Somewhere along the line, as stock car racers, we've made a deal with the devil from which there is no turning back. Our lust for ever-increasing power levels is insatiable. To that end, inventive parts manufacturers and engine builders have been successful in squeezing power levels previously thought impossible from a carbureted pushrod V-8 small-block. Amazingly, as power has increased, so has durability.
But the price we have to pay for added power and durability is the need to have every part and piece within the engine meet very exacting tolerances. Too much or too little in either direction will lead to a costly failure. Take, for example, pushrods. In the "good old days," finding the perfect length for an engine wasn't as much a priority as it is now. That's because solid, non-roller rockers have a large flat that makes contact with the valve stem tip. Missing the pushrod length by a little bit wasn't much of a problem, because the flat on the rocker tip covered for the error. But today, almost every race car in every class can run roller-tip rocker arms. With the roller, the contact point between the rocker and the valve stem is very narrow. If you miss the mark, even by a little, you will get accelerated valveguide and valve stem wear. Also, big-lift camshafts and high-ratio rocker arms only exacerbate the problem. The point at which the rocker tip contacts the valve stem is controlled by the pushrod length, so getting that correct in today's racing engines is absolutely critical.
The problem is that when it comes to custom handbuilt engines, it is almost impossible to predict the correct pushrod length before the engine is actually assembled. Factors that can affect pushrod length include, but aren't limited to, the following: block deck height, cylinder head deck height, camshaft base circle, lifter design and height, valve stem length, and even the design of the rocker arms and corresponding mounting systems. Changing any of these variables can require significant length changes, so it is often difficult to predict exactly what you will need ahead of time. Instead, the easiest and most reliable plan is to wait until the engine is assembled to determine how long the pushrods need to be. Then, order the pushrods from your performance parts distributor and have them shipped overnight to your location.
So What? So what if I install the wrong length pushrods, you say? The engine still runs and the power seems to be OK. Yes, it might be in the beginning, but improperly sized pushrods-especially in an engine equipped with roller-tipped rockers-can be disastrous. When the valvetrain geometry is correct, the tip of the rocker makes contact with the top of the valve stem just north (the intake side) of the center of the valve tip, moves across the center of the valve tip at mid-lift and to the exhaust side of the tip at maximum lift, and then returns as the valve closes. The distance the roller tip travels on either side of the center of the top of the valve stem is equal. This maximizes the downward force the rocker places on the valve and minimizes side-loading on the stem.
When the pushrod length is not correct, the rocker arm tip won't be centered over the valve stem. When the camshaft activates the valvetrain, part of the force created by the motion of the rocker arm will push the valve stem laterally as well as vertically. When this happens, the valve stem scrubs against the valveguide in the head, causing excessive wear on both the valve stem and the guide. This wear will open up the valveguides and allow the valve to flop around. If you are lucky, the only problems will be poor valve control (engine power will drop sooner at the higher rpm levels), a loss of compression, and an engine that mysteriously starts burning oil. If it isn't caught, however, it can lead to a broken valve head and a junked engine.
Finding Center There are several ways to find proper pushrod length, but when it comes to stud-mounted rocker systems, the simplest way is to use a Sharpie and a wrench to turn the crankshaft. Even the most sophisticated engine builders use this method. Keith Dorton of Automotive Specialists says it is the most reliable and simplest way to determine correct pushrod length.
Begin by setting up the components for one cylinder. Because he builds hundreds of high-end engines a year in all different racing classes, Dorton keeps pushrods of multiple lengths and thicknesses on hand and simply test fits different ones until he finds exactly what he needs. Most of us, however, don't have that luxury. Instead, you can use an adjustable-length checking pushrod designed especially for this task.
Checking pushrods are sold by different manufacturers, but most operate on the same principle. They are typically two-piece units that are threaded to allow expansion in length. You can adjust the checking pushrod's length until you have what you need, then use that as a guide to order the real set. Some, such as Comp Cams' Hi-Tech checking pushrods, are threaded so that each revolution is equal to 0.050 inch of length. The pushrod's collapsed length is marked, so after you find the correct size, simply count the number of revolutions the pushrod has been expanded and you will know the right size to order the actual pushrods.
On stud-mounted rocker systems, finding correct pushrod length is critical because it determines where the rocker tip contacts the valve stem. In this example, the pushrod is too short, and the roller tip isn't centered over the valve stem. |
Engine builder Keith Dorton of Automotive Specialists begins the process by covering the valve stem tip with ink from a Sharpie. This will be worn off when the rocker makes contact with it after just a few revolutions of the crank. You can also use machinist's dye. |
With the lifter on the base circle of the cam, install either a pushrod that you think is the best length or a checking pushrod and the rocker arm, and then zero out the lash (or if you are using a hydraulic lifter, tighten to the recommended preload). Now, turn the motor over a few times by hand. |
Remove the rocker and check the result. The line where the ink has been worn away shows where the rocker's roller tip has contacted the top of the valve stem. It should be centered, but the line is a little too high here. That means the pushrod is too short. |
A checking pushrod, such as this one from Comp Cams, is made in two threaded sections that allow it to expand. You can expand or collapse the pushrod until you find the correct length and then use that information to order your real set. |
A checking pushrod, such as this one from Comp Cams, is made in two threaded sections that allow it to expand. You can expand or collapse the pushrod until you find the correct length and then use that information to order your real set. |
After another try with a slightly longer pushrod, Dorton has the mark centered over the tip on the valve on the right. |
From the side, you can see how the rocker is going to contact the valve tip and at least get close before beginning your tests. |
During this stage, you should also check to make sure your rocker studs are the correct length. When the rocker arm is in the correct location, you should be able to see most of the studs' threads for the adjuster nut. If too many threads are covered by the fulcrum, the lock nuts won't get good purchase on the stud and the system will be weakened. |