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Determining Proper Rocker Arm Geometry

Discussion in 'COMP Cams Technical Reference Articles' started by jjamros, Apr 20, 2011.

  1. jjamros

    jjamros Guest

    You may have just installed the latest and greatest camshaft and valve train components, but if you haven’t taken the time to determine the proper rocker arm geometry, you won’t be getting the most out of your engine. Proper rocker arm geometry is crucial to achieving optimum durability, performance and stability, as well as maximum valve lift.
    If you think of a valve stem tip as a circle, optimum rocker arm placement is directly in the center of the circle. If the roller tip does not hit in the center, the amount of surface area contacted is reduced because roller tip edges will come off of the sides of the circle. The further away from the center the rocker arm is when it hits, the more likely it is for damage to occur. Two of the most common issues that occur are breaking the roller tip on the end of the rocker and “mushrooming” the valve tip. The pushrod can also end up contacting the edge of the rocker cup, causing unnecessary wear on the pushrod and rocker arm. In addition, if the rocker is not touching the center of the valve, it will not be pushing straight down, but at an angle, which can induce valve guide wear issues. The harder the engine is pushed, the worse the damage will be.

    Staying dead center in the circle throughout the entire cycle is not possible because the rocker arm actually travels across the circle as it pushes down – this is referred to as the rocker arm “sweep” or “pattern”. When rocker arm geometry is set up correctly, 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.

    There are a lot of factors that are involved in determining proper rocker arm geometry. Pushrod length, block deck height, head gasket thickness, rocker arm design, cam base circle size, lifter design and valve stem length are just a few of these. The only way to account for all factors is to use an adjustable checking pushrod that will allow you to find the exact length of pushrod your setup needs.

    An adjustable checking pushrod installs just like a normal pushrod. However, it can be lengthened and shortened by twisting it. With each adjustment you can turn the motor over and watch the rocker arm as it comes into contact with the valve tip. Unlike other measuring tools, adjustable pushrods allow you to quickly and easily make small adjustments until you find the perfect geometry and wear pattern.
    Using an adjustable pushrod to measure for proper rocker arm geometry is easy. First, install your adjustable checking pushrod just like a regular pushrod. Lengthen or shorten it until the rocker arm tip is just inboard of the valve center. Now, use a dry erase marker to cover the valve stem tip with ink. Next, install the rocker arm and then turn the engine over while watching the roller wheel as it travels across the face of the valve. After several rotations, remove the rocker and look at the pattern on the valve tip. If the pattern is on the center of the valve, you’re loading the middle of the valve and not off to one edge, which means your geometry is correct. If the pattern is not centered, shorten or lengthen the pushrod until the sweep is correct. Once it is, you can remove the checking pushrod and measure it.

    Once you have calculated the pushrod length you need, it’s almost time to order. Before you actually place an order, you need to understand that not all companies measure pushrods the same way. Pushrods can be measured in three different ways: theoretical length, actual length and gauge length. Before ordering new pushrods, be sure that you are using the same measurement system as the catalog from which you are ordering.

    The most difficult of the three techniques is gauge length. However, it is also the most reliable because the oil holes and their chamfers are eliminated, which is why the engineers at COMP Cams use this method to measure all of their pushrods. The COMP Cams Hi-Tech™ Adjustable Checking Pushrods remove all the variables and make the measuring processes as simple as possible. And, if you used a COMP Cams Hi-Tech™ Adjustable Checking Pushrod, the length you came up with while measuring will match perfectly with the lengths listed in the COMP Cams Master catalog.

    When building a new engine, finding perfect rocker arm geometry is a quick step that often gets overlooked. But maximizing your valve train stability will net better performance and optimum durability. And the more aggressive your setup, the more important it becomes because the number one killer of rocker arms is valve train instability.


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    Last edited by a moderator: May 28, 2014

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