In the intricate world of high-performance engine building, each element holds significance, and the pushrod is no exception. This seemingly straightforward component wields a profound influence on engine performance, endurance and overall robustness. Engine builder professionals recognize that the careful selection of pushrods can make all the difference in the relentless pursuit of peak performance.
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The bedrock of pushrod selection resides in the materials employed, a critical factor in pushrod performance. Three primary materials are used: steel, chromoly steel and aluminum.
Steel pushrods are ubiquitous in stock engines, are cost-effective and are characterized by limited strength and unimpressive weight characteristics, making them suboptimal for high-performance demands.
Chromoly steel pushrods are the preferred choice for performance engines because they outperform their conventional steel counterparts in both strength and weight considerations. Their reduced mass translates to valvetrain efficiency and durability.
Finally, aluminum pushrods are the lightest weight option, so they serve with distinction in racing applications where shedding valvetrain mass is paramount. However, they may exhibit diminished endurance compared to both steel or chromoly.
Material selection must be matched to the engine’s precise requirements, considering variables such as power output, engine rpm range and budget.
Moving on from material considerations, figuring out proper pushrod length warrants meticulous attention. Precision length determination entails measuring the distance from the lifter cup to the rocker arm cup during the lifter’s dwell on the base circle of the camshaft lobe. Many people opt to use adjustable pushrods to fine-tune this critical dimension. Pushrods that deviate from the “green zone,” whether too short or too long, introduce instability into the valvetrain, which can undermine engine performance.
Determining the correct pushrod length is often regarded as one of the most intricate aspects of the selection process due to the variability in valvetrain geometry and design. To tackle this challenge, let’s clarify some terminology.
First, overall length of the pushrod is the measurement encompassing the very tip of the ball or cup on each side. It’s essential to note that the presence of oil holes in the pushrod ends can slightly affect this measurement, depending on the hole’s diameter.
Next, theoretical length represents the pushrod’s length as if it had no oil holes in the ends. Finally, effective length is a measurement that holds particular relevance for cup and ball-style pushrods due to potential differences in cup depths among manufacturers, which can be as much as .060”. The effective length is determined by measuring from the contact point on both ends of the pushrod, from the bottom inside the cup to the tip of the lifter end. To achieve precision, you can calculate it by subtracting the cup’s depth from the overall length. Alternatively, by placing a ball bearing of the correct size inside the cup and measuring the overall length over the bearing, you can obtain the same result.
“Perfect pushrod length is better than compromised pushrod length,” says Trevor Manton of Manton Pushrods. “Now more than ever it’s critical to get that length right the first time to save time and money. If the motor doesn’t start because of a lack of compression, you didn’t choose the proper length pushrod to preload the lifter and you hang the valve open because the pushrods are too long.
“If you just use the .025” increments in length, when a lifter like a Johnson calls for a tolerance within .010” a +/-5, you’re creating yourself a much higher likelihood of having a problem with something else related to the lack of perfect pushrod length. And, make sure your camshaft is at base circle. Also make sure that if you have an adjustable rocker arm that you have the adjusting screw where you would desire the screw to be at when you actually set lash.”
Manton can create and produce a pushrod or rocker arm adjusting screw stronger, more durable and suitable for a customer’s engine needs. All of their products are made from quality materials – tubes are made of / seamless chromoly or cold working tool steel and are available in a variety of heat-treated conditions. This allows Manton to manufacture a pushrod to enhance specific applications.
Another crucial aspect of pushrod selection is determining the appropriate ball and/or cup configuration. To determine the correct radius for each end of the pushrod, precision measurements using radius gauges are recommended. Measure the radius of the lifter’s pocket and the ball or cup on the rocker arm that will be employed. Another technique involves checking whether the ball and cup ends match by fitting a known-correct pushrod’s ball end into another pushrod’s cup end.
Selecting the proper pushrod diameter is also important. Diameter selection should mirror the engine’s specific needs, with larger diameters allowing heightened rigidity and robustness. The wall thickness indicates the pushrod’s weight and durability, with thicker walls augmenting strength at the expense of mass. This delicate balance must be struck to avoid unwarranted weight penalties while ensuring structural integrity.
“Diameter and material grade will drastically increase the strength, while wall thickness will incrementally increase the strength,” Manton says. “And heat treatment is just another option to make your setup even better. Outside of added cost, there aren’t really too many negatives to heat treating.”
Surface treatments such as heat treatment and black oxide coating hold the potential to confer elevated durability and heightened wear resistance upon pushrods. These treatments mitigate friction within the valvetrain and extend the operational lifespan of the pushrod.
Builders must also consider what design of pushrod they prefer. One-piece pushrods are created from a solitary material piece that imparts these pushrods with unparalleled strength and endurance, making them the favored choice for high-performance racing engines. Two-piece pushrods are composed of two distinct sections conjoined. These pushrods, while lighter, sacrifice some structural integrity, finding their niche in less demanding applications.
