Understanding hydraulic valve lifters

Understanding hydraulic valve lifters

By: Ray T. Bohacz:

The most irritating aspect of valve-lash adjustment is its awkwardness; too many components require removal to perform a ten-minute task.

Hydraulic valve lifters, on the other hand, require no adjustment for the most part. When adjustment is necessary, instead of setting lash, as you would with solid or mechanical valve lifters, a hydraulic system requires pre-load. There is no lash. This is usually required only when the cylinder head is being reinstalled.

The need for lash or free play

The camshaft is responsible for the valve’s timing, its lift and its duration—the periods it remains open and closed. In a cam-in-block engine, this is accomplished by the camshaft working with intermediate components: valve lifter (or tappet), pushrod, and rocker arm. With an overhead cam design, the intermediate components differ, using some style of follower in lieu of a pushrod and possibly a tappet. This discussion focuses on the hydraulic tappet employed in cam-in-block engines.

It’s the profile of the camshaft lobe that determines the valve action, and that motion is first transmitted to the valve lifter and onto the pushrod and finally the rocker arm that contacts the stem of the valve.

When the parts are cold, they shrink and as heat is generated they expand. For this reason, free play is required to prevent parts binding when heated. Free play is created between the rocker arm and the valve stem tip.

Valvetrains that required lash were often identified as employing a solid lifter or mechanical camshaft. Today’s engines provide either a hydraulic or mechanical lifter, based on the manufacturer’s decision.

Improvements in metallurgy and valvetrain design now allow a mechanical tappet to stay in adjustment much longer and work efficiently with less operating clearance or lash. Often these are referred to as a tight-lash design.

When the engine is cold, operating clearances recede and when hot they expand, depending upon engine materials. If the engine block and heads are entirely of cast iron, expansion will be minimal. Alternatively, if they are of aluminum, expect expansion because aluminum expands twice as much as steel—and both lifter and pushrod are made of steel. Aluminum block and heads will increase the lash 0.010in – 0.020in cold to hot.

In addition, the lash setting means that the effective valve lift is less than the height of the cam lobe. This is the result of the multiplicative effect of the rocker arm ratio, which is the offset of the fulcrum from the rocker mounting.

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. However, if it were a mechanical design with 0.020-inch lash, then the valve lift would be 0.540 inch.

This decrease may sound inconsequential, but it represents approximately six percent less valve travel and a corresponding impact on air flow both in and out of the cylinder. Furthermore, as the parts wear from constantly colliding as the lash reduces, the performance of the engine degrades and emissions output is altered.

Also, it’s a false impression to assume a solid lifter camshaft makes more power than a hydraulic design. A solid lifter has the potential to follow a more aggressive camshaft lobe and also work effectively at higher engine speeds. But racing engines aside, this argument is irrelevant.

The difference in lifter design

For our discussion a solid lifter is as its name implies: one piece of metal. It can be considered simply a means to transfer camshaft lobe action to the pushrod. In contrast, a hydraulic lifter is hollow, has an internal piston, spring, and allows oil to enter and exit.

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. The internal piston is now at its maximum travel upward since the oil is below it.

As the camshaft rotates and opens the valve, the piston is forced down and a check ball is usually employed to close the oil inlet orifice. 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.

During the lift of the camshaft and due to valve spring pressure, oil is forced from 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 its at-rest position. The cavity is now replenished with oil.

Diagnostics and adjustment

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. The remedy is to replace the tappet.

By changing the engine oil regularly and avoiding over-revving the engine, hydraulic lifters will work as designed indefinitely. Most hydraulic lifters fail due to poor maintenance.

To determine which lifter is noisy, remove the valve cover, start the engine, and let it idle. Anticipate oil spray: take precautions. Then, using a long extension 3/8-inch drive, push down gently on the rocker arm where it connects to the pushrod. This will absorb some of the internal piston slap in the lifter and should change the sound.

Due to the effort required to replace a failed lifter, it’s wise to renew all of them. If one is worn the remainder will soon follow. Also, during starting, avoid dry lifters operating against camshaft lobes by coating their bottom surfaces with engine assembly lubricant before installation.

Some engines use a threaded nut on the rocker stud to adjust preload while others place a shim under the rocker stand. In further designs employing a rocker shaft, adjustment is self-regulating if the valve installed-height is correct and pushrod of the proper length.

Regardless of the design, a good rule is to rotate the pushrod between your fingers and when rotation is no longer possible, correct preload has been achieved. If a stud-mounted rocker is used, add one-quarter rotation to the nut once the pushrod preload is established.


  1. This was a VERY helpful and informative article. I have a 1997 Jeep 4.0L that suffers from a stumbling idle; the vehicle has never demonstrated adequate vacuum at idle since I bought it with 125k miles. I’ve tested for vac leaks, checked compression and done a number of things to try to remedy the situation. The 4.0’s are known for some ticking that normally disappears as they warm up, which mine does as well. Recently, I tried to measure my valve lift at the pushrod side of the rockers. Each appraised within the range of 0.122 to 0.162in. I let the lifters “bleed” down at the top of the camshaft lobe travel, so that each would come to a stop. However, with this wide range of values and even adding 0.060 or 0.080in of piston travel, valve lift is far from factory specs. Could this be the reason for my poor idle manifold vacuum and irregular idle? Would you replace both the camshaft and the lifters or just the lifters if these are the problem? Again, thanks for this informative article. It’s the best thing I’ve read yet, indicating that maybe my camshaft\lifters have been causing the problem all along.

