HOW TO SELECT THE BEST PUSHROD FOR YOUR APPLICATION
OPEN VALVE SPRING PRESSURE
There are a few terms used in determining which pushrod would best fit your needs that would be beneficial for you to understand. One major factor for determining the correct tube diameter and wall thickness is “open valve spring pressure”. Open valve spring pressure is the pressure, in pounds, exerted against the rocker arm when the valve is in the fully opened position. Some factors affecting this load number are the valve train geometry (angles of lifter to pushrod and pushrod to rocker arm), rocker arm ratios, whether the engine is naturally aspirated, and the normal operating RPM of the engine.
RADIUS OF ENDS
Determining which ball and/or cup you require is also very important. We will quiz you to help us determine what ends you need on your pushrods. For many of the common engines we know the radius of the stock pushrod tips, but when rebuilding and/or replacing components with aftermarket parts, it’s never safe to assume. The most accurate way of determining the correct radius (curve) that is needed for each end of the pushrod is by measuring with a set of radius gauges. Measure the radius of the puck (pocket) of the lifter and the ball or cup of the rocker arm that will be used. Most machine shops will have radius gauges if you do not. One way to determine, on a ball and cup style pushrod, whether the ball and cup are the same radius is to take two pushrods that you know are correct and fit the ball end of one pushrod into the cup end of the other and check the fit. In some cases the best way to get the correct ends for your project is to send us samples of the parts being used and we will measure them.
This can be one of the hardest measurements to determine. Because of varying valve train geometry and design, determining the correct pushrod length is a science unto itself. First, let’s tackle some terminology.
There’s “overall length”, which is the length measured from extreme end to end. This length is from the very end of the ball to the very end of the other ball end or cup end. A couple factors affect this measurement. If the pushrod has oil holes, then the measurement could be off by the amount of material machined from the pushrod end for the oil hole subtracted from the overall length of the end. This usually doesn’t adversely affect the pushrod length depending on the diameter of the oil hole.
The “theoretical length” is the actual measurement of the pushrod if there were no oil holes in the ends. Don’t let theoretical and overall length measurement confuse you because the difference in length measurement is usually very slight.
The “effective length” is the most accurate measurement for cup and ball style pushrods due to varying cup depths (as much as .060” difference) from one manufacturer to another. The effective length is measured from the contact point on both ends of the pushrod (bottom of the inside of the cup to the tip of the lifter end). You can take an overall measurement (outer top edge of cup to tip of lifter ball) then measure how deep the cup is and subtract that measurement from the overall length to get the effective length. To get a more accurate length measurement, place a ball bearing the proper size inside the cup, measure the overall length over the ball bearing, and subtract the full diameter of the bearing to get the same result.
We're sure questions will arise, so please don't hesitate to contact us for assistance.