.
Always consult your lifter manufacturer's recommendations for the ideal preload range.
When to Recalculate Pushrod Length
Any change to the valvetrain or cylinder head components can alter the required pushrod length. Always re-measure if you change:
- Camshaft (different base circle diameter)
- Cylinder Heads (different valve seat depth, valve stem length, rocker stud height)
- Head Gaskets (different compressed thickness)
- Lifters (different pushrod cup height)
- Rocker Arms (different pushrod cup location relative to pivot)
- Deck Height (block milled)
- Valve Job (changes valve stem height relative to spring seat)
Conclusion
Calculating pushrod length is not a step to be skipped or guessed. Taking the time to accurately measure and select the correct pushrods will contribute significantly to your engine's longevity, performance, and overall reliability. Follow these steps carefully, and you'll be rewarded with a quiet, efficient, and powerful valvetrain.