“They were created to get rid of the need for combustion engines to have mechanical clearance in the engine's valve train… So, in effect, hydraulic valve lifters keep the valve healthy and protected from an engine's normal thermal expansion process.” “The hydraulic valve lifter is pressurized by the oil gallery right at the start of motion in the engine. The pressure from the oil is only just enough to remove whatever clearance there is in the valve train, but not enough to actually open the hydraulic valve itself.” “The check ball isn't under a lot of pressure then and is ready to be shoved into the spring by the oil pressure, which lets in oil into the check ball cavity and starts the whole cycle again.”

“Hydraulic lifters eliminate the clatter produced by solid lifters because the valvetrain runs with zero lash (clearance).” “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 acts like a solid lifter to push the valve open.”

“A hydraulic tappet, also known as a hydraulic valve lifter or hydraulic lash adjuster, is a device for maintaining zero valve clearance in an internal combustion engine.” “The hydraulic lifter was designed to compensate for this, allowing the valve train to operate with zero clearance at all operating temperatures once the oil is warm—leading to quieter operation, longer engine life, and eliminating the need for periodic adjustment of valve clearance.” “During operation, it fills with oil, which provides hydraulic resistance to compression, eliminating ‘play’ (also known as ‘lash’) from the system and creating an effective zero-tolerance system… Oil pressure will build quickly upon startup and they will set themselves to the proper height. As the whole process is actuated by hydraulic pressure at engine start, there is no need for service or adjustment.”

"When the valve is closed the lifter fills with oil. When the camshaft lobe begins the lift phase it compresses the piston which shuts the oil inlet. Since oil does not compress, this high pressure makes the lifter effectively solid during the lift phase." The paper notes that hydraulic lifter piston travel "is enough to allow the elimination of that clearance and provide more precise valve control, silent operation and reduced wear characteristics," contrasting with mechanical lifters that require valve lash clearance.

“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.” “Hydraulic lifters will normally make some noise when an engine is initially fired up, but should soon quiet down as **oil fill the lifters and the lifters expand to tighten up the slop in the valvetrain.**”

“Hydraulic lifters eliminate the clatter produced by solid lifters because the **valvetrain runs with zero lash (clearance).** … 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 acts like a solid lifter to push the valve open.**”

In a modern **hydraulic lifter**, a hardened steel push rod cup sits on top of a **plunger mounted inside the hollow lifter body**. Under the plunger is a spring that **holds the plunger up so oil can fill the cavity between the plunger and lifter body**. A **one-way check valve in the bottom of the plunger allows oil to enter the plunger cavity but traps the oil inside when the lifter moves up. This prevents the lifter from collapsing**, which would not allow it to open the valve fully. Oil pressure acting on the lifter plunger exerts a force that **forces the rest of the valvetrain to push against the valve stem**, and because of the check valve, when the lifter is moved upward by the camshaft, the **oil is essentially incompressible and the plunger cannot collapse** inside the lifter body.

“When the valve is closed the lifter is on the base circle of the cam (the round part of the lobe) and its cavity fills with oil. The internal piston is now at its maximum travel upward since the oil is below it.” “For example, if the cam lobe is 0.350 inch and the rocker arm ratio is 1.6:1, the valve lift would be 0.350 X 1.6 = 0.560 inch, if the engine used a hydraulic lifter, which has no lash.” “In contrast, a hydraulic lifter is hollow, has an internal piston, spring, and allows oil to enter and exit… engine oil flows to the cavity in the hydraulic lifter.”

Hydraulic valve lifters rely on **engine oil to maintain zero valve clearance** in the engine. Oil enters and **fills the empty space behind the plunger when the valve is closed, this results in zero valve lash as the oil pressure pushes on the plunger** so it makes constant contact with the camshaft or camshaft follower. All hydraulic lifters are equipped with **one-way valves that allow oil in, but not out**. When the valve starts opening and the camshaft presses on the lifter, the one-way valve prevents oil from being pushed out. Because engine oil is virtually incompressible, when the camshaft pushes on the lifter this **makes hydraulic lifters act like solid lifters** and allows the camshaft to open the valves.

“a typical lifter new out of the box dry, has it's internal plunger pushed upwards against the retaining lock by an internal spring. When you install it and tighten down on a rocker arm, it pushes down on the pushrod and depresses the plunger in the lifter, and how far it depresses the plunger is called preload correct?” “When the cam is on the base circle the lifter sees hydraulic pressure and the cavity is filled… The oil is trapped under the cup. (The cup itself can be empty) This is what pushes the pushrod up; a fluid is not compressible.” “The more you preload the lifter the more lift you lose at the valve. The further you push the plunger down with preload the smaller your making…”

The explanation notes that with near zero valve spring pressure on the lifter seat, "the hydraulic oil pressure lifts the push rod seat taking up the valve train clearance." It emphasizes that the oil pressure itself "just maintains contact at all times" and is not enough on its own to overcome the valve spring without the trapping mechanism, implying that the hydraulic lifter’s design allows the plunger to automatically maintain zero lash while transmitting cam motion.

“body okay that's basically it yeah so what happens how this actually works is it's meant to take up any clearance zero clearance okay and that's accomplished by oil pressure from under here uh that spring tension and then the locking of that pressure in there through the check valve and turns it into a solid lifter…” “this spring plus the oil pressure will push the plunger back up against the push rod and take up that clearance and this all happens very very quickly and it happens quietly you don't hear any rattle in your lifter when uh there zero lash… take that gap up yeah so straight away this plunger will pump go higher up into its [body] and take out all that lash…” “you can use your hydraulic lifter to adjust geometry… plunger sitting deeper into hydraulic lifter it still works you will not notice any difference guys nothing at all will change…”

“Valve lifters, sometimes called cam followers or tappets follow the profile of the cam lobe and produce a reciprocating motion within the valve train.” “In an overhead valve engine…the lifter transmits this motion through a pushrod and rocker arm, opening and closing the valves. Hydraulic lifters incorporate an internal plunger and spring with engine oil to automatically maintain proper lash.” “Leakdown and plunger travel are engineered so the lifter can compensate for minor changes in valvetrain geometry and still maintain quiet, zero-lash operation.”

