Low friction roller lifters replaced flat tappet lifters many years ago in stock production engines, but there are still plenty of hot flat tappet cams bumping the valves open in vintage muscle cars, street rods and race cars. Even NASCAR is still using flat tappet cams to actuate the valves.
The advantage of flat tappet lifters is that they are relatively simple and cheap compared to roller lifters. With solid lifters, there’s not much that can go wrong. The lifter is nothing more than a hollow steel bucket with a slightly convex bottom. The lifter rides on the cam lobe and transfers the rotary motion of the lobe into vertical lift. That motion passes up through the pushrods to operate the rocker arms and open the valves.
Solid lifters work well in high-revving engines, but do require constant valve lash adjustments. A solid lifter valvetrain is also quite noisy because of the clearances between the rocker arms and valves. That’s not an issue in a race car, but for a street car it may be a consideration. However, solid lifters do provide the ability to change the valve lash by adjusting the rocker arms or pushrods. This allows cam lift and duration to be tuned slightly to adapt to changing track conditions.
With hydraulic lifters, there’s an oil-filled bucket inside the lifter. Oil pressure enters the lifter through a small hole in the side and fills the piston cavity. A check ball and spring in the lifter trap the oil temporarily, allowing the piston to move upward and take up slack in the valvetrain. This eliminates valve lash, noise and the need for frequent adjustments. But the trade off is added complexity, cost and a tendency to “pump up” at higher rpms. Hydraulic lifters are usually the best choice for stock engines, street performance applications and low rpm torque motors that don’t rev much beyond 5,500 to 6,000 rpm.
The taper on the cam lobe and the curvature of the bottom of a flat tappet lifter cause it to rotate as it rides on the lobe. This reduces friction, but still creates a lot of pressure between the cam lobe and the bottom of the lifter. Proper lubrication is absolutely essential to prevent cam and lifter failure.
Never reuse old lifters if you are replacing a cam. A new camshaft requires a new set of lifters.
To prevent rapid camshaft and lifter wear, the oil must contain adequate levels of anti-wear additive, such as ZDDP (zinc dialkyl dithiophosphate). ZDDP dates back to the 1950s, and has long been used to protect flat tappet cams in pushrod engines. But with overhead cam engines and pushrod roller cam engines, valvetrain friction has been greatly reduced. Consequently, there is less need for ZDDP in today’s motor oils.
The oil companies have been gradually reducing ZDDP levels in recent years to prolong the life of catalytic converters. The problem is that ZDDP can contaminate the catalyst and reduce its efficiency if an engine is using oil. “SM” rated motor oils and newer oils (including both conventional and synthetic) have much less ZDDP than they once had.
There is still enough of the anti-wear additive to provide marginal protection for older pushrod engines with stock flat tappet cams and valve springs, but not enough to protect the cam and lifters in modified or high performance engines with a flat tappet cam (especially if the engine has stiffer-than-stock valve springs). For these applications, a ZDDP supplement or other anti-wear oil additive must be used. Or, you can fill the crankcase with a racing oil that is formulated with adequate levels of ZDDP or another anti-wear additive.
Something else every engine builder has to keep in mind with flat tappet cams and lifters is the need to properly lubricate and protect a new camshaft and lifters when an engine is assembled, then started. All cam lobes and lifter bottoms must be pre-coated with high pressure assembly lube to provide protection during the first critical moments following the engine’s initial start up.
Pressurizing the engine’s oil system prior to cranking it over for the first time is also recommended. Once the engine starts, keep it above 2,000 rpm for 20 to 30 minutes. Don’t let it idle or you run the risk of wiping out the new cam. Also, advise your customer of the importance of using a ZDDP additive or racing oil.
Roller Lifters
The main advantage of roller lifters is that they provide a huge reduction in friction compared to flat tappet lifters. This not only reduces parasitic horsepower loss in the valvetrain, but also wear on the cam lobes.
Lubrication is less of an issue with roller lifters, but is still important in performance engines with high spring pressures. At higher rpms, the rollers on the bottoms of the lifters may skip and slide rather than rotate. So the need for a good anti-wear additive in the oil is still important (though less critical than with flat tappet lifters).
Roller lifters also provide another important advantage over flat bottom lifters: they allow more radical lobe profiles and faster valve opening and closing rates. That means more “area under the curve” (valve opening) for more horsepower and torque.
