LS Cam Upgrades - Matching Push Rods To A Cam

One of the   most  common  questions that we get asked  is “which push rods match this cam?” This is a very important decision, as the push rods are the link between the cam and the valves, and they affect the performance and reliability of your engine.

There seems to be a common misconception that's possibly left over the days of reground cams where you were left with a smaller base circle than factory and a longer push rod was used to make up for the removed material.  

Here’s some of the information that we provide our customers, that can help you to find the right push rods for your own cam and engine.

What’s In A Base Circle Size?

All of our cams have a standard base circle size, so you might think that a stock push rod length of 7.4 inches would work fine, but that’s not always true. There are other variables that can affect the optimal push rod length for your engine.

Some of the variables that can alter the required push rod length are:

• Valve train wear: Over time, the valve train components can wear out and change their dimensions, such as the valve seat, the valve guide, the lifter bore, and the rocker arm pivot. This can affect the valve lash and the lifter preload, which are the clearances between the valve train components.
• Machine work: Some machining operations can change the distance between the cam and the valve, such as milling the cylinder heads, decking the block, or installing thicker or thinner head gaskets. This can also affect the valve lash and the lifter preload.
• Aftermarket parts: Some aftermarket parts can have different specifications than the stock ones, such as the rocker arm ratio, the valve stem length, the valve spring height, or the lifter design. This can change the valve lift and the lifter preload, which are the amount of valve movement and pressure generated by the cam lobe.
• Lifter type: There are different types of lifters available, such as hydraulic, solid, roller, or flat tappet. Each type of lifter has a different amount of plunger travel, which is the range of motion of the lifter plunger inside the lifter body. The plunger travel determines the lifter preload, which is the amount of pressure that the lifter exerts on the push rod and the valve when the valve is closed.

All of these factors can influence the optimal push rod length for your engine, and they need to be taken into consideration when choosing your push rods.

Aim For The Lifter Manufacturer’s Specified Lifter Preload

The main goal when selecting a push rod length is to achieve the correct lifter preload.

This lifter preload can vary depending on the complexity of your build, but for a typical cam swap, it is usually somewhere in the middle or slightly lower than the middle of the lifter’s total plunger travel.

The lifter preload is important for several reasons:

• It ensures that the valve train components are in constant contact with each other, eliminating any unwanted noise or vibration.
• It prevents the valve from opening too early or too late, which can affect the engine’s performance and efficiency.
• It prevents the valve from bouncing off the seat or floating at high RPM, which can cause a loss of power, engine damage, or even a catastrophic failure.
• Therefore, achieving the correct lifter preload is essential for the optimal operation of your valve train and your engine.

How to Measure the Lifter Preload

The best way to measure the lifter preload is by using a dial gauge, which is a device that measures the amount of movement of a metal tip. The dial gauge is attached to the rocker arm or the valve spring retainer, and the metal tip is placed on the valve stem or the push rod.

The lifter preload is measured by following these steps:

• Rotate the engine until the cam lobe is on the base circle, which is the lowest point of the lobe. This is when the valve is fully closed and the lifter is at zero lift.
• Adjust the rocker arm bolt until there is zero lash, which is when there is no clearance between the valve train components. You can check this by spinning the push rod with your fingers, it should feel snug but not tight.
• Zero the dial indicator, which is when the needle points to zero on the dial.
• Tighten the rocker arm bolt by the amount specified by the lifter manufacturer, usually between 1/4 and 3/4 of a turn. This will depress the lifter plunger and create the lifter preload.
• Read the dial indicator, which will show the amount of lifter preload in thousandths of an inch. For example, if the needle points to 0.060", that means the lifter preload is 0.060".
• The lifter preload should be within the range recommended by the lifter manufacturer, usually between 0.040" and 0.080". If the lifter Pre load is too high or too low, you will need to change the push rod length accordingly.

It’s Worth Doing Right

Choosing the right push rods for your cam is not as simple as matching the factory base circle size.There are other factors that can affect the optimal push rod length, such as valve  train wear, machine work, aftermarket parts, and lifter type.

The main goal is to achieve the correct lifter preload, which is the amount of pressure that the lifter exerts on the push rod and the valve when the valve is closed. The lifter preload can be measured by using a dial indicator, and it should be within the range recommended by the lifter manufacturer.

The best way to choose the right push rod length is by using a push rod length checker which in length adjustable push rode and measuring the lifter preload until you find the one that gives you the best performance and reliability.

Of course, if you have any questions about any of this, you can ask the Malex Motorsports team. Our expertise is available to our customers to help them get the most out of their vehicles.