    • Rob, Thanks for reading my article and the kind words about it. I too am confused about your measurements at the pushrod. But first I need to know a little more. How does the engine run off idle? The idle instability: is it there when the engine is cold? Is it a misfire that you can hear and feel at the tailpipe? What is the mileage on the engine now? I do not want to jump to conclusions until I hear back from you with some answers but I think the issue may be with an injector(s) and not internal to the engine. Please send me an email at: hotrodfarmer@farmmachinerydigest.com so that we can open a dialog on this and get the 4.0 L humming as it should! Thanks, Ray

  2. There is severe metal-to-metal knocking noise in the 2 liter engine of my 2007 Nissan Xtrail. It occurrs only on starting, then stops. I’ve been told it is caused by faulty hydraulic lifters, and to add a Wynns’ product to the oil and after 500km the noise will stop. Is this true?

  3. What is throwing me off base is the term that you use to describe the sound… metal-to-metal knocking. A slow to pump-up noisy valve lifter will make a tapping and not a knocking sound.
    I need to know how long it takes for the sound to go away and if it happens on only the initial start of the day or on subsequent restarts?
    Your engine is an OHC design and does not employ a traditional valve lifter as the article is about. It uses a follower. It can be a hydraulic follower and will share the operating principles in the text.
    Adding something to the oil would not hurt but most likely will not cure the issue if the lifters are slow to hydraulically pump up.
    Listen to the sound again and let me know about the restart and how long it takes for the sound to diminish. If the truck has an oil pressure gauge, see if you can determine if the sound is linked to a slow rise in oil pressure on initial start-up.
    If the engine needs an oil filter with an anti-drain back valve and the one you are using does not have it, that can cause a noisy valvetrain on starting, especially with an OHC engine.
    Let me know and we can go from there.

  4. I have a Chevy 402 engine with cam lobes and lifters that don’t match. Is this normal? So far, the lobes and the lifters of the cams I’ve tried don’t match.

    • Reg, When you say that they do not match, are you referencing the contact point on the lobe. Please expand on this and I will gladly provide my best advice. Thanks! Ray Bohacz

  5. My 2018 Ssangyong Korando 2L petrol engine suffers dry start in the morning. It sounds like cams / tappets dry of oil. Ssangyong insist this is a feature of the engine. I say, as do others who are mechanics, this rattling noise is caused by lost of oil returned to the sump overnight or cam wear. What do you think? Regards Rob.

  6. I agree but I wouldn’t jump to the conclusion of cam wear. As we don’t have Ssangyong vehicles in America, I assume the engine is an OHC design. Most modern OHC engines, if not all, use an oil filter with a check valve to prevent the entire lubricant returning to the oil pan. In some applications, the check valve may be in an oil gallery and not the filter. So, my first suggestion is to examine your oil filter. Even with a check valve, the system will not prevent total drain-back over a long period of time. Here in the States, most modern engines use either 5W-20 or 0W-20 engine oil. My four Ford vehicles are of OHC design and employ an oil filter with a check valve. Thus, they can remain stationary for weeks and on starting the low viscosity oil travels to the top of the engine so quickly they never exhibit dry-start. My next suggestion is to determine the viscosity of your oil. If Ssangyong is correct and it is the nature of the engine, it does not speak well about their engineering prowess. If I can help further please let me know. Ray in America.

  7. Hydraulic lifters big-block Chevrolet: When I preload and adjust, I lose compression on whatever cylinder I’m working on. Can you diagnose my trouble?

    • Please supply me with more information. How are you determining the loss of compression? What is your procedure to set the preload? Is it by twisting the pushrod? With more data, I will do my best to assist you. Thanks! Ray

  8. Ray – Very informative article, thanks for providing it.
    I have a 1967 Ford F100 with 352 FE V8 engine. We removed the heads due to a burnt valve and performed a complete valve job. As we had the heads off, I replaced both the push rods and lifters with Melling parts. There isn’t any adjustment capability of the rocker arms. Now, the engine is making a clattering noise. When you wrote about rotating the push rods by hand to assess the preload, was this executed at cylinder TDC, or at any point in the cam rotation? And, should the lifters be full of oil to perform the test?


    • Dear Sir,

      In previous decades, Ford engines, though excellent, had many different variations even in the same family. Today, that is not the case.

      Ford engineered for the best application and was not concerned with cross over to other powerplants in their family. As an engineer, that is the best way to do it: make it what it needs to be. But to the enthusiast and those working on the project, it brings frustration that one would not find with a Chevrolet. Nonetheless, these are my thoughts but please don’t accept them as gospel, as many capable aftermarket companies provide these products today.

      Ford recommends that the lifters be full of oil but that is often hard to accomplish unless you either remove the distributor or keep bumping the engine around with the starter.

