If you have ever rebuilt an engine or replaced a noisy valvetrain component, you may have heard the advice: “Soak the hydraulic tappets in engine oil before installation.” Some mechanics recommend soaking them overnight, while others say that a quick oil bath is enough.

So, should hydraulic tappets be soaked before installation? How long should they remain in oil? And does soaking mean the lifters must be completely filled with oil before they go into the engine?

The answer depends on the design of the component and the manufacturer's installation instructions. Pre-soaking can provide useful lubrication, but soaking, priming, and air bleeding are different procedures. Understanding the difference can help prevent installation errors, unnecessary valvetrain noise, and potential engine damage.

1. What Is a Hydraulic Tappet?

A hydraulic tappet, also commonly called a hydraulic lifter or hydraulic lash adjuster, is part of an engine's valve train. Its job is to compensate for changes in valve clearance by using engine oil and an internal hydraulic mechanism.

Many hydraulic tappets contain an internal plunger, spring, check valve, and oil chamber. Engine oil enters the mechanism and helps the tappet maintain the required operating position as the engine runs.

The exact construction varies with the engine and component design. Hydraulic lifters used with pushrods, hydraulic roller lifters, and hydraulic lash adjusters in overhead-camshaft engines should not automatically be treated as identical.

That distinction matters because the correct preparation procedure is determined by the specific lifter and engine—not simply by the fact that the component is hydraulic.

2. Why Are Hydraulic Tappets Soaked in Engine Oil Before Installation?

Pre-soaking is primarily a pre-lubrication procedure. Depending on the lifter design, it may also help reduce initial operating noise.

2.1 To Lubricate the Outer Surface Before Installation

A hydraulic tappet must fit correctly in its bore or valve-train assembly. Before the engine starts, applying clean engine oil helps establish a lubricating film on the component's external surfaces.

This is particularly relevant during an engine rebuild, when new lifters, a replacement camshaft, or other valvetrain components are being installed.

COMP Cams explains in its hydraulic lifter installation instructions that pre-soaking is a good idea, although it is not mandatory for the products covered by that particular instruction sheet. The stated benefit is adequate lubrication of the lifters' outer surfaces before installation.

2.2 To Help Reduce Initial Valvetrain Noise

A dry or air-filled hydraulic tappet may make ticking or clattering noises during initial operation while the lubrication system supplies oil and the hydraulic mechanism reaches its operating state.

Pre-soaking may help displace some air from the internal plunger reservoir, which can contribute to a quieter initial start. However, soaking alone does not guarantee that the lifter is completely filled or free of trapped air.

Some initial noise can occur even when a new hydraulic lifter has been installed correctly. Persistent or excessive ticking should not automatically be blamed on a lack of soaking; incorrect preload, oil pressure problems, restricted oil passages, contamination, or a defective component may also be responsible.

2.3 To Prepare the Component for Installation

A clean oil bath allows the mechanic to prepare the lifters before assembly and keep them lubricated until they are ready to be fitted.

Use clean engine oil of the specified type, keep the container and components clean, and follow any special preparation requirements supplied with the lifters.

Pre-soaking is a preparation step, not a substitute for correct assembly, valve adjustment, or the engine's normal lubrication process.

3. Soaking vs. Priming vs. Air Bleeding: What Is the Difference?

These three operations are often confused, even though they serve different purposes.

Soaking: Pre-Lubrication

Soaking means immersing the lifter in clean engine oil so that its outer surfaces—and any internal areas accessible through its design—can come into contact with oil.

It does not necessarily fill the high-pressure chamber completely or remove every air pocket.

Priming or Pre-Filling: Filling the Internal Oil Chamber

Priming refers to introducing oil into the internal hydraulic mechanism. Some designs can be primed using a prescribed procedure that involves operating the internal plunger.

Other lifters should not be fully pumped up before installation or valve adjustment.

For example, COMP Cams warns that fully pre-pumping the hydraulic lifters covered by its instructions is unnecessary and undesirable before adjustment. A lifter that is pumped up can cause valves to open during the adjustment process instead of allowing the plunger to sit in its intended operating position.

