In both cases the valve train can take a beating if the clearance is out of specification. This clearance must be checked and adjusted over time to compensate for component wear. The hydraulic tappets allow for zero or minimal valve clearance by firming up only when pressed by the camshaft. The hydraulic tappet allows for a small amount of compression to maintain the correct valve clearance at all temperatures.
The tappet has a reservoir oil chamber and high pressure oil chamber separated by a check valve ball and spring. The cam lobe applies pressure to the tappet when it is time for the valve to be opened. This forces the check valve closed and seals the high pressure oil chamber making the tappet rigid so it can transfer the motion of the camshaft to the valve.
The check valve opens and the tappet becomes pliant again when the valve closes and the the tappet returns to its resting position. While hydraulic tappets exist to reduce the time and cost of engine maintenance, they do have their own service needs.
Engines with hydraulic tappets have additional small oil passages for the tappets. If the oil passages are clogged or restricted the tappets will cease to function correctly.
Improper oil viscosity could also cause problems. The tappets themselves may wear over time. This makes the lifters rotate as the cam turns, which helps to reduce friction and wear. Advertisement The area of contact between the lifters and cam lobes is the highest loaded surface inside an engine, with as much as , to , PSI at the point of contact depending on valve spring pressure!
Consequently, it is critical that both components have the correct geometry both convex and taper , that both surfaces have adequate hardness to resist premature wear and failure, and that the point of contact receives good lubrication with a motor oil that contains sufficient levels of high pressure anti-wear additive such as ZDDP.
Lubrication has been a problem in recent years because the amount of ZDDP in motor oil has been significantly reduced to prolong the life of catalytic converters. Reducing ZDDP to less than ppm has not created a problem for most late model engines because they have low friction roller lifters or overhead cam followers.
But in older engines with flat tappet cams, using a low ZDDP motor oil may not provide adequate wear protection for the cam and lifters — especially if stiffer valve springs are installed.
Some suppliers offer lifters that have a small pinhole burned through the bottom center of the lifter body to direct oil right to the cam lobe.
Another supplier grinds several small flats only a few thousandths of an inch deep down the sides of their lifters so more oil can flow down onto the cam. A big improvement came about with the invention of roller lifters. By placing a small wheel on the bottom of the lifter, friction between the cam and lifter is greatly reduced. Roller lifter also allow the use of more radical cam lobe profiles with faster opening and closing ramps that allow more total valve opening for a given lift and duration.
Advertisement Mounting a wheel on the bottom of the lifter also changes the dynamics between the lifter and cam. This requires the addition of a linkage bar between adjacent lifters to keep them straight, or machining the lifter body and lifter bores with a flat to prevent them from twisting. One of the differences between a roller cam and a flat tappet cam is that the lobes on a roller cam are truly flat whereas those on a flat tappet cam have a slight taper.
If the wrong type of cam or lifters are used together flat tappet cam with roller lifters, or a roller cam with flat tappet lifters , the mismatch will cause bad things to quickly happen.
Advertisement Something else that should never be done when rebuilding an engine is to install a new cam with used lifters. The cam and lifters develop a specific wear pattern to each other as they seat in.
If a high mileage cam is worn, or one or more lifters show concave wear on the bottom, the cam and lifters all need to be replaced. If the original cam and lifters are still in good condition and are being reused, make sure all of the lifters are reinstalled in their original holes same location as before. However, if the original cam is worn and needs to be replaced, replace the lifters too. Advertisement The only exception to this rule is with roller cams. Because the cam lobes are flat and the lifters have rollers rather than a convex surface, a new roller cam can be installed with used roller lifters provided all of the lifters are in good condition with no damage, pitting or cracking.
Hydraulic lifters were first developed back in the s and became common in production engines in the s. Hydraulic lifters eliminate the clatter produced by solid lifters because the valvetrain runs with zero lash clearance. Solid lifters require a small air gap between the tips of the rocker arms and the tops of the valve stems to compensate for thermal expansion in the engine as it heats up. Lash adjustment is critical because too much clearance makes the valves noisy and reduces valve lift, duration and performance.