      Given your non-adjustable valvetrain, the pushrod length sets the preload. It is my professional opinion (and I may be mistaken) that you are suffering a stack-up of tolerances. During the valve job, the seats were cut which affected the installed height of the valve. If the valves were not renewed, the tips were ground square which affects the length. Also, I am quite sure that the replacement pushrods and lifters, though correct, are of different lengths (albeit minor) than the factory parts. With these factors combined, the preload and often the rocker geometry becomes skewed and, in your case, results in excessive clearance.

      The prescribed Ford method is to obtain the proper length pushrod. I recall that Jeg’s and others sell shims to insert under the rocker shaft but, in theory, that would increases the clearance, and based on what you’ve said, it is already excessive.

      I would talk to the machine shop that performed the valve job and hear what they suggest. Candidly, they should have made you aware of this potential problem and calculated the equation to provide you the correct pushrods or at least make a suggestion.

      Engine building shops use an adjustable pushrod that determines the proper length with the twist method I describe. Then with the accurate measurement the parts are ordered.

      You could also guess the length and purchase a pushrod that is 0.060in longer. Ford (the company) used to offer 0.060in longer and shorter pushrods to compensate the effects of machining operations.

      I am sorry that I cannot offer more help and hope I have steered you in the proper direction.

      Being a car guy and a farmer I wondered if your truck is a resto or a vehicle that is being used. When young, a country neighbor (about 2 miles away) also had a 1967 model with a 390, 4 WD, stamped steel wheels and skinny tires… nothing could stop that beast!

      Please let me know what you find.

      Have a blessed day,

      Ray Bohacz

  9. Hi Ray,

    Firstly, thank you for taking the time to publish your article. I found it very informative and it may have helped answer some of my questions regarding hydraulic flat tappets. I posted my questions recently on a forum but, so far, haven’t had any replies. So, I’d greatly appreciate if you could read through my notes below and offer any further insight?

    Regards Ben

    Hi All,

    I’m hoping to get some technical advice regarding hydraulic flat tappets. I have a Chrysler 318 engine with a Comp Cams CRS XE268H-10 utilizing their 867-16 Pro Magnum lifters. I’ve previously had a low-idle oil pressure issue with this engine and as a result had lifters ticking. Currently, the engine is removed from the car and have rectified the oil pressure issue. However, I’m hoping to get some further advice on the typical characteristics of automotive hydraulic lifters. My experience has been predominately in the aviation field where our hydraulic lifters remain constantly pumped up regardless of whether the engine is operating or not. As many would know, aircraft engines operate at a low and constant RPM, and I’m told the lifters in my 318 are of the anti-pump style for higher rpm ranges.

    I have emailed Comp Cams with my tech question but have yet to hear from them. Basically what I would like to know is the following: Currently, I have the intake manifold removed and when rotating the oil pump with a drill, the lifters appear to pump up somewhat. However, the ones that are positioned with a valve open don’t appear to, or at least not fully. Also, some of the lifters seem to bleed down pretty quickly. Is this normal behavior when operating the oil pump with the drill?

    I appreciate any advice or suggestions, and I thank you in advance.


  10. Hi! Thanks for reading my article and reaching out to me. I will do my best to provide my opinion.

    From what I gleaned from your letter, you are receiving oil to each lifter but some are responding differently than others.

    Based on your description, I believe all is fine. I would be concerned with each lifter being fed oil and not necessarily how it receives the oil under the current conditions. Also, how did you set the preload? I would set it for zero lash (pushrod twist method) and maybe 1/2 turn based on the camshaft design. It does not surprise me that some lifters are bleeding down at a quicker rate and that those working against valve spring pressure are not filling to capacity. The speed and thus oil flow from the pump along with all of the other dynamics are skewed and, thus, the hydraulic response will not mimic a running engine.

    I now have a question for you. How do the lifters stay pumped up with the engine off on an aviation design? I would assume that they must bleed down, over a period of time, and especially those with an open valve. If you could acquaint me with a resource, I would greatly appreciate it. Are they truly hydraulic lifters (on the aviation engine) or a solid lifter that has an oil cavity for cooling and as a conduit to the rest of the valvetrain?

    I trust you will hear from CompCams. Please let me know what they tell you so that I can learn too.

    Sorry I was not more help.

    Ray Bohacz, http://www.farmmachinerydigest.com

  11. Hi Ray,

    Thanks for the article. I have a 1974 SBC 350 flat tappet, and I’m confused about how high the lifter should be outside of block and full of oil primed. Should the plunger be locked solid or should it be able to be depressed slowly to bottom of it’s travel? I ask because when I prime and install and then set pre-load (3/4 turn past 0 lash) all the valves are open because lifters are locked solid.

    • Jon,

      Thank you so much for reading my article. Please reach out to me at hotrodfarmer@farmmachinerydigest.com so that I can try to help you with this issue.

      I have a few questions such as are you setting the pre-load with the lifter on the base circle of the lobe? Are the lifters the same length as the ones you replaced? Are you using the pushrod twist method to determine zero lash?

      Thanks so much, Ray


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