The pushrods and lifters work with the camshaft and rocker arms to open the engine’s valves. This basic setup has changed little since the earliest days of pushrod engines. The only major change in these components has been roller lifters replacing flat bottom lifters on late model engines. Even so, there’s still a market for flat bottom lifters in older street performance engines and circle track engines (including NASCAR).The market for pushrods has also been declining, but is still a long ways from disappearing anytime soon. One domestic pushrod supplier we spoke with said it is currently making 62 million pushrods a year for everything from motorcycles to passenger cars, light trucks and diesels.
Original equipment pushrods have not changed much, but there is a growing demand in the aftermarket for stronger, stiffer pushrods for performance engines. Stock pushrods only have to deal with relatively light spring loads (less than 200 lbs.) and lower engine speeds (less than 6,000 rpm). Pushrods in performance engines, by comparison, may be working to overcome spring pressures of 800 lbs. or higher, and engine speeds of up to 9,500 rpm. That’s an entirely different operating environment that requires much greater strength and stiffness.
One-piece 4130 chrome moly pushrods have long been the standard for upgrading valvetrain performance. This material has a tensile strength that ranges from 140,000 to 240,000 psi depending on the heat treatment it receives. But nowadays, you can get 4130, 4135, 4140 or even bimetal pushrods with an aluminum inner liner that adds reinforcement with little increase in weight. Pushrods made of 4140 chrome moly can provide up to 275,000 psi of tensile strength with the right heat treatment.
Pushrod wall thicknesses of .080" to as much as .188" are also available today for performance engines that demand increased stiffness. For some Top Fuel drag racing applications, solid pushrods are even available (the rocker arms are lubricated by other means).
Larger diameter pushrods are also available to reduce flex and increase strength. Standard sized 5/16" and 3/8" diameter pushrods can be replaced with 7/16", 1/2", 9/16" and even 5/8" pushrods where clearances allow it. Most of the circle track racers are limited to a maximum pushrod diameter of 7/16", but many drag racers are now using the larger 1/2" to 5/8" diameter pushrods in their motors. Most are using straight tubing, but others are using tapered pushrods to get increased strength and stiffness in the critical lower area where loads are greatest.
For performance applications, many engine builders prefer to use one-piece pushrods rather than three-piece pushrods that have the ends welded on. The ends on a one-piece pushrod are CNC machined to conform to the recessed cups in the rocker arm and lifter. It works as well as a welded steel ball, but adds cost.
The hardened ball on the end of a stock three-piece pushrod or even a chrome moly pushrod can break off under racing conditions. But there are alternatives to going with a more expensive one-piece pushrod. One supplier said it has changed the way it welds the hardened steel ball onto the end of the pushrod. Instead of just cutting the tubing off flat and welding on the ball, they now radius the end of the tubing to match the circumference of the ball. The result is a weld that has about 300 percent greater contact area and increases the shear strength to 12,000 psi – which is about three times stronger than before with only a small increase in cost.
Another pushrod supplier said it is switching to a new centerless grinding process to finish its pushrods, resulting in a much straighter pushrod. The new pushrods will be available around the beginning of 2008 in .050" length increments for all popular applications.
How Important Is Weight?
Common sense tells us that reducing the weight of the pushrods increases the rpm potential of the valvetrain, and reduces the spring pressure needed to maintain valve control at high rpms. But as racers have learned, weight is much more critical on the valve side of the rocker arm than the pushrod side. Why? Because of the leverage effect of the rocker arm.
If a rocker arm were a straight 1-to-1 ratio with no multiplication in lift, any increase in weight on either side of the rocker would have the same net effect. But most rockers have a 1.5, 1.6, 1.7 or even higher lift ratio. This means the valve end of the rocker travels much further vertically than the pushrod end. The leverage effect of the rocker arm ratio multiplies the force exerted by the spring as it shoves the pushrod back down to keep the lifter from jumping off the cam lobe. So if the pushrod is a little heavier but a whole lot stiffer, it doesn’t really hurt anything. In fact, it really helps high rpm power by reducing pushrod flex and valvetrain harmonics that can cause the valves to bounce and float.