By contrast, Rhoads Lifters instructs installers to fully fill certain Rhoads lifters by submerging them upright in oil and repeatedly compressing the inner plunger. Its instructions explicitly state that soaking alone will not fill those lifters.

These are different product-specific procedures, not conflicting universal rules.

Air Bleeding: Removing Trapped Air

Air bleeding is a separate operation intended to remove trapped air from the hydraulic mechanism.

A Subaru service procedure for certain hydraulic lash adjusters, for example, calls for immersing the adjuster in engine oil, operating the plunger according to the specified method, and continuing until air bubbles disappear. The procedure also specifies leaving the adjuster in oil until it is ready for installation.

This illustrates why simply leaving a component in an oil bath for a set number of hours may not accomplish the same result as a prescribed bleeding procedure.

The key point: soaking, pre-filling, and air bleeding are not interchangeable. Always use the procedure specified for your exact lifter and engine.

4. How Long Should Hydraulic Tappets Be Soaked?

There is no universal soak time for all hydraulic tappets.

The required duration depends on the component design and the manufacturer's installation instructions. Examples from different manufacturers show why a single rule such as “always soak overnight” is unreliable.

• Some COMP Cams hydraulic roller lifters: A specific set of Evolution hydraulic roller lifter instructions calls for soaking the lifters in COMP Cams Break-In Oil or equivalent for at least two hours to lubricate their outer surfaces.

• Other COMP Cams hydraulic lifters: The High Energy and Hi-Tech hydraulic lifter instructions describe pre-soaking as beneficial but not mandatory.

• Certain Subaru hydraulic lash adjusters: The service procedure focuses on bleeding air until bubbles disappear, rather than prescribing a universal soak duration.

These examples apply to the particular products and procedures cited. They should not be used to establish a minimum or maximum soaking time for every engine.

Is Overnight Soaking Necessary?

Not necessarily.

If the manufacturer specifies an overnight soak, follow that instruction. If the product requires only pre-lubrication or provides a different preparation procedure, there is no reason to assume that an overnight soak is automatically better.

Longer soaking does not replace correct air bleeding, preload adjustment, or lubrication-system preparation. It also does not justify pumping a lifter full of oil when the manufacturer advises against doing so.

For an engine rebuild, the most reliable reference is the engine service manual together with the installation instructions for the exact hydraulic tappets being used.

5. What Does the 12-Hour Rule Actually Mean?

This is one of the most important distinctions in hydraulic valvetrain installation.

A 12-hour wait after engine assembly is not the same as soaking hydraulic tappets in oil for 12 hours.

In its September 2025 Technical Messenger, MAHLE addresses valve replacement in valve trains operated by hydraulic followers. It states that the hydraulic followers should either be drained before installation or the engine should not be started until approximately 12 hours after assembly.

The reason given is the risk of valve impacts: filled hydraulic followers can push the valves down too far, creating a risk of contact or collision.

This is a specific assembly and startup precaution. It is not a universal instruction to soak every hydraulic lifter in an oil bath for 12 hours.

Why Can an Incorrectly Filled Lifter Cause a Problem?

A hydraulic tappet that is excessively extended or filled in a way that is incompatible with the adjustment procedure may prevent a valve from seating correctly.

Depending on engine design, valvetrain geometry, and piston-to-valve clearance, this can cause poor compression or potentially create a valve-to-piston contact risk.

The appropriate remedy depends on the engine and the component. MAHLE identifies draining the hydraulic followers before installation or observing the specified post-assembly waiting period as alternatives in the valve-replacement situation it discusses.

Do not automatically transfer this procedure to unrelated engines, and do not assume every hydraulic lifter must be drained or allowed to bleed down for 12 hours.

If the engine or camshaft manufacturer specifies a different startup or break-in procedure, resolve the applicable instructions before assembly rather than improvising a generic waiting period.

6. Practical Installation Guidelines for Hydraulic Lifters

Whether you are replacing noisy hydraulic tappets or rebuilding an engine, use the following checklist.