The drawbacks with roller lifters are their added complexity and cost. Roller lifters can be heavier than flat tappet lifters, and may require a hinged lever between adjacent lifters to keep the lifters in proper alignment (if the lifters don’t have a flat on one side to prevent them from rotating out of position).
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 they size their needle bearings to the nearest micron and carefully match all the needle bearings to improve the durability of their roller lifters.
Pushrods
Stock pushrods are okay for stock engines, but are usually not stiff enough to handle stronger-than-stock valve springs. If you’re building a performance engine, you want the valvetrain on Viagara: stiffer is better and will get your to the finish line.
Reducing weight in the valvetrain obviously increases an engine’s rpm potential. But the weight of the pushrods is something you don’t want to minimize because it will increase flex and the risk of bending or breakage. The pushrods have to be stiff enough to handle the valve springs, so the stiffer the valve springs, the stiffer the pushrods need to be.
Weight on the pushrod side of the rocker arms has much less effect than weight on the valve side of the rocker arm. The lift ratio of the rocker arms has a leverage effect that multiplies spring pressure when the valves close.
Chrome-moly pushrods are a step up, and for serious racing the pushrods will also have to be thicker walled (.080? minimum), larger diameter and possibly tapered (if they’ll fit and you are not using guide plates). Some aftermarket performance pushrods are available in wall thicknesses up to .125?, and others use a double wall construction with an aluminum tube inside a steel outer tube. The inner tube adds stiffness with adding much weight. And if you’re worried about weight, don’t be because added mass on the pushrod side of the rockers is much less important than mass on the valve side.
If you’re not sure how much pushrod stiffness a motor needs, check with your pushrod supplier. They can advice you on what pushrods to use based on the application, cam and valve springs. As one pushrod supplier put it, “The same cam and valve springs in one kind of racing application may require pushrods that are different from those that would be the best choice for a different type of application. Everything is very specialized today, so you have to consider all the variables, not just the cam grind and valve springs.”
Pushrod Fit
Pushrod lengths are often custom-sized to fit a performance engine. The length of the pushrod will depend on a number of factors, including the installed height of the valves, the centerline and geometry of the rocker arms, and the position of the cam base circle relative to the block and heads.
Pushrods have to be correctly sized to keep the tips of the rockers centered over the valves as they open and close. Wrong length pushrods can result in side loading on the valves, uneven valve stem and guide wear, and pushrod bending and breakage.
The tip of a roller rocker arm should be centered over the valve stem when the cam is at mid-lift. If it is off-center, it will produce side loading on the valve stem. An adjustable-length pushrod or similar pushrod measuring device can be used to determine the correct size. Install the adjustable pushrod, adjust valve lash as it will be set on the engine (zero lash if you are using hydraulic lifters), then measure the length of the pushrod.
Round-off the length to the nearest .050?, since that’s the increment that most custom pushrods are made to size. Check with the pushrod supplier to see if they measure pushrod length end to end, or if they add a compensation factor for the oil holes in each end.
If an engine has cup-end pushrods rather than rounded end pushrods, measuring the length can be tricky because the size and shape of the cup can vary depending on the manufacturer. One trick is to place a 5/16?(.3125?) steel ball in the cup end, measure the overall length of the pushrod with the ball in place, then deduct the diameter of the ball to get the true length of the pushrod.
Valve Springs
With high-revving engines, stiffer is not only better, it’s a must. Stock valve springs can keep up with the demands of the valvetrain to about 5,500 to 6,000 rpm, but beyond that the engine will need stiffer springs, double springs or possibly even triple springs depending on the engine’s redline, the lift and duration of the cam, and the weight of the valvetrain components on both sides of the rocker.
Increasing the rocker arm lift ratio generally requires increasing the spring rate to maintain the same rpm potential. Going from a 1.5 rocker ratio to a 1.6 ratio may require a valve spring that is about 6.5 to 7 percent stiffer to maintain the same redline as before.
Spring harmonics also play a big role in how many rpms a set of springs can handle before the valves start to float. Beehive springs with their tapered profile tend to be more resistant to harmonics than conventional coil springs. But the shape of a beehive spring means you can’t use a double spring – so there’s no backup if you break a spring (as is the case with double springs).
