The lifter (or "tappet") of an overhead valve engine rides on the cam lobe, actuating the push rod that directs the movement to the valve stem. It is an essential part of the valve train and the importance of a proper break in between the lifter foot of a flat tappet and the cam cannot be overstated.
 Types of lifters
Lifters are either hydraulic or solid. They can be either a "flat tappet" type, or a roller type. These two types can be either hydraulic or solid (also called "mechanical").
 Flat tappet
Flat tappet lifters are flat on the bottom. Actually they're not perfectly flat, they have a very gradual radius ground onto them to help the lifter to rotate (along with the angle of the cam lobe), and resemble a cylinder of steel with a snap ring on one end holding in a pushrod cup.
Flat tappet cam/lifter valve trains were the primary form of valve actuation for domestic engines from the early '50s until about the mid-'80s. They were reasonable durable and reasonably reliable, although there were better designs. Not until the EPA mandated a change in the motor oil formulations did they fall out of favor.
 Flat tappet lifter foot
The lifter foot of a flat tappet lifter is not perfectly flat, instead it is slightly convex. This is done to allow the lifter to rotate, and the shape of the lifter combined with the shape of the cam lobe is what causes this rotation. The rotation of the lifter keeps the highly loaded interface between the lifter and cam lubricated and cooled so as to not cause excessive wear during operation.
 Mushroom flat tappet lifter
Mushroom lifters were used in high performance flat tappet applications. The larger diameter of the mushroom lifter's foot allowed it to handle more aggressive cam lobe profiles. They weren't used on production engines much at all, the assembly process when using them is labor intensive- something the OEM always tries to avoid.
 Hydraulic lifter
The hydraulic lifter has been a mainstay for over 40 years in domestic automobile engines. Its design allowed a basically maintenance free valve train. They are usually quiet and reliable.
The hydraulic lifter was invented as a means of reducing maintenance and quieting the valve train. Generally the pressure from the engines oil pump would provide the "self adjusting" hydraulic action and keep the lifter tight to the camshaft and push-rod. This action was well controlled by design within the normal operating ranges of most passenger vehicles although over-extension, known as lifter "pump up" of the lifter can occur at elevated RPM, allowing the valves to be held off the seat causing a loss of power. This means mechanical lifters are the preferred type of lifter for high RPM applications. Later versions of performance hydraulic lifters included anti-pump up-types and roller lifter types as well.
- Click here for a video that shows an animation of a hydraulic lifter.
 Fast bleed down hydraulic lifter
An example of a fast bleed down hydraulic lifter is the Rhodes lifter. The Rhodes lifter accomplishes the fast bleed down by machining a precision groove on the plunger, possibly by using EDM (electron discharge machining) or laser. This groove allows the oil to bleed out of the lifter at a faster- yet controlled- rate. This allows the lifter to lower the lift and duration of the cam at lower RPM, typically from idle to ~3000 RPM or so, depending on the valve spring pressure and oil viscosity. At higher RPM, the lifter behaves as a normal lifter would.
Below, the groove on the plunger can be seen at the arrows. The plunger is partially removed from the lifter body; the retainer clip and pushrod cup having been previously removed.
 Solid lifter
Originally, virtually all lifters were a solid or "mechanical" lifter design that required regular maintenance to check and set clearance in the valve train to account for wear during normal engine operation. This insured that the valve would fully seat against the valve seat, which is important not only for good engine performance but for the valve (especially the exhaust valve) to transfer heat from the valve into the seat then on to the cooling system to prevent the valve from rapidly eroding (aka a "burned valve").
With a solid lifter valve train, a predetermined amount of play is introduced between the tip of the closed valve and the rocker arm. This clearance is sufficient to allow for heat expansion of the components during operation.
The solid lifter valve train requires periodic maintenance to account for wear, else the valve could eventually be damaged by insufficient clearance. That causes the exhaust valve to overheat and/or burn and a loss of performance can result from the intake not seating and sealing as it should. Excessive clearance can cause valve train damage and reduced performance and mileage.