Too little clearance can also create problems because it causes the valves to open sooner and close later, reducing heat dissipation through the valve seats when the valves are closed. This can make some valves especially exhaust valves run too hot and fail. If the lash is way too tight and closes up entirely, it may hold the valve open causing a loss of compression and possibly contact between the valve and piston. Advertisement Solid lifters require regular valve lash adjustments to compensate for wear in the valvetrain.
With racing engines, valve lash adjustments may also be required to fine tune the engine for prevailing weather and track conditions. Changing valve lash has the same effect as changing valve lift and duration. Less lash increases lift and duration for more high end power, while opening up the lash adjustment decreases lift and duration to improve low RPM torque and throttle response.
Hydraulic lifters eliminate the clatter and the need for periodic adjustments by maintaining zero clearance when the engine is running. They do this by using oil pressure against a spring-loaded plunger inside the lifter body. Oil fills the cavity under the plunger when the valve is closed. This pushes the plunger up to take the slack out of the valvetrain and hold it tight. A one-way check valve inside the lifter holds the pressure inside the lifter as the valve opens. Since oil is incompressible, the oil trapped under the plunger prevents the plunger from compressing and the lifter act like a solid lifter to push the valve open.
Advertisement Hydraulic lifters are also kinder on valvetrain components than solid lifters because zero valve lash reduces the hammering effect that occurs when the valves slam shut at higher engine speeds. This also reduces noise and helps extend the life of the valvetrain components. But at high engine speeds say over 6, to 6, RPM , hydraulic lifters experience some limitations. This overextends the plunger and prevents the valve from closing all the way. The same thing can happen if the oil inside the lifter does not bleed down quickly enough between cycles to maintain normal valve lash.
This creates too much lash in the valvetrain, which results in noise and loss of power. Advertisement Hydraulic lifters are precision fit assemblies. The plunger is closely matched to the housing to provide minimal clearance so the leakdown rate is not too great or too small.
Do each lifter individually so the original assembly tolerances are maintained. You may be under the false assumption that a solid lifter camshaft makes more power than a hydraulic design. That is not true in a pure sense. A solid lifter has the potential to follow a more aggressive camshaft lobe and also to work effectively at higher engine speeds.
Other than a racing engine or one in a pulling tractor, it is not relevant. For this discussion, a solid lifter is as its name implies: one piece of metal. It can be considered just a means to transfer camshaft lobe action to the pushrod. In contrast, a hydraulic lifter is hollow and has an internal piston and spring, and it allows oil to enter and exit.
In many ways, it has similarity to a hydraulic piston on a tractor bucket. When the valve is closed, the lifter is on the base circle of the cam the round part of the lobe , and the lifter cavity fills with oil. The internal piston is now at its maximum travel upward since the oil is below it. As the camshaft transitions through rotation into opening the valve, the piston is forced down and a check ball usually is employed to close the oil inlet orifice.
Since oil is considered incompressible, the piston can no longer move since the oil is trapped below it and the bottom of the cavity. This now makes the tappet work as a solid lifter and transfers the motion from the camshaft lobe to the pushrod. Over the lift of the camshaft due to valve spring pressure, the oil is pushed out of the lifter cavity by the time the lifter dwells on the nose of the lobe.
Once the travel of the lifter on the lobe is complete, the pressure from the pushrod is decreased on the piston and it enters at rest position. Fresh oil now enters the cavity. If an engine with hydraulic lifters is noisy, then either the internal spring has lost some tension or the check ball is not sealing or allowing the oil to fill the cavity. For all practical purposes, the tappet needs to be replaced.
Most hydraulic lifters fail due to poor maintenance. If you want to try to determine which lifter is making noise, pull off the valve cover, start the engine, and let it idle. Keep in mind that oil will be spraying, so take the proper precautions. This will take up some of the internal piston slap in the lifter and should change the sound.
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