“Though the strength of a one-piece pushrod might seem obvious over a two-piece pushrod, a multi-piece pushrod is often stronger than a one piece because you can tailor your engineering choices for the center section to be stiff and the ends to be a good bearing surface,” Jordan Manton says. “With a one piece you have to find a middle ground that will be rigid but also a good bearing surface. The main advantage of a one piece is cost.”
Pushrods are the weakest link in the upper valvetrain. Pushrods transfer and redirect the upward motion of the lifters, which goes in one direction, to the rocker arms which move in another direction. Consequently, they are subjected to bending forces as the load and engine speed increases.
For stock engines, mild steel stock pushrods are more than adequate. But for performance applications with stiffer valve springs and higher valvetrain loads, significantly stronger, stiffer pushrods are needed to withstand the punishment.
Terry Manton of Manton Pushrods, Lake Elsinore, CA, says compression and deflection are the two fundamental forces pushrods must withstand in racing. Pushrods deflect under high loads because of differences in angularity between the lifters and the cups or adjustment screws in the rocker arms. Oblique angles contribute to side-loading that leads to deflection if the pushrod is not rigid enough to remain straight. If the pushrod deflects, it reduces net lift at the valve, which hurts breathing and the engine’s power potential. It also shortens valve duration and retards valve timing.
Manton says most of the deflection occurs at the lower end of the pushrod where the side loads are typically the greatest. He recommends going with larger diameter pushrods (his company makes ones up to 9/16? in diameter!) and/or using pushrods with a heavier wall thickness (.120? up to .188?). The pushrods must also be made of a stronger material. Stock pushrods are typically made of steel tubing, which is not strong enough for springs that have more than 400 psi of open pressure, or engines that exceed 7,000 rpm. Pushrods made of or chromoly tubing are much stronger and are recommended for almost all types of racing.
Chromoly is a tough material with a tensile strength that varies from 140,000 psi up to 240,000 psi depending on the heat treatment. For all out racing applications such as Top Fuel, Pro Stock, Pro Modified, Blown Alcohol, NASCAR Cup racing, marine racing and tractor pulling, chromoly tubing can provide up to 275,000 psi of tensile strength.
Pushrod deflection can also be reduced by using single taper or offset dual taper pushrods. The thickest part of the pushrod should be positioned closest to the lifters to take advantage of the larger cross-section. Manton says it is especially important to use tapered pushrods in engines that are equipped with roller lifters, increased valve lash, high ratio rocker arms, double or triple springs and operate at high rpm.
“Weight is not as much of a concern as many people think it is,” says Manton. “The difference between a stock 5/16? diameter pushrod in a small block Chevrolet and a 7/16? to 3/8? taper pushrod represents a difference of approximately 2.5 percent of effective weight. This is because the pushrod is on the slow side of the valvetrain. The effective increase in weight between the two pushrods is small but provides a huge increase in valvetrain stability. The valve side of the valvetrain is the critical side where any weight savings will make marked improvements.”
Something else to consider when choosing pushrods is whether the engine has guide plates or not. Pushrods that are used with guide plates must be hardened to withstand wear. Guide plates also require straight pushrods.
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Manton says he custom makes all his pushrods to order. The tips are CNC machined in house to ensure quality and are made from high impact, wear resistant barstock that is case hardened, cryogenically treated and tempered, or H13 tool steel that is heat treated and salt nitrided.
Dennis Marshall of Smith Brothers, Bend, OR, is another custom manufacturer of high performance pushrods. “The number one misconception engine builders have about pushrods is that lighter is better. That’s absolutely wrong! The last thing you want to be concerned with is weight. If the pushrods are too light duty, they will deflect or bend and cost you horsepower.”
Marshall says a lot of engine builders say they want 5/16? thinwall pushrods for hydraulic lifters, thinking lighter pushrods are the way to go. But the smaller size does not work very well in performance applications because they deflect too much. He says you can actually see the pushrods flexing if you watch the engine while it is running on a valvetrain dyno.
“We sell a 7/16? diameter pushrod with a .120? wall thickness for hydraulic and solid roller lifters. It is a very strong pushrod and works great in the most demanding applications. We also make pushrods up to 1/2? diameter with the .120? wall thickness, which are being used in more and more racing applications. Professional race teams are running the heavier wall pushrods to eliminate deflection.”
Marshall explains on Chrysler applications with ball and cup pushrods, correct geometry is very important. If the pushrods are too long, the cup will sit above the oil hole screw and they will burn up the cups. If they are too short, the angle of the adjusting screw to the pushrods becomes severe and you can get into a bind situation and break cups or adjusters. He says Chryslers have an oiling problem so it’s important to use good quality adjuster screws with proper hardening and radius. “On our pushrods, we maintain our radius tolerances to less than plus .? and minus .?. We also use chromoly tubing that is heat treated and case hardened to Rockwell 60 RC for guide plate applications, or RC 42 for non-guide plate applications.”
Marshall says he also custom builds pushrods to order, and can inspect and straighten used Smith Bros. pushrods for customers, also. “In addition to competition applications, we produce a lot of custom pushrods for antique and agricultural engines.”