Solid flat tappet lifters are still used in some high performance engines, solid roller valve trains are the choice for racing when allowed and street/strip engines used at high RPM. They are as durable as a hydraulic lifter in flat tappet form, but suffer the same wear problems due to the decrease of wear additives that have been removed from current motor oil.
Solid lifters are not used in modern engines because they make enough noise that they could confuse knock sensors, and the need for periodical maintenance is seen as too costly for the manufacturers to do under warranty.
 Roller lifter
 GM hydraulic roller lifter
Shown below is a GM hydraulic roller lifter. The arrow shows the location of the cup, spring and check ball assembly mentioned below.
The Cadillac CTS-V lifters, GM-88958689 (a set of 16 are ~ $300.00 as of mid-2013) will not withstand excessive valve spring pressure. The Cadillac CTS-V lifters are lighter but are not able to handle high spring pressure any better than standard Chevrolet LS-7 lifters (GM-12499225, set of 16 go for ~ $140.00 as of mid-2013). Either of these lifters are designed to be used with </= 100 lb. seat pressure and </= 300 lb. open pressure. If using more pressure with standard weight valve train components (no titanium, etc.), you should consider using an aftermarket hydraulic roller lifter like Comp Cams 'Reduced Travel' p/n 875 or Comp Cams p/n 15850 'Short Travel' hydraulic roller lifters (use with OEM dog bone retainers), made for OEM hydraulic roller blocks. These lifters use a very small preload (0.000 to 0.005") and the total travel is just 0.050".
GM uses the p/n 12499225 (AC Delco p/n HL-124) roller lifters as service replacements in all Chevrolet 1991 to 2013 (and likely beyond) engines. Don't use lifters designed for use in net lash valve trains (example is AC Delco p/n HL-105, GM p/n 5234670); severe engine damage can occur due to lifter loft/pump up.
 How to tell a hydraulic lifter from a solid lifter
The rule of thumb for visually discerning a solid lifter from a hydraulic lifter is the fact that hydraulic lifters contain a spring loaded plunger assembly that is retained with a snap ring or wire lock. A hydraulic lifter will compress, but this can be difficult to see once the hydraulic lifter is filled with oil unless the lifter is placed into compression long enough for the oil to bleed out of it, allowing the lifter to compress.
Some solid lifters are one piece construction. Others look much the same as a hydraulic lifter, but good quality solid lifters will usually have an internal C-clip type retainer rather than a wire clip. The clip is visible when looking at the lifter from the top where it contacts the pushrod.
On a solid lifter valve train, usually if the lifters all have about the same lash while on the base circle of the cam, and the lifters do not compress either by placing a thumb over the pushrod seat of the rocker and pressing down or by cranking in some preload and waiting to see if the spring pressure bleeds the lifter down, they're solid. To clarify, the intake and exhaust lash measurement may well be different from each other, but similar among themselves, i.e. intake valve lash all about the same and the exhaust valve lash also all about the same. Like seeing 0.016" intake clearance and 0.020" exhaust clearance as an example.
Solid lifters will have a distinctive "ticking" sound at idle caused by the clearance between the tip of the rocker arms and the valves.
If you adjust the rocker arm and lifter to "zero lash" using a valve whose lifter is on the heel of the camshaft lobe, and you then adjust the rocker arm nut down further than zero lash, a hydraulic lifter will compress and allow this to happen. If it's a solid lifter, you will be compressing the valve spring and you will feel considerable resistance to turning the adjustment nut down further. A hydraulic (in most cases) should allow at least a full turn of the adjustment nut, if not two or more turns before the valve spring starts to compress. If the engine has just been running, there will be resistance caused by the hydraulic lifter being pumped full of oil, but after waiting after turning down the adjustment nut, a hydraulic lifter will "bleed down". A solid lifter won't do this, and the valve spring will remain compressed.
