Pushrods: Measure Twice, Order Once. Do It Right the First Time

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Major engine assembly should be left to a professional or at least a seasoned hobbyist. It takes knowledge, practice and expensive specialty tools. That said, setting up a valve train is something a newbie can do and feel good about. It takes very few tools and is simple to understand. Here we will show you how to measure for correct pushrod length, the most vital part of correct geometry.

Start with an adjustable pushrod. Never assume that your engine will require a standard-length pushrod. It’s a good length to start at, but with so many variables changing the distance between the cam and the valve, it’s not a sure thing. For example, this engine has had .030-inch cut off the block and another .020-inch off the cylinder heads. This modification will require a shorter pushrod to accommodate. Other factors include varying head gasket thicknesses, small base-circle cams, aftermarket rocker arms or anything else that’s changed from the factory between the cam to the valve seat.

Most valve train component manufacturers have a set of adjustable pushrods available to fit your engine. We started with the 8.5-9.8-inch threaded adjustable pushrod that works for small-block Chevys. It has narrow body and threaded adjustment that allow you to install the pushrod and rotate the engine to verify the correct length. There are plastic pushrods for this as well; they are less expensive but don’t allow you to turn the engine over to run a pattern.

The next step is running a pattern. This process starts with marking the valve top with permanent marker. Then we install the adjustable pushrod, rocker and polylock. There’s no need to lash here with a solid lifter; the rocker tip just needs to make firm contact with the valve stem surface. A hydraulic lifter will need the preload recommended by the manufacturer. It takes several valve actions for the roller tip to wear away the marker, so give it some time. The worn section should lay right in the middle of the valve ensuring that the rocker doesn’t put too much pressure on the far inside or outside edge. A pushrod that’s too long will bring the roller tip closer to the outside of the valve tip, and too short will bring it to the inside. The proper length pushrod will place the roller tip just inward of center while the cam is on the base circle, in the middle at mid-lift, and just outward of center at maximum lift. Keep in mind that a pushrod that is too short will minimize clearance between the rocker arm body and valve spring retainer.

Now that we’ve verified the adjustable pushrod’s length, we need to measure it. Take care to note that different manufacturers use different measurements to order. There are three basic measurements: theoretical, actual and gauge length. Theoretical is the length of the pushrod as if it had no oil hole, actual length is the measured length from a dial caliper, and the third measurement is specific to the manufacturer’s checking device. Check your pushrod manufacturer’s measuring method before you attack this project. We used a 12-inch caliper to measure, but if you don’t have one that large, you can measure the difference between a known-length pushrod and the adjusted pushrod.

We’re fast-forwarding in time to when the correct pushrods we ordered have arrived. There’s nothing more to do than dab each end with assembly lube and drop them in. When you place the rocker arm on top make sure the pushrod fits into the cup of the rocker.

The last step is to set the adjustment referred to as “lash.” This is the distance between the roller tip and valve stem head for a solid lifter. This gap effectively delays the opening and closing of the valve and should also be considered in the piston-to-valve clearance step. Hydraulic lifters require a preload specified by the manufacturer and don’t need to be checked hot. The lifter needs to be on its base circle or the measurement will be inaccurate. The best way to know that it’s on the base circle is to watch what its buddy is doing. If you’re working on the intake, watch where the exhaust valve of the same cylinder is. We know the intake is absolutely closed when the exhaust valve is opening and the exhaust is absolutely closed when the intake is closing. Getting this wrong can get you into valve overlap, so be careful!

Most camshaft instructions call for a hot lash, which is a lash adjustment performed at or near operating temperature. This is important because the distance between the camshaft and the rocker arm changes as heat affects the materials.

Many manufacturers recommend doing your initial lash at the recommended hot lash measurement, which will be loose when the engine is hot for the final adjustment. We performed a test to see how much the engine grows from room to operating temperature. A safe starting point for this engine was .018-inch cold at 77°F. With the engine at operating temperature, around 180°F, the gap grew to .023-inch. We opted to set all of our valves .005-inch tight to get the most consistent adjustment. The problem with doing your final adjustment hot is that the engine cools so quickly you won’t be able to ensure each valve was adjusted at the same temperature unless you warm it up after each set. We recommend going by their rules for your first try, since a valve train that’s too tight is much more dangerous than one that is too loose. To make setup and checking easier, we’ve installed a starter button under the dash that can be easily reached through the driver’s side window. It allows us to rotate the engine without turning on the ignition.

With the valve cover back on and the engine at operating temperature, it’s time to do the final check/adjustment. If you did it by the book, each valve should be between .003-.007-inch loose. Tighten these up to the cam card’s spec while keeping an eye on the engine temperature. An infrared temperature gauge does a great job of keeping tabs.

It’s a common misconception that a solid lifter camshaft engine requires constant lash adjustment, but the truth is they don’t. They don’t magically make space between the parts unless something is broken. Nearly all of the perceived change in lash is due to inconsistent adjustment. The key to consistent valve lashing is consistent temperature and pressure of the feeler gauge between the two parts. A general rule is to use the largest feeler gauge you can possibly squeeze in before it bends. How often you check lash depends on how much and how hard you run your engine. Some racers check every couple of races and street cars once or twice a year.

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Tools you need: an adjustable-length pushrod, dial caliper large enough to measure it, permanent marker for the valve, feeler gauge for lash measurement, assembly lube and a special polylock tool that combines a box-end wrench and Allen T.

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To see where the rocker arm tip rides we cover the valve stem face with permanent marker.

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Setting lash with a misplaced pushrod can be catastrophic. Save them by verifying they are sitting completely in the rocker arm cup. We ran a mirror behind each one.

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It’s a huge time saver to put a starter button at the bottom edge of the dash on the driver’s side. This allowed us to reach in and turn the engine over so that we could get the rockers where we wanted them.

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With the adjustable pushrod in place and rocker arm installed, we turned the engine over so the valve opened about eight times. With the rocker removed, the pattern is exposed. Here you see the wear mark is heaviest in the middle, where the roller is at mid-lift.

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We used the adjustable pushrod length to order our pushrods. Here you see the actual measurement as it came out of the engine after running the pattern.

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Image 7 shows a .018-inch feeler gauge under the rocker tip at room temperature. 7A shows that after warming the engine up, the gap grew to .023-inch. All-iron engines won’t grow as much, but you still need to be aware of the difference in hot and cold lashing.

Text and Photos by Liz Miles

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