We all know that pushrod engines come in two different lifter or “tappet” configurations – roller or flat faced. And each of those configurations can be had in hydraulic or solid (mechanical) varieties. Flat tappets have been around forever and a day. No secret. Generally speaking they’re hollow and house a pushrod-operating cup (typically manufactured in steel). The cup is held in place by way of a steel-retaining ring. Using a Chevy as an example, some types of lifters make use of an oil-metering valve that controls the amount of oil fed to the pushrods. Hydraulic lifters make use of an internal self-adjusting mechanism. Oil under pressure flows into the lifter by way of one opening and enters the hollow body of the plunger through another. Solid lifters obviously do not contain such a mechanism and as a result, lash clearance is set at the valve tip. Hydraulic lifters, on the other hand, require preload, or distance between the retaining snap ring and the plunger seat when the valve is closed, but once they’re set, hydraulic lifters require no regular maintenance.
All types of flat tappets are allowed to rotate in the bore as they move up and down. In order to facilitate this, each lobe of the camshaft is slightly tapered. Ditto with the camshaft contact face of the lifter. The idea behind flat tappet lifter rotation within the bore is to even out the wear of the lifter, especially at the edge where the lifter comes in contact with the cam lobe.
Fair enough, but isn’t a flat tappet lifter pretty much obsolete? It’s no secret, there isn’t one modern production car built in North America that uses flat tappets. And it’s been that way for decades. Roller lifters for pushrod engines and overhead camshaft configurations are the order of the day. But there’s a catch: There are some places where flat tappets still see considerable use (some by racing rules). Case-in-point is NHRA Stock Eliminator. Here the rules specify what lifter can and cannot be used. Plenty of Stockers came with either solids or hydraulics of the flat tappet kind. And so do plenty of vintage vehicles – musclecars, hot rods and street machines. That’s not all either. At one time, NASCAR Cup Car rules mandated flat tappets (that has now changed). But those rules literally drove development of flat tappet cam technology.
Does the technology from racing trickle down to the little guy? Yes and no. Take a typical NASCAR camshaft. Back in the old days, a Cup cam could cost upwards of $2,000. A set of tool steel lifters for that same camshaft could run from $600 to $3,000 for the set, depending upon how trick they really were. If you had a cup team at the time, that was probably economical. The reason being, they could go for three events before replacing the cam and lifters. Before the debut of tool steel lifters and special cam cores, cup cars would use one cam and lifter set for practice, another set for qualifying and yet another set for the event.
Is there any technology that has trickled down? While that old NASCAR technology is pretty much on another planet, there are a few things that came out of it that you can use. A good example is the EDM hole in the base of a lifter (Electrical Discharge Machining – it provides a tiny portal for added lubrication between the lobe and the lifter face). There may be a reliability advantage when using a lifter with the EDM holes. Two different EDM arrangements are offered today by various cam companies – one centered in the lifter base and another that is offset. As the lifter spins, the offset hole will lose contact with the cam lobe. But as it turns out, some builders have a preference for the offset hole while others prefer the on-center EDM hole. Jury is still out.
Another technology born from competition is camshaft finishing. Parco Lubrite or “Parkerizing” has been the standard for final surface treatment on cast iron cams for years. Lubrite is a Manganese Phosphate Conversion Coating, which is formulated to produce nonmetallic, oil absorptive coatings on iron and steel bearing surfaces. These corrosion-resistant coatings consist chiefly of iron and manganese phosphates, and are engineered to reduce wear on camshaft lobes and tappets as well as other bearing surfaces. Today, some cam manufacturers are micro finishing cam journals and lifters. The idea behind micro finishing is to provide a fine, almost polished surface for the cam and lifter contact points. Others offer Nitrided surface treatments for flat tappet camshafts. Here, the cam is heat treated, and in the process, the cam core is case hardened. Another technology is DLC (Diamond Like Carbon). Here, very hard, man-made coatings are bonded to the surface of the something like a lifter face to provide hard surface properties. DLC coatings are extremely slick (and have hard surface properties). They are more resistant to abrasive wear in extreme applications, such as the old NASCAR cams. Typically, DLC lifters are used with steel core camshafts. All of these technologies are available for use on flat tappet cams and in some cases, lifters, but for many street engine applications, some of the costs may prove prohibitive.
Have camshaft profiles changed over the last few years to reflect the gains we’ve seen in lifter technology? For the most part yes, but gains have been incremental. And little actually has to do with lifter technology. Flat tappet cams that were once considered state of the art, are now, well, old school. Typically, what you’re seeing today are camshafts with more radical opening and closing rates – more like the square nose cams developed years ago for NHRA class racers. These camshaft profiles typically place more area under the curve, which translates into improved cylinder filling along with enhanced exhaust pumping. Its possible today to use these style of cams, not because lifters have progressed so rapidly, but because springs have improved, valve train components have been stiffened and valve train hardware has been lightened. For example, years ago, a 5/16-inch diameter pushrod was considered sufficient for almost any cam in a small block Chevy. Today, most engine builders will tell you to use the biggest (diameter), stiffest pushrod you can get away with in your engine – hence the move toward large diameter, ellipsoidal or tapered pushrods. Remember, pushrod column strength is key. Spring technology has also improved. A good example is the Beehive-style spring. Here, the oval/multi-arc wire shape places the maximum area of the wire at the point of highest stress to handle valve train stress more efficiently and allow better heat dissipation for longer life. Because of the unique design, the valve train is said to handle more RPM and more aggressive cam profiles. These valve springs provide a double weight savings in your valve train by reducing weight of the actual spring and allowing a smaller, lighter retainer. We’re simply scratching the surface here, but as you can see, there’s plenty of modern technology available to breathe new life into an old engine combination.
When it comes to valve springs, the use of flat tappets certainly limits the spring pressure you can safely use (particularly in comparison to a solid roller). Remember that spring rates for flat tappets vary by application. For example, the seat pressure on a hydraulic grind will be under 135 pounds with an open pressure of 350 pounds maximum. A solid can have a seat pressure under 160 pounds and an open pressure of 385 pounds. On the other hand, those old NASCAR Cup cam combinations with tool steel components saw open pressures of 450 pounds and 220 on the valve seat. You sometimes numbers approaching those figures in NHRA Stock Eliminator
That’s a wrap for this issue. Next time around, we’ll look at lifter bore machining, examine a series of tips to help ensure success with flat tappet cam break in and we’ll also look at zinc additives. Watch for it.