Determining The Correct Length
The length of a pushrod may vary from stock dimensions depending on the base circle of the camshaft, 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 heads or block have been milled.
Russ Flagel of Indy Cylinder Heads, Indianapolis, IN, a builder of custom pushrods mostly for Chrysler applications, says the only person who can accurately determine pushrod length is the person who has all the parts to assemble – in other words, they guy who is building the engine.
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 geometry is off, the rocker arms can push the valve stems sideways causing abnormal loading and wear. If the pushrods are too long, the valve spring coils may bottom out and bind, causing damage to the valvetrain.
Determining the correct length of a pushrod can be tricky, so many pushrod suppliers recommend purchasing an adjustable “checking” pushrod, installing it and then measuring the pushrod’s length before ordering a full set of custom length pushrods. All the pushrod manufacturers we interviewed for this article said it’s very important to make sure all the valve stems are trimmed so the installed height of all the valves are the same – otherwise you’ll end up requiring a different length pushrod for each valve.
The method you use to determine the overall length of a pushrod is also important because methods vary. With pushrods that have balls at both ends, the length can be measured end to end. You can use the actual length as measured, but remember that the overall measurement will be affected by the size of the oil holes in the end of the pushrod. Because of this, some people use the “theoretical length” of the pushrod as if the oil holes were not there. Determining the theoretical length requires using a special gauge that compensates for the oil hole, or estimating the amount the oil hole reduces the ball radius at the ends of the pushrod (figure .017? less for a .100? oil hole in the end of a 5/16? pushrod).
With pushrods that have a cup at the top end, the overall length of the pushrod can be measured – but you’ll also have to determine the “effective length” of the pushrod by measuring the depth of the cup and subtracting it from the overall length. 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 you and your pushrod supplier are on the same page and they understand which method you used to determine length. The supplier may also need to know how much spring pressure the engine has so the pushrods will have sufficient strength to handle the load.
Another variable is the radius of the balls at the ends of the pushrod. This may vary depending on the type and brand of rocker arms used. Measuring the radius requires special gauges, or obtaining the required radius dimensions from the rocker arm supplier.
Installation Tips
The best advice for selecting performance pushrods is to use the largest, stiffest, strongest pushrods that will physically fit the motor. As a rule, you only need to allow about .010? of clearance at the closest spot for the pushrods. This will determine the maximum diameter pushrods that can be used as well as single or double taper.
Also, don’t worry about the weight. Thicker wall pushrods are much stronger than thin wall pushrods and will be less apt to deflect under extreme loads and speeds. The added weight will have little effect on the valvetrain’s rpm potential and will actually improve power by maintaining a rigid column between the lifter and rocker arm.
Tapered pushrods should be used where possible (especially in high load, high rpm engines) but not in guide plate applications. Straight hardened pushrods should only be used with guide plates.
Make sure there is plenty of oil flow through the lifters and pushrods to the rocker arms. With high lift cams, it may be necessary to modify the oil hole in the lifter cup so oil is not cut off at maximum lift.
If pushrods are being reused, they should be inspected with a magnetic particle inspection machine to check for cracks, and checked for straightness. If bent, the pushrods should be replaced or returned to the manufacturer for straightening (if possible).
If an engine has experienced repeated pushrod failures, it may indicate one of two things: physical interference in the valvetrain (too much valve lift and/or insufficient valve spring clearance), or excessive deflection in the pushrods. The former requires checking clearances and machining as needed to provide the necessary clearance, while the latter indicates a need for stronger, stiffer pushrods.
LIFTERS & CAMS
Cam failures can occur if there are lubrication problems in the engine. Lifters create a lot of pressure and friction on the cam lobes, so the lobes and cam bearings must receive lots of oil. If oil pressure is low or the oil is dirty, the cam may suffer accelerated lobe wear and ultimately lobe failure resulting in a dead cylinder (no valve action).
This type of cam damage can also be caused by using the wrong viscosity motor oil. In overhead cam engines, it’s a long ways from the oil pump to the top of the cylinder head. On cold mornings when the oil is thick, it can take quite a few seconds for adequate oil pressure to reach the cam. That’s why most vehicle manufacturers recommend using 5W-30 oil rather than 10W-30 or 10W-40 for cold weather driving.
If an engine is being reassembled with the original camshaft and lifters, the lifters must be put back into the same bores as before so the wear patterns will match up. If the camshaft is being replaced, new lifters should be installed also if they are the flat tappet variety. Flat bottom lifters are not really flat but have a slight convex curvature so the lifter will rotate and spread the wear over the lifter surface. If the lifter doesn’t spin due to wear, it can wear down the cam lobe causing a cam failure. With roller lifters this is not necessary because the lifters run on low friction rollers.
The type of lifters used can also affect the effective lift and duration of a cam. Some aftermarket street performance lifters are designed to “leak down” and reduce lift and duration at low rpm to improve low end torque and throttle response.
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