"A hydraulic lash adjuster includes a body having a reciprocating plunger moving therein. The plunger includes a central cavity portion which defines an oil reservoir. Inlet ports are provided through the body and the plunger to thereby place the internal reservoir in fluid communication with the engine oil galleries.""The internal or operating pressure of the adjuster [oil reservoir] be approximately one-third to one-half of the gallery operating pressure. Such a pressure differential will insure sufficient oil pressure to permit operation without plunger pump-down due to oil starvation, and yet prevent plunger pump-up."

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.[1] 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.[1] Oil under constant pressure is supplied to the lifter via an oil channel, through a small hole in the lifter body.[1]

"Hydraulic lash adjusters (HLAs) use oil pressure to maintain constant contact between the camshaft and valves. HLAs have inner and outer plungers that fit closely together to trap oil between them when pressured by the camshaft lobe. This hydraulic lock transmits the camshaft force to open the valve.""The HLA reservoir top is designed just like plungers, one little hole flowing oil in (under oil pressure) and no holes out. So this oil is essentially stagnant inside the HLA… Not enough oil pressure means that the chamber between the plungers shrinks by enough to allow a small space between the valve and lifter or cam and lifter."

Hydraulic valve lifters are cylinders, or bucket-type shapes, and you can tell they're hydraulic as they have a small hole for the oil passage. They use engine oil pressure internally to automatically take up the clearance between the valve follower and the valve stem, maintaining zero lash under operating conditions. In contrast, solid lifter engines require manual adjustment of valve clearance using feeler gauges and shims or screws.

Most modern engines use **hydraulic lifters**, which use a **small reservoir of engine oil to zero out valve lash automatically**. This design allows the lifter to maintain essentially zero clearance between the camshaft and valve train components during operation, resulting in quieter operation and reduced wear. Hydraulic lifters continually adjust using pressurized engine oil so that any clearance caused by wear or thermal expansion is taken up by the oil-filled plunger system.

NOTE: If your 235/261 is equipped with hydraulic lifters, the adjustment procedure is the same, except you carefully adjust each valve to zero lash when it is in position, then give the adjusting screw 1/4 turn more and lock the nut. Hydraulic lifters use engine oil supplied through internal passages as a reservoir to automatically maintain zero valve clearance during operation. If oil is not present or passages are blocked, the hydraulic action cannot function properly and valve train noise or improper operation can result.

M10 engines in 2002s have a valve clearance, or lash, spec of 0.006 to 0.008 inches. M20 motors in E30s are specced at 0.010 to 0.012, but you carefully adjust them manually because they use mechanical (solid) rocker arms. In engines without hydraulic lifters, a fixed clearance is required to allow for thermal expansion of valve train components; the clearance is set cold and checked with a feeler gauge between the rocker and valve stem. Hydraulic lifter engines do not require periodic valve clearance adjustment because oil-fed lifters automatically compensate to maintain near-zero lash.

Hydraulic lifters eliminate the clatter and the need for periodic adjustments by maintaining zero clearance when the engine is running.[9] 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.[9] 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.[9]

Hydraulic tappets "are provided with a **small reservoir of oil, and a spring-loaded plunger**" that bears against the pushrod. When the engine is running, "oil pressure keeps the reservoir full" and the plunger moves to compensate for any clearance. As a result, "hydraulic tappets **maintain zero valve clearance (zero tappet clearance)** under normal operating conditions," removing the need for periodic manual adjustment.

The Society of Automotive Engineers paper describes a valve train design using **hydraulic lash adjusters** where "engine oil is supplied to a plunger-and-reservoir assembly". The plunger moves to take up clearance, and when the check valve closes, "oil is trapped in the internal reservoir, making the lash adjuster act rigid and thereby **maintaining zero lash** during steady-state operation." The internal reservoir is explicitly identified as part of the hydraulic system for zero-clearance maintenance.

Similar in operation to the hydraulic piston of a tractor bucket, engine oil flows to the cavity in the hydraulic lifter. 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.[4] As oil is considered incompressible, the piston cannot move because oil is trapped below it and the bottom of the cavity. The tappet now operates as a solid lifter and transfers the motion from the camshaft lobe to the pushrod.[4] If an engine with hydraulic lifters is noisy, either the internal spring has lost some tension or the check ball is not sealing or allowing the oil to fill the cavity.[4]

"Each bucket contains a one way valve where the oil enters so that when the cam comes to open the valve the oil cannot escape and the little 'hydraulic piston' holds the valve bucket tight against the cam lobe, keeping zero lash.""Oil pressure regulator feeds oil to them that pushes them up in the bore to create a constant zero valve lash… basically there is an oil reservoir in the lifter which is supplied from the engine oil gallery through a small hole and a non-return valve."