“In operation, **there is a constant bleed-off of oil from the lower cavity out along the gap between the plunger and the bore, which is replenished by oil being admitted by the ball valve when there is any slack in the valvetrain.**” “Wear is negligible in this area because during engine operation **the tolerance is always filled with slowly-flowing oil and the range of movement is very small…** The movement is also not rapid oscillation but slow, **resulting from the bit-by-bit increase and decrease of the volume of oil trapped under the plunger.**”

The technical note on Harley-Davidson Sportster engines explains that hydraulic lifters (hydraulic tappets) supply oil through tight clearances: "Due to the tight oil seal between the lifter body and plunger, oil pressure can be transferred through the path to the rocker arm." It describes the lifter as a hydraulic element that, when supplied with oil pressure, maintains constant contact in the valvetrain and compensates for thermal expansion, acting as an automatic lash adjuster to eliminate mechanical clearance under normal conditions.

“At the top of the plunger of some hydraulic lifters is a metering valve or plate, which supplies oil to the pushrods for valvetrain oiling.” “Hydraulic lifters use an internal spring-loaded plunger and engine oil to maintain valve lash automatically, so the valvetrain runs with little or no clearance during normal operation.” “Adjusting hydraulic lifters involves setting the rocker arm so that there is zero lash, then tightening further to depress the plunger to its operating position, after which the lifter will self-compensate during engine operation.”

(narration) “…you still have **oil moving from the oil gallery into the plunger** however the ball check closes and you get a **high pressure area that can't be compressed underneath the plunger and that's what makes the lifter body and the plunger move together as a solid unit…**” “Now I'll elaborate more on the lifter at rest and **the ball check is open that's going to allow the oil to move freely through the bottom and it also allows the plunger to move independently… and that's going to remove any valve lash in the valve train.** So when the lifter is actuated…the ball check at the bottom closes which forces the plunger and the lifter body… to move together as a solid unit…”

“Oil enters the lifter: Oil enters the lifter through a small hole. This pressurized oil fills a chamber inside the lifter called the plunger.” “As the engine warms up and the metal components expand, the plunger inside the lifter automatically adjusts its height to maintain zero clearance (lash) between the camshaft, pushrods, and valve stems.” “The check valve keeps the oil trapped under the plunger when the valve is opening, making the lifter act as a solid piece so the valve motion is precise, while still allowing automatic adjustment when the lifter is on the cam’s base circle.”

Hydraulic valve lifters, also called hydraulic lash adjusters, employ an internal plunger and engine oil pressure so that “the plunger automatically extends or retracts slightly to take up lash as components expand and contract,” maintaining near-zero clearance in typical passenger car valvetrains. In manufacturer service literature, hydraulic lifters are described as “self-adjusting” elements that use oil pressure and spring force to keep the plunger positioned such that valve clearance is effectively eliminated during normal operation, reducing noise and wear. These descriptions are consistent across major OEM technical manuals for common hydraulic lifter designs used in pushrod and overhead-cam engines.

“Hydraulic lifters are like a **hydraulic cylinder**. By tightening the adjuster past 0 lash, you're **pushing the plunger towards the center of its travel**.” This preload allows the internal **spring and oil pressure to move the plunger automatically to take up changes in clearance** during operation, keeping lash effectively at zero. Users note that with hydraulic lifters "there shouldn't be any lash" in normal operation, distinguishing them from solid-lifter engines that run a specified clearance.

The post explains the function of hydraulic lifters: "These components automatically adjust the clearance (valve lash) in the valvetrain to account for thermal expansion, eliminating the need for manual adjustments." It notes that "Oil is fed from the engine into the lifter's reservoir, where pressure allows it to maintain constant contact with both the camshaft and the valve, ensuring smooth and quiet operation." A check valve operation is described: "Oil pressure forces the lifter's check valve closed, creating a rigid connection between the lifter and pushrod."

A forum answer notes: "High oil pressure has no affect on hydraulic lifter operation. The clearance between the lifter body and the plunger is measured in millionths of an inch." This indicates that the hydraulic lifter plunger runs with extremely tight clearances in the lifter body so that, with normal oil pressure and internal spring action, the plunger can maintain contact and effectively eliminate lash, rather than relying on unusually high system pressure.

"If it's hydraulic lifters there **shouldn't be any lash**. If it's a solid lift engine it'll be @ .017"." Commenters explain that **zero lash is crucial for preventing valve float damaging the pistons at higher rpm**. They discuss lifter plunger position: "If the lifter has the **plunger at the top of the lifter** it will hang the valve open less during valve float," implying the plunger position automatically changes with conditions. Adjustable hydraulic valve trains and proper rocker geometry are recommended to ensure the lifter can **correctly take up clearance via its internal plunger and oil pressure**.

"Run nut down on rocker by hand while turning push rod until you get zero lash, then turn nut 1/2 to 3/4 turn more and then tighten the lock." The additional turn **preloads the hydraulic lifter plunger**. A user notes that without the lifters depressing, "it's holding my valves open rather than pushing the lifter down," illustrating that in correct operation the **plunger in the hydraulic lifter should move automatically to absorb clearance**, not hold the valve open. This procedure reflects how the plunger's position, set by preload, engine oil pressure and its internal spring, **self-adjusts to maintain minimal or zero lash** during running.