I must be off my rocker…. So to speak

By Jok Nicholson
10/7/05

his month in “Back-2-Basics” I want to go over the process of installing Comp Cams Pro-Magnum rocker arms on the Mopar Stage VI aluminum heads. These same procedures will apply to any stock big-block Mopar cylinder head. I have used and broken about every type and style of aluminum rocker arm on my big-block Mopars over the years and the Comp Cams Pro-Magnums looked like a stronger option. They are part number 1321-16 and come with 16 rocker arms, thick wall shafts and all the attaching bolts, clamps and spacers.

Before I go any further with this article I do want to mention we have worked closely with Comp Cams on a couple things we encountered while using the Pro-Magnums. Read closely and you will see there are some things to consider before the assembly starts.

First, you need to realize the original design for this assembly is intended for high performance street/strip use. That does not mean the rocker arms are not tough enough for race cars. What it does mean is that the factory clearance between the Pro-Magnums and the highly polished shaft is between .0015” and .002”. This offers enough clearance for cams with lift up to about .600” and will operate quietly in street machines. For race applications with lift over .600” and all roller camshafts, this clearance must be increased to .004” to .006”.

We had our Pro-Magnums honed at a local machine shop to get the clearances correct. This cost us $80 (which I imagine is about average). Failure to do this on a race engine could result in part failure and severe engine damage. How do I know this is a necessary step? I experienced the damage because I did not have the correct clearance on the first set I installed. The tech staff at Comp Cams and I gave this a lot of thought and the only thing that could have caused the problems we encountered was lack of clearance between the rocker arm and the shafts with our .705” lift roller cam.

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As with all of my tech articles I always give my readers the “whole story”. I do want to be clear that what happened is nobody’s fault and actually ended up being a blessing as Comp Cams is now changing the Instructions in the Chrysler Pro-Magnum rocker arm kit. As my editor Jeff always says… “No good deed goes unpunished.” I took a bit of punishment here, but in the end Comp Cams stood up tall and took care of the problems I encountered and for that I am glad to see big companies still care about their customers.

The installation is pretty straight forward. I took a pair of rockers from the kit, lubed the shaft with ARP moly and slid the rockers on. I then installed two Comp Cams “checking pushrods.” These are threaded and extend about an inch so you can find the exact length pushrod you need. The Pro-Magnums use ball-type ends rather than the normal cup-style most Mopar rocker arms use. I adjusted the checking pushrods to approximately the length of the pushrods I pulled out. This would get me close to the right length. I set the lash on the two rocker arms and then looked to see if the roller tip stayed centered on top of the valve. It did not and we played around with checking pushrods until we found the length that was just right.

The method I use to check this is to take a piece of carbon paper and put a sheet of paper under it and stick in between the roller tip and the valve and turn the engine over by hand. The impression the roller tip makes on top of the valve can be seen on the paper under the carbon paper. At first ours was off center to the inside. We shortened the pushrods about .100” and it was almost perfect. It took about an hour but if you are picky about this it makes the geometry better and takes the side loads off the valves.

Now that we had the right length for the pushrods we simply called Comp Cams and told them what we needed. They were at my shop two days later. We then used the ARP Moly on every moving part and assembled the rocker arms and pushrods. Be sure to check side clearances between the rocker arms. We wanted .035” to .045” per pair. All we had to do was grind a little off the hold-down spacers to get it just right. We then marked the spacers with a small punch so we could get them back together again in the same spot. My suggestion to you is if this is the first time you are installing custom parts just take your time, make sure you lube everything well and if the engine is all new or just rebuilt be sure to pre-lube it until you get oil out of the rocker shafts. Time spent doing this will save you big bucks later on.