Clearance should be set when the engine is cold and the cam follower is off the cam peak. Amount of clearance should be monitored as excessive clearance leads to noisy operation and reduced performance. Zero valve clearance typically refers to hydraulic lifter systems where the lifter, supplied by engine oil, keeps the clearance effectively at zero during running, unlike mechanical systems that require a specified gap. If a mechanical valve train is set to zero clearance cold, valves may not fully close when hot due to thermal expansion, risking loss of compression and valve damage.

The technical description of hydraulic valve lifters notes that each lifter includes "a **body, plunger, and oil chamber (reservoir)** supplied with pressurized engine oil." During operation, "the plunger position self-adjusts so that the clearance between cam lobe and valve train components is effectively zero." The trapped oil in the chamber prevents collapse under load, so the lifter behaves like a solid element and **keeps valve lash at zero** without manual setting.

The valvetrain overview notes that "many modern engines use **hydraulic lifters or lash adjusters**" which "use **engine oil pressure acting on an internal reservoir** to maintain essentially **zero valve lash** across operating conditions." This eliminates the clicking noise associated with mechanical clearance and reduces the need for periodic adjustment, since the hydraulic mechanism continuously compensates for wear and thermal expansion in the valve train.

The hydraulic lifter uses pressurized oil from the engine's oiling system to change the effective height of the lifter in order to keep the valve train adjusted to zero lash or as close to zero lash as possible.[5] Pressurized oil flow from the lifter oil feed passages into the lifter reservoir through the lifter body feed hole. This pressurizes the lifter's oil reservoir. This pressurized oil causes the piston in the cylinder to pump up until the valve lash is taken up.[5] When the lifter's high pressure chamber is under pressure, the oil escapes through the clearance between the plunger and the lifter body. As the volume that was supporting the plunger is reduced, the lifter plunger is forced farther into the high pressure chamber, which reduces the effective height of the lifter.[5]

Yes, valve clearances do get smaller when the engine starts to heat up, due to the expansion of the metals but not to the degree of where the valves cannot close. This is why mechanical valve trains are set with a specified clearance cold; thermal expansion reduces the lash toward zero as the engine reaches operating temperature. Hydraulic lifter systems use oil-fed internal mechanisms to automatically adjust for these changes and keep effective clearance at or near zero while ensuring valves still fully seat.

The oil enters the lifter from block oil passages under pump pressure while the lifters on the cams base circle, with near zero valve spring pressure on the lifter.[8] This allows the lifter’s internal plunger to move and adjust so that the clearance in the valvetrain is taken up by the hydraulic action of the oil trapped inside the lifter.[8] If the lifter pumps up excessively with oil, it can hold the valve off its seat until some oil leaks down and clearance returns.[8]

"There's an oil-fed hydraulic 'spring' that keeps the tappet lightly pressed against the cam at all times (zero clearance, or zero 'valve lash').""In the situation of a hydraulic lash adjuster, there's a slightly different setup, but basically there's an oil-fed hydraulic 'spring' that keeps the tappet lightly pressed against the cam at all times (zero clearance, or zero 'valve lash').""The downside is they're more complicated to design (requiring oil passages to the HLA's and the appropriate design so that they get fed oil when needed and won't leak oil when being actuated)."

With zero clearance there is a chance that valves could have kissed the piston tops while you turned the engine by hand. Not good. Something is definitely wrong if you measure zero valve clearance in a mechanical (non-hydraulic) valve train; there should always be some lash to allow for expansion. Zero clearance is only acceptable in the sense of hydraulic lifter "zero lash" systems, where oil pressure inside the lifter body keeps the follower in constant contact with the cam while still permitting full valve closure.

In the patent for a "valve train lubricating structure," the internal combustion engine includes oil galleries that feed both bearing surfaces and "hydraulic valve lash adjusters." These adjusters contain "oil pressure chambers" that receive engine oil and expand to remove clearance. The document states that when the chamber is filled, the adjuster "acts as a rigid member and **eliminates lash** between the cam and valve." The adjuster design uses engine oil as a working fluid to keep valve clearance at or near zero.

A mechanical engineering textbook section on valve gear states: "Hydraulic tappets incorporate a **small oil reservoir and plunger mechanism** which is continuously supplied with lubricating oil from the engine gallery." As the components expand with temperature, "the plunger position is automatically altered, thereby **maintaining zero clearance between cam follower and valve stem** during steady running." The text contrasts this with solid tappets that require periodic manual adjustment of valve clearance.