A lot of engine builders say they have found gains of 15 to 25 horsepower on the dyno by just changing the pushrods in an engine to a stronger, stiffer design. Others are finding even more power by playing around with pushrod lengths and different rocker arm ratios and styles.
Increasing the lift ratio adds horsepower with little or no loss in low rpm torque, idle quality or vacuum. By opening and closing the valves at a faster rate, the engine flows more air for the same number of degrees of valve duration. High lift rocker arms also reduce the amount of lifter travel needed to open the valves, which reduces friction and the inertia of the lifters and pushrods that must be overcome by the valve springs to close the valves.
On the other hand, increasing the rocker ratio also increases the effort required to open the valves because of the leverage effect. The higher the rocker arm ratio, the greater the force the camshaft, lifters and pushrods have to exert to push the valves open, and the stronger the pushrods have to be to keep from bending. That’s why everyone is clamoring for stronger, stiffer pushrods today.
Many pushrod suppliers offer custom-made pushrods in special lengths, diameters or materials. Ordering a set of custom pushrods may be just the thing to squeeze a little more horsepower out of an engine, but they are not cheap. A set of custom-made pushrods can cost as much as $50 to $60 or more each!
The finishing that goes into a pushrod can also vary, depending on the supplier and how much you are willing to spend. Some suppliers polish their pushrods to give them a nice, chrome-like finish. Others may apply special coatings to reduce friction in engines that use pushrod guide plates. One supplier said it inspects every pushrod it makes for NASCAR applications with magnetic particle inspection as a final quality control check.
The length of a pushrod will depend on a variety of factors: the base circle of the camshaft (higher lift cams with reground smaller diameter base circles will require longer pushrods) and the height and location of the lifters; whether the pushrod cups in the lifters are offset or centered; the location of the pushrod cups or adjuster screws in the rocker arms; the geometry of the rocker arms and rocker arm ratio; the installed height of the valves; and how much the cylinder heads or block have been milled.
With so many variables, it’s important to get the correct length pushrods so the tips of the rockers will be properly positioned on the tops of the valve stems. If the pushrods are too short, the rocker arms may exert a side thrust against the valve stems as they push the valve open. This can increase friction, valve stem and guide wear, and increase the risk of valve stem breakage. If the pushrods are too long, the valve spring coils may bottom out and bind at maximum valve lift, causing damage to the valvetrain (typically a bent pushrod).
Most professional engine builders know how to measure pushrod length, and how to determine the correct length pushrod for a given camshaft, cylinder head and valvetrain combination. Adjustable length pushrods are available to make the job relatively easy, assuming you know where and how to measure.
To determine the right length, an adjustable pushrod is installed in the engine. The length of the pushrod is then adjusted so the roller on the end of the rocker arm is over the exact center of the valve stem when the rocker arm is at the halfway point of its maximum lift. It’s also important to make sure all the valve stems are installed at the same height so the same length pushrod can be used for every cylinder. If valve heights vary, each pushrod will have to be measured individually for each valve.
With pushrods that have balls at both ends, the length can be the overall length of the pushrod end-to-end, or it can be the "theoretical length" of the pushrod (which is measured as if the oil holes were not in the balls). Determining the theoretical length requires using a special gauge that compensates for the oil hole, or estimating how much the oil hole reduces the radius at each end of the pushrod.
With pushrods that have a cup at the top end, the overall length can be measured end-to-end, or you can use the "effective length" which is the overall length of the pushrod minus the depth of the cup on the one end. This can be done by placing a ball in the cup, measuring the overall length of the pushrod with the ball in place, then subtracting the diameter of the ball.
Whatever method you use, make sure your pushrod supplier knows how you are measuring pushrod length so you can get the correct length pushrods from them. You may also need to discuss spring pressure and rocker arm ratio to determine how much stiffness is needed to handle the load.
Flat tappet lifer technology hasn’t changed much in recent years, but there is more interest lately in friction-reducing coatings to minimize the risk of premature cam failure. Nitriding adds hardness and wear resistance to the lifters.