 How to adjust a solid lifter
 Flat tappet cam and lifter break in
The motor oil formulation has been changed due to concerns for the environment. These changes involve a reduction of the wear additive essential for a flat tappet cam to survive the harsh conditions they operate under. Because of this, special steps must be taken at break in and at oil changes to prevent premature wear or failure of the flat tappet cam and lifters.
For more on the steps that need to be taken to avoid a cam and lifter failure, see Camshaft install tips and tricks.
 Repairing or replacing lifters
Caution: Lifters and cam lobes become wedded as soon as the engine fires up and the cam is broken in. From that point forward, the same lifter has to be kept on the same lobe, preferably in the same engine. To replace a faulty lifter with a new lifter (or to replace a used cam with a new cam, reusing the same lifters) is to risk cam and lifter failure requiring the engine to be torn down to remove the shrapnel left from the destruction of the cam lobe(s) and lifter(s). This will certainly cost more than what a new set of lifters and a cam would have cost from the beginning, considering that a new cam and lifters will be needed, along with gaskets, fluids and filters, down time, aggravation, etc.
If the cam lobes and lifter bottoms are not excessively worn, before replacing a stuck or noisy lifter with a new lifter (which should be considered only in an emergency situation), or replacing the cam and lifters with new parts, disassemble, clean and inspect the lifter that is in question- there might be hope for it after all.
 Disassembling a hydraulic lifter
Use clean lint free rags or paper towels. To clean, use carb spray, acetone, lacquer thinner, or WD-40.
To disassemble a lifter, depress the lifter cup using a wooden dowel or a pushrod. This allows the retainer clip to be easily removed from the groove in the lifter body. Use a pick and lift one side out of the groove and the clip will come out.
Be careful to not accidentally shoot the clip across the shop.
Once the clip is out, the pushrod cup and metering disc can be removed, followed by the inner plunger assembly. The plunger can be stubborn, tapping the open end against a cloth-covered wood block will usually bring the plunger to the top of the body allowing it to be coaxed out the rest of the way.
On the bottom of the plunger you will see a cup and a stiff spring. Remove the spring and set it aside. Inside the cup is a small spring and check ball (left). Some designs use a small disc instead of a ball, however check "ball" will be used herein. The cup is a press-fit onto the plunger, it snaps off and back on. With a lifter that has collapsed or is noisy, often all that has happened is the cup assembly isn't fully seated into the recess on the bottom of the plunger, or a speck of dirt has lodged between the check ball and seat.
Remove the cup using a pick and see that the tiny spring isn't bent, kinked or missing, and that the check ball is there and is smooth and shiny, and that the seat on the plunger that the ball seals doesn't have any ridges, burrs, or imperfections that would prevent the ball from seating properly.
- Return to Roller lifter.
 Lifter reassembly
Lightly lube the plunger using motor oil, put the stiff spring over the check ball cup on the bottom of the plunger, then insert the plunger assembly into the lifter body (again, no lint). Work the plunger up and down a few times (it'll take a minute to bleed the air from it) to verify it works smoothly and doesn't hang up or have any grittiness.
Next replace the metering disc and then the pushrod cup. Using a wooden dowel or a pushrod, depress the pushrod cup and reinstall the retainer clip. Again- be careful to not launch the clip into orbit.
At this point the lifter can be depressed and should return smoothly to being fully extended.
Note: If trying to repair a bad lifter and the lifter's inner bore is OK but the plunger is found to be unrepairable, replace the plunger assembly from another identical lifter and reuse the lifter body, then replace the lifter back on the same lobe it came from.
The lifter is the most precisely machined part in the engine. The plunger OD is matched to the lifter body ID; the close tolerances are required to give the correct amount of bleed down. For that reason, there is a chance you might need to go through more than one lifter to find a good match to the old plunger OD. The MAIN thing is that the plunger not be too tight. Slightly loose will still perform OK, although there's a chance there could be some slight tapping at idle. But if the lifter and cam cannot for whatever reason be replaced as a set, this is STILL preferable to putting a new lifter on a used cam.