This diagram illustrates the internal components and function of a hydraulic lifter (or lash adjuster) used in internal combustion engines. Function: These components automatically adjust the clearance (valve lash) in the valvetrain to account for thermal expansion, eliminating the need for manual adjustments.[7] Operation: 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.[7]

"The oil feed groove location machined in the HLA body will allow oil pressure to feed into the plunger assembly for proper operation when the HLA is raised in the bore to correct the valve lash.""Oil pressure is fed into the HLA and trapped by an internal check valve. This trapped oil forms a hydraulic spring that maintains contact between the cam lobe and valve, effectively eliminating lash."

Ensure the engine is completely cool before beginning adjustments to account for thermal expansion, which affects valve clearance accuracy. Noisy operation may indicate loose valve clearances; overheating or poor performance may suggest tight clearances. This procedure applies to engines with mechanical valve trains where lash is set manually; engines with hydraulic lifters are designed so that engine oil maintains zero lash automatically, and routine clearance adjustment is not required.

Some just soak them overnight in oil so it will be drawn into the tappet to lubricate the piston in the body, rather than leaving it dry, initially. What one should bear in mind is that the normal operating position will be somewhere between fully compressed and fully extended (but not actually reaching either).[2] If one fully primes the oil reservoir, it will mean the valve may be held off the seat until the tappet "leaks down" (loses enough oil to allow the valve to close), whereas with the option of installing them relatively dry meaning there will be excess clearance until the tappets receive enough oil to fill their reservoir and bleed out any trapped air.[2] As an aside, the first part of a/ is basically what happens when hydraulic tappets suffer valve float - the oil takes up the momentary increase in clearance.[2]

Fluid Storage & Supply: Reservoirs store the hydraulic fluid (or system fluid) for the system. They make up for fluid lost during leaks or when actuators cycle.[6] The reservoir’s functions are beyond storage; it has more critical uses: Thermal reduction, Air Separation, Contaminant Settling, and Mounting Platform.[6] The hydraulic reservoir is not only a tank, it’s an engineered life-support system.[6]

"Just bleed em in a cup of oil so there's no air or old oil in them, then install as normal. You'll want crank it over for a bit to prime the oil system and fill the lash adjusters with oil.""Lash adjusters should be stiff, not squishy, and only move up and down when filled with oil under pressure. When filled with oil they act as solid lifters to maintain proper valve clearance."

"With the lash adjuster upright with the plunger at top, push the plunger firmly until it makes a full stroke… return the plunger slowly and release the steel ball to allow pressure chamber to fill with diesel fuel. When you've done this right, push on top of the lash adjuster firmly and the plunger should not move at all, should be firm.""Then let's oil it up and prepare to put it on in the head… this process removes air and fills the internal reservoir so the adjuster can maintain proper valve lash when supplied with engine oil."

I've seen videos quoting the “rule of 9” for a 4-cyl, 4-stroke engine - when valve 8 is fully open, adjust 1, when 7 is fully open, adjust 2, and so on, for mechanical valve trains. In the demonstration, the instructor uses a feeler gauge between the valve and rocker to set a specified clearance; this is necessary because the engine shown does not use hydraulic lifters. Hydraulic lifter engines differ in that the lifter, filled with engine oil, automatically keeps the follower against the cam, eliminating the need for regular manual clearance adjustment and maintaining effective zero lash.

"Is there anything wrong with pulling the HLAs out, bleeding them in a container, and filling each rocker arm with oil before reinserting the HLA?""The goal is to get the air out and have the adjusters pre-filled with oil so that when the engine starts, they immediately take up the lash and prevent valve train noise."

In traditional pushrod or overhead cam mechanical valve trains, the clearance is measured between the cam follower (rocker arm, bucket, or tappet) and the valve stem using a feeler gauge. If the clearance is set to zero on a cold mechanical engine, thermal expansion as the engine warms can hold valves slightly open, causing loss of compression and potential valve damage. Hydraulic tappet systems were developed to avoid this problem by incorporating an internal oil reservoir and plunger that automatically adjust for expansion and wear to maintain zero lash during operation.

Hydraulic tappets, or hydraulic valve lifters, are typically integrated into the engine valvetrain and contain an internal oil chamber or reservoir that is filled from the engine’s pressurized oil supply. This internal reservoir allows the tappet plunger to move to take up lash and then act as a solid column when oil is trapped, maintaining near-zero clearance between cam, lifter, and valve components across operating conditions.[1][3] The combination of the oil reservoir, plunger, spring, and check valve within the lifter constitutes a small hydraulic system whose purpose is to automatically adjust and maintain valve lash close to zero without external adjustment.[1][3]

"Just bleed em in a cup of oil so there's no air or old oil in them, then install as normal. You'll want crank it over for a bit to prime the oil system and fill the lash adjusters with oil before they can properly take up valve lash.""This bleeding and filling process ensures the internal reservoir and pressure chamber are full of oil so the adjuster can maintain zero or near-zero clearance during operation."