Step 1: Confirm the Correct Components

Verify the lifter part number and application before installation. Hydraulic lifters that appear similar may have different dimensions, internal designs, oiling arrangements, or adjustment requirements.

When replacing worn or noisy units during a rebuild, always match your engine displacement and valvetrain configuration. For example, common replacement options include 16-piece valve lifter tappet sets for 1.4L–1.6L engines and 24-piece valve lifter tappet sets for 2.5L–2.7L V6 engines.

Step 2: Read the Specific Installation Instructions

Determine whether the manufacturer requires soaking, active priming, air bleeding, draining, or a post-assembly wait before startup.

Do not assume that instructions for one brand or engine apply to another.

Step 3: Use Clean, Appropriate Engine Oil

Where an oil bath is specified or recommended, use clean engine oil appropriate for the application. Keep the container free from dirt, metal particles, and other contaminants.

Clean or prepare new components only as instructed. Avoid introducing unapproved solvents, lubricants, or cleaning procedures that might affect the component.

Step 4: Apply Assembly Lubricant Where Required

An oil bath is not a substitute for camshaft assembly lubricant when that lubricant is specified.

For flat-tappet camshaft installations, COMP Cams instructs users to apply its specified Cam and Lifter Installation Lube to the bottoms of the flat-tappet lifters. Other systems, including roller lifters, may have different lubrication requirements.

Always follow the instructions for the camshaft and lifters being installed.

Step 5: Set Valve Clearance or Hydraulic Lifter Preload Correctly

Incorrect hydraulic lifter preload can contribute to valvetrain noise, poor engine performance, or hydraulic component damage.

Follow the manufacturer's specified valve adjustment and preload procedure. Do not fully pump up lifters before adjustment unless the relevant instructions explicitly require it.

Step 6: Prepare the Engine Lubrication System

Pre-soaking an individual lifter does not prime the entire engine's oiling system.

Where required, prime the lubrication system according to the engine service manual. Use the specified procedure for the engine rather than assuming one cranking or priming method is correct for all applications.

Step 7: Follow the Correct First-Start Procedure

Before starting the engine, confirm that the installation, valve adjustment, oil supply, and any required waiting period have been completed.

For engines with specific camshaft break-in requirements, follow the applicable manufacturer's procedure. If an assembly or valve-replacement instruction appears to conflict with a camshaft startup procedure, resolve the conflict before starting the engine.

7. What If New Hydraulic Lifters Still Make Noise?

A short period of initial ticking does not necessarily indicate that a new lifter is defective, but continued noise should be investigated.

Potential causes include:

• Incorrect hydraulic lifter preload or valve adjustment.

• Low engine oil pressure or an oil supply problem.

• Restricted oil passages or contamination.

• Incorrect lifter application or installation.

• A damaged or defective hydraulic lifter.

• Camshaft or valvetrain wear.

Do not assume that leaving noisy lifters in an oil bath longer will solve these issues. Once the engine is assembled, diagnose the actual cause according to the service manual and appropriate manufacturer guidance.

Conclusion: Follow the Lifter's Instructions, Not a Universal Soaking Rule

An oil bath before installation can be a useful form of pre-lubrication for hydraulic tappets. Depending on the component, it may also help displace some air and reduce initial valvetrain noise.

However, there is no universal rule that every hydraulic lifter must be soaked overnight or for 12 hours. Some products recommend a specified soak duration; others make soaking optional, require active priming, or specify a separate air-bleeding procedure.

The 12-hour instruction found in MAHLE's valve-replacement guidance is a post-assembly startup precaution for a particular hydraulic-follower situation—not a general oil-bath requirement.

The safest approach is to verify the part, follow the correct installation and adjustment procedure, lubricate the specified contact surfaces, and prepare the engine's oiling system as directed. Correct preparation is more important than simply leaving a lifter in oil for longer.

Need replacement valvetrain hardware for your project? Explore CelerParts' selection of hydraulic tappets and engine components, including popular 16-piece valve lifter sets for 1.8L and 2.0L engines, built to ensure quiet operation and reliable oil pressure retention.