Shaft Rockers Series Part I


Roughly forty-five years or so ago, I was at the local speed shop and in the service bay was a big block powered Chevy Nomad. The hood was up and the rat motor wore a set of valve cover spacers sandwiched between the M/T valve covers and the cylinder heads. Valve cover spacers meant only one thing: There was no room under the valve cover for a stud girdle. And if it had a stud girdle, dollars to donuts the thing also had roller rockers. Bottom line here was, one of my guys at the speed shop suggested I steer clear of it, because it could give my big block Corvette all it could handle. I never did see that Nomad run, but wow, the look of it made an impression upon me.

Spin the clock back to today: Valve cover spacers, are for the most part relegated to the antique racing pile at the local swap meet. Tall valve covers changed that, but the truth is, the use of stud girdles isn’t as common as it was once either. The reason is the world of valve trains has progressed to a point where sophisticated rocker arm systems are the norm rather than the exception. In fact, thanks to today’s rapid CNC technology, many of them are made to order, but we’ll get to that later.

When examining today’s crop of roller rockers, it becomes clear that a considerable amount of research and development has occurred (particularly when you reflect upon yesteryears technology). Where, at one time, it was sufficient to replace something like a stamped rocker with a rollerized example, now we’re dealing with shaft rocker setups complete with  countless upgrades and options. Sure those old valve cover spacers simply meant a car was a bad-to-the-bone piece. It was a combination to be reckoned with. But it also meant the guy (or gal) spinning the wrenches pretty much went to war with the entire top end package when it came time to set the valves. Typically, the drill went like this: First you loosen the stud girdle, then you lash the valves. The girdle is re-tightened and finally most folks check the valve lash once more. If the studs are the least bit out of alignment, tightening the stud girdle changes the valve lash. Not fun. But you also have to be extremely careful with clearances on some combinations – the rockers and girdle can touch, which obviously spells more (and bigger) trouble.

Given the pain of adjustment, that’s likely sufficient for most people to reconsider the valve train layout, but that’s not the end of it: In something like a small block Chevy application, the stock rocker has a relatively short pivot length. This means the arc it travels in is comparatively small, particularly in contrast to other pushrod engines (Jesel notes that a stock small block Chevy has a rocker pivot length of 1.40-inches. In comparison, a big block has a pivot length of 1.65-inches). What this all means is the stock small block rocker arm tip more or less scrubs across the tip of the valve as it opens. This isn’t much of an issue at lower gross valve lifts such as those experienced with stock or mild camshafts (such as those used in production engines). But when you increase the lift dramatically (as seen in today’s race engines) and simultaneously increase the spring pressure you’re soon faced with another quandary. An increase in friction. As it turns out, decades ago, Dan Jesel (the owner of Jesel Valvetrain Innovation) relocated the rocker studs away from the valves on a small block Chevy. The idea here was to incorporate big-block rocker arms in the mix (which made perfect sense considering the rat motor’s 1.7:1 rocker ration). But something else happened during the R&D: While checking the rotating torque of the engine, the small block fitted with big block rockers consumed 80 ft-lbs less torque to rotate when compared to a stock rocker small block. That’s a lot.

In order to extend the pivot length on a rocker arm, it’s not exactly easy to relocate rocker studs (on any engine – small block Chevy or otherwise). On the other hand, you can’t change the rocker pivot length and correct the rocker geometry unless you move the pivot point. Instead of physically moving the rocker stud mount, it is far simpler to build an entirely new rocker arm arrangement such as today’s shaft rocker.

In a (simplified) nutshell, that’s how today’s new rocker arms were born. It involved a huge amount of work by some very dedicated (smart, too) racers. But that certainly wasn’t where the story ended. Rocker arm technology continued at a rapid pace. Next issue, we’ll show you how high technology entered the equation. It’s interesting and it has allowed the RPM level of pushrod engines (even those used in endurance events) to increase manifold. Watch for it!

There are all sorts of different rocker arms available today. Some are stamped, soem are extruded, and some are cast. many are “rollerized”. But there are huge differences between them.
Rocker arms are a critical component in a pushrod engine. In fact, they may be “THE” critical component. More in text.
The way a rocker arm contacts (or scrubs across) the tip of the valve has a huge effect upon horsepower, efficiency and reliability.
Shaft rockers, and the mounting system they mandate opened the door toward superior valve train geometry. Basically, the designer can move up, the rocker pedestal down or sideways on the head.

5 Comments on Shaft Rockers Series Part I

  1. 80 ft/lbs lost to rocker tip scrub???
    That just doesn’t seem right. That’s a difference of 160 ft/lbs at the camshaft.
    I can’t really believe it even takes 160 ft/lbs to turn the cam, much less that much difference.

    • You are right Cobra, but I think they are mistaking rocker scrub with rocker ratio which is the only way that kind of hp/tq could possibly be lost.

      • They’re not talking about torque output, they’re referring to the torque required just to rotate the crankshaft.
        I’ve rotated a flat-tappet camshaft when replacing a timing set, and I’d guess it takes around 50 ft/lbs, which would only be 25 ft/lbs at the crankshaft, due to the 2:1 ratio of the timing set.

  2. No, it’s on the crankshaft where you measure from. I have a torque wrench from the aerospace industry and all I have to do is push on the torque wrench and measure the how much force it takes to get the crankshaft to move. I can build a SBC and turn it with a 3/8 drive ratchet, that easily. In part by using the same principles that is talked about in this writing.

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