One of the biggest issues engine builders say they have found with some flat bottom lifters is a poor finish on the bottom of the lifter. The finish may be too rough, not hard enough, or not have the right amount of convex to rotate the lifter. Flat lifters are not really flat on the bottom but have a slight taper, which can be seen by placing them on a flat surface and rocking the lifter slightly side to side.
One of the keys to getting proper lifter rotation is to make sure the lifter bores are properly aligned with the lobes on the camshaft. One supplier has recently introduced a new scribing tool that can help engine builders accurately determine lifter alignment. The tool has a pointed nylon scribe on the bottom that leaves a slight mark on the cam lobe. The cam can then be pulled out to see where the center of the lifter is in relation to the cam lobe. Ideally, the center of the lifter should be about 20% "uphill" from the center of the tapered lobe for optimum rotation. If the lifter is on the low side of the lobe center, it won’t rotate and will probably wipe out the cam lobe causing premature cam failure. A shim kit that installs behind the cam gear can be used to reposition the cam forward or backwards in the block to correct any misalignment with the lifters.
Another concern with flat bottom lifters is getting enough lubrication to the cam lobes. Splash lubrication may be okay for a low rpm stock engine with stock valve springs, but in a high revving engine with stiff valve springs, there’s a lot of pressure between the lifters and cam lobes. By cutting a small "dribble" groove in the lifter bores, oil can dribble down and help lubricate the cam lobes. One aftermarket lifter supplier also sells a lifter that has a small hole in the face that helps lubricate the cam.
With few exceptions, most late model pushrod engines use roller lifters instead of flat bottom lifters. Roller lifters provide a significant reduction in friction, and also allow steeper lobes and faster valve opening and closing rates than flat bottom lifters. What’s more, roller lifters reduce cam wear to the point where the cams just don’t wear out, unlike cast iron, flat lifter cams that often round off lobes. The tradeoffs, though, are higher cost, more weight and an increased risk of lifter failure due to failure of the roller bearings.
The most critical area in a roller lifter is the roller bearing. If the needle bearings are not perfectly matched in size, the largest one will bear most of the load and eventually fail. One supplier said it sizes its needle bearings to the nearest micron and carefully matches all the needle bearings to improve the durability of its roller lifters.
Solid Or Hydraulic Lifters?
Hydraulic lifters, either flat bottom or roller, are fine for street engines that don’t rev much higher than 6,000 rpm. Above a certain point, the lifter can’t handle the spring pressure and collapses. On the other hand, hydraulic lifters are nice for street engines because they use oil pressure to maintain valve lash. This keeps the engine quiet and eliminates the need for constant lash adjustments.
Solid lifters, either flat bottom or roller, are usually preferred for racing unless rules prohibit them. A solid lifter is rigid with no oil cavity or spring-loaded plunger under the pushrod. Because it has no "give" it can’t take up valve lash and requires adjustments to the rocker arms to set the proper valve clearance. That makes for a noisy valvetrain, but also allows a racer to "tune" his engine by changing the lash adjustment for more or less valve lift and duration. A solid lifter can also handle as much rpm as the valve springs can withstand, so that’s why most high revving pushrod engines use them.
As for rebuilders of stock engines, costs can be minimized by rebuilding rather than the original lifters. It’s more economical to rebuild roller lifters than to replace them with new ones, but that’s not the case with flat bottom lifters. New lifters can usually be purchased for less that what it would cost to rebuild them.
One supplier of remanufactured roller lifters said its customers have fewer problems with remanufactured lifters than it does with new lifters. There are serious quality issues with many lifters that are sourced off-shore. "There has been a great deal of upheaval in the lifter industry over the past four or five years," the supplier said. "A lot of people have learned the hard way that cheaper parts from overseas aren’t necessarily the best parts to buy."
Latest posts by Larry Carley (see all)
- Picking a Racing Oil - Jan 16, 2015
- Consider a Dyno/Flow Bench for Your Shop - Dec 23, 2014
- Selecting Pushrods, Rockers and Springs for Your Build - Dec 23, 2014