Theoretical Length: This assumes that the pushrod has no oil hole in the end of it. Therefore, the radius at either end is complete, which lengthens the pushrod approximately .017” in the case of a 5/16” pushrod with .100” diameter oil holes, minimally chamfered.
Actual Length: This is what you would measure if you had a set of calipers large enough to measure over the oil holes at each end of the pushrod. This is the measurement that most people can relate to.
Unfortunately, this measurement is affected not only by the diameter of the oil holes but also by the entrance chamfer for each oil hole.
Gauge Length: Although the most difficult to measure (it requires a special length checking gauge), this measurement is the most reliable. This is because the oil holes and their chamfers are eliminated from the measurement.
The only problem is that not all companies use the same gauge diameter. COMP Cams® uses a .140” gauge diameter. All of the Magnum and Hi-Tech™ Pushrods listed in this catalog are measured using this technique. See Diagram B on the following page.
4. Simple measurement techniques
The above was not meant to confuse you needlessly. We know that most people don’t have access to the special gauge required for these measurements or even to a dial caliper large enough for most pushrods.
We’ve developed two techniques to help you determine exactly how long the pushrod is that you so diligently played with until the perfect valve train geometry was achieved in your engine.
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Pushrod Measurement Techniques
Technique #1: This technique assumes that you have purchased one of our Hi-Tech™ Pushrod Length Checkers. These are marked with a standard length stamped in them. This number represents the gauge length of the part (.140” gauge diameter) with the two halves screwed completely together.
Extending the pushrod one rotation lengthens the gauge length .050”. For example, a pushrod stamped 7.800 screwed apart one rotation would be 7.800” + .050” = 7.850” gauge length.
Therefore you would order the part number from the catalog that matches this gauge length, since gauge length is how they are listed.
Technique #2: This technique assumes you have purchased one of our Magnum Pushrod Length Checkers. Once fixed, you don’t need to have an expensive gauge or a pair of calipers to measure it.
You just need a pushrod of a known length to compare it to (a standard). Then use a pair of common 6” calipers to measure the difference between the standard and yours.
A few final hints about pushrods in general. It is always a good idea to buy a few spares when purchasing a set of custom length pushrods, and stick them in your toolbox. If one ever fails at the track and you need a replacement, it would be nearly impossible to borrow one from a fellow racer.
Another hint involves cup end pushrods. Measuring them for length is especially difficult, no matter which technique above you choose to use. The size and shape of the cup end varies greatly from manufacturer to manufacturer, so measuring from the ball end to the cup end over the cup surface is a dangerous practice. The best strategy is to drop a 5/16” diameter steel ball into the cup end, and do all your measuring over this ball, subtracting the 5/16” diameter (.3125”) to figure the length. |