How much spring pressure do you actually need? If you are building a mild performance small block engine with a flat tappet cam and no more than .450? lift, single springs with 80 to 90 lbs. of closed seat pressure should work just fine. For a hotter street/strip engine with a flat tappet cam, single springs with 100 to 120 lbs. of closed seat pressure (300 to 330 lbs. open pressure) are usually recommended. If the engine has a roller cam with heavier lifters, you might need springs with 120 to as much as 250 lbs. of closed pressure depending on the cam grind, the weight of the valvetrain components and peak engine rpm.
For a high-revving circle track or drag racing engine that is running a flat tappet cam, double springs with closed seat pressures of 130 up to 200-plus lbs. may be needed to handle the rpms. Many Pro Stock drag racers are using triple springs with up to 475 lbs. of closed seat pressure, and over 1,000 lbs. of open valve pressure. In most instances the open spring pressure will be two to three times the closed seat pressure, so the valvetrain must be strong enough to handle it.
The main disadvantage with higher spring pressures (besides the load they create on the valvetrain) is that stiffer springs don’t last as long as springs with less spring tension. That might not be a big deal on a drag motor where the springs can be replaced often, but on a street car or endurance engine, short spring life would not be desirable.
An important point to remember is that good quality springs are expensive. Watch out for cheap springs that seem like a bargain, but won’t hold up and will lose pressure quickly or break.
Valve Spring Installation
Always check spring pressures to make sure they meet specifications and vary less than five percent from one spring to the next. Some engine builders recommend compressing a new spring several times to where the coils bind, then checking its pressure. Installed spring pressure can be adjusted by changing the installed height of the valve stems, and/or inserting shims under the springs.
If you’re after weight savings to reduce the risk of valve float at higher rpms, replacing conventional springs with beehive springs can save some weight. The taper at the top allows the use of a smaller and lighter spring retainer. However, if cost is not an issue, titanium springs are the way to go. Titanium is lighter than steel and the springs are typically made with larger diameter wire but fewer coils. This allows a titanium spring to handle more valve lift without coil bind. The reduced mass and inertia of a titanium spring also increase the natural frequency of the spring to lessen harmonics at higher engine speeds. Titanium also has a lower torsional modulus than steel which makes it more springy than steel. Consequently, the springs hold their pressure longer and resist taking a set at elevated temperatures.
With conventional steel springs and beehive springs, the quality and purity of the wire are extremely important for spring reliability and longevity. Most valve springs are now made from high silicon wire or chrome vanadium wire. Ovate shaped wire can also provide some longevity advantages over round wire, and a spring made of ovate wire can handle more valve lift without coil bind. Surface treatments that micropeen and/or polish the springs reduce stress that can lead to spring breakage. One supplier of valve springs who uses a special microfinish process on its springs says it improves spring durability at least 10 percent. Springs can also be nitrited to improve durability.
Some spring suppliers also say that cryogenically treating springs (freezing them to 300 degrees below zero) is worthwhile because it extends the life of the springs. One supplier of cryogenically treated valvetrain components says the process allows heat-treated base materials to achieve their maximum hardness. It also improves geometric stability and uniformity, reduces stress and increases wear resistance 25 to 30 percent on most parts.
Various types of surface coatings for springs can also improve spring reliability and durability be dissipating heat and reducing friction. Coatings that reduce friction between inner and outer valve springs, and the spring and dampener help the spring run cooler so it will last longer. Be cautious, though: coatings that trap heat rather than dissipate heat should never be used on valve springs.
Clearances also have to be carefully checked to make sure a set of valve springs deliver the specified seat pressure at the installed valve height, and that the coils won’t bind at maximum valve lift. Most camshaft manufacturers say to allow at least .060? of clearance between the coils at maximum lift, but some racers are running much closer tolerances (some as little as .015?!) and getting away with it.
There also has to be adequate clearance between the bottom of the spring retainer and the top of the valve guide at maximum lift so the valvetrain doesn’t bottom out.
One final tip: oil helps cool and carry heat away from the springs. So don’t restrict the oil flow to the cylinder heads. The springs need that oil.
For more information and a list of suppliers of valve springs, pushrods and lifters, check out our upgraded online engine builders buyers guide.
