Length & Rocker Arm Geometry
Before you order custom length pushrods for your new camshaft, please
read this important information.
A large number of variables are involved
in determining the correct length pushrod for your application.
Pushrod length is affected by any one or all of the following:
block deck height
head deck height
head stud boss height
rocker arm brand/design
cam base circle size
lifter design/brand/pushrod seat height
valve stem length
assume anything in determining the right pushrod for your new engine.
A pushrod that fits one engine may not necessarily work in another.
Any number of items can be different on your engine, requiring you
to use a different pushrod length. Following the steps below will
streamline the pushrod selection process, ensuring that you get
the right parts the first time.
Buy a checking pushrod
Do not buy pushrods when you buy the cam, lifters, and the rest
of the valve train components. As much as we would like to sell
you pushrods at this time, nobody can predict ahead of time what
length a given engine needs, unless it is bone stock. Instead, invest
in one of our checking pushrods at this time. They are on page 277
of the catalog. They come in two different designs, the more expensive
of the two being easier to measure once you have it adjusted to
the proper length for your valve train. Neither is particularly
expensive if you consider time lost and freight costs when returning
pushrods. Other companies offer their own versions of pushrod length
checking devices, funny little plastic things and such with a sheet
of complicated instructions to calculate the length. The main disadvantage
with these is that you have to order the pushrods and receive them
before you know if your calculations are correct. With a checking
pushrod, you can actually rotate the motor over and check the rocker
arm/valve tip relationship as you adjust the pushrod length. When
you get the correct geometry, it is a simple matter then to measure
the length and place an order. COMP Cams ® carries a large number
of various length and diameter pushrods in stock. So you can have
them overnight if you want, and the correct length the first time.
correct valve train geometry
What is the correct length pushrod for your application? The one
that produces correct valve train geometry. What is correct valve
train geometry? When the rocker arm roller tip rolls from the intake
side of the valve tip, across the center of the tip (at approximately
mid-lift), to the exhaust side of the valve tip (at full lift) and
back. See Diagram A.
the resulting pushrod
Measuring the length of a pushrod is really rather simple, but it
can be confusing at times. The most important thing to remember
is that different manufacturers measure pushrods differently. So
not all pushrods of a stated length will measure exactly the same.
The three most common pushrod measurements are shown in Diagram
B on the next page.
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
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.
Although the most difficult to measure (it requires a special length
checking gauge part #4997), 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 custom length pushrods listed
on pages 270-275 of this catalog are measured using this technique.
See Diagram B on the following page.
The above was not meant to confuse you needlessly. We know that
most people dont have access to the special gauge required
for these measurements or even to a dial caliper large enough for
most pushrods. Weve developed three 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.
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.
#2: This technique assumes you have purchased one of our less
expensive pushrod length checkers. Once fixed, you dont 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.
#3: In either of the above cases, if you are unsure as to how
to measure your checking pushrod, wrap it up and send it to us.
We will measure it and send it back to you along with a set of Hi-Tech
pushrods to match. Youll know when you receive them how long
they are by the length that is written on each one. Then if you
ever have to reorder, youll know what length to ask for. In
addition, these can act as a standard for future pushrod measuring
jobs that you may have.
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 you ever fail one at
the track and 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.