Engine Parts: Pushrods - The Long And Short Of It

How long is too long? We learned... 

   

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How long is too long? We learned how to properly measure for new pushrods with help from Lunati, Powerhouse, and Ferry's Aluminum Cylinder Head Repair.

When it comes to assembling a host of go-fast engine parts, pushrods are rarely thought of as being anything more than a simple link between the cam and the valves-that is, until one of them breaks or you're putting a new combination together and you find that your stock pushrods won't work anymore. Why wouldn't they work, you might ask then?

There are many factors that determine whether or not a pushrod will fit and operate correctly. Unfortunately, there's little information out there showing you how to determine if yours are correct. We wanted to know what was involved in properly measuring for new pushrods and then see if those measurements would translate correctly into a real, working engine. To find out how to measure pushrods we called Lunati Cams, and they supplied their Pushrod Length Checker, which is a practically foolproof way to determine the proper length. Then, to see how accurate our measurements were we used a Rocker Arm Geometry Checker from Ferry's Aluminum Cylinder Head Repair to see if the proper length pushrods put the rocker arms in contact with the right spot on the valves.

Vital Measurements
Stock engines are easy to order new pushrods for. Just tell the cam company what type of cam you're running (i.e. hydraulic roller, flat-tappet, etc.) and what diameter you'll need to fit your guideplates (i.e. 5/16, 3/8, or the big 7/16). But what about engine blocks that have been decked or heads that have been milled? Machine operations like those can require custom-length pushrods, and there's really no way to know exactly how long they'll need to be without a trial assembly first.

That's where Lunati's kit comes in. The parts included with the tool are: an adjustable length pushrod (two different lengths for big-blocks), a modified roller rocker that's missing the trunion bearing assembly but has a regular roller tip, and a long poly-lock nut without the locking set-screw. Instructions included with the kit tell you to first put the cam lobe of the valve you're checking on its base circle as if you were going to set the valve lash. Insert the adjustable pushrod with its end turned most of the way down to clear the rocker arm. Then, screw the modified rocker arm on by hand until all clearance between the valve and roller tip is taken up. It might help to use some light lube on the rocker nut and adjustable pushrod tip to free things up.

Then, simply turn the pushrod until the tip contacts the rocker arm cup and creates a tight assembly. You'll know just by the feel of things when you've got all the slack removed. Then, secure the pushrod's adjustable lock nut finger tight before removing it. It might be necessary to remove the pushrod guideplates when you perform this check. If you do, make sure to keep the rocker arm tip centered on the valve. It's then a simple matter to measure the adjustable pushrod with a large dial caliper or machinist's rule to determine your new length.

Checking Things
How will you know if the new pushrod length you just came up with is ideal? Even though the Lunati tool is utterly foolproof, there's always room for human error. That's where the guys at Ferry's Aluminum Cylinder Head Repair can help out. They make a tool to check roller tip placement with any length pushrod. The tool works by measuring the roller tip placement relative to the valve stem tip, and a little math tells you if you've got it right. Centering the roller tip on the valve when the cam is at mid-lift is the best way to ensure longevity and increase horsepower, as well.

The increase in longevity comes from a decrease in side-loading of the valves that happens when the roller tips are too far off center. The horsepower increase comes from the reduced frictional losses from having the tip off center also. Ferry's testing has shown that it can take several foot-pounds more torque to turn the engine over by hand when the rocker tips are off center. Consider measuring for new pushrods an integral part of building a performance engine. After all, you wouldn't assemble an engine without checking bearing clearances or degreeing the cam, would you? This is just one more step towards perfection, and it's an easy step at that.

Incorrect pushrod length can...        read full caption Incorrect pushrod length can cause a number of problems. This diagram from the Reher-Morrison Upper Engine Assembly book shows three of the most common mechanical problems associated with incorrect pushrods.

Lunati's Pushrod Length Checker...        read full caption Lunati's Pushrod Length Checker is a simple tool to use. Note the absence of the roller trunion that allows the rocker arm to properly determine pushrod length.

A lot of "feel" is involved...        read full caption A lot of "feel" is involved when properly measuring for pushrods. Even with Lunati's tool, you still have to determine when all the slack has been removed by spinning the adjustable pushrod lightly between your fingers.