KNOW THE DIFFERENCE BETWEEN MECHANICAL AND HYDRAULIC VALVE LIFTERS

Ray Bohacz
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I personally have a love-hate relationship with valve lash adjustment. I love adjusting anything mechanical, getting my hands on it and fine-tuning it to perfection.

The part that I hate is how awkward, burdensome, and complex it often is to get to the lash adjustment. It seems that half of the engine and its accessories need to be removed to perform a 10-minute valve lash.

For this reason, I like engines with hydraulic valve lifters that, for the most part, require no adjustment. If the valve cover never has to come off an engine, that is a good day for me.

There are times when the hydraulic valve lifter needs to be adjusted. But instead of setting lash (as you would with a solid or mechanical valve lifter), a hydraulic system needs to have the preload set since there is no lash. This is usually only required if the cylinder head has been removed and is now being reinstalled.

THE NEED FOR SETTING LASH

The camshaft in an engine is responsible for the timing, lift, and the duration the valves stay open and closed. To accomplish this, it works through the intermediate components of the valve lifter (or tappet), pushrod, and rocker arm (in a cam-in-block engine).

With an overhead cam design, the intermediate components are different using some style of follower in lieu of a pushrod and possibly a tappet. This guide will be focused on a hydraulic tappet employed in an engine that has the camshaft in the block.

The profile of the camshaft lobe determines the valve action. That motion is first transmitted to the valve lifter and onto the pushrod and finally, the rocker arm, which contacts the stem of the valve. When the parts are cold, they shrink; as heat is generated, they expand.

For this reason, there needs to be free play or lash so that the parts do not bind when heated. The lash is created between the rocker arm and the valve stem tip.

A valvetrain that requires lash is often identified as employing a solid lifter or mechanical camshaft. Today, an engine can have either a hydraulic or mechanical lifter based on the manufacturer’s decision.

Most small utility engines (such as those found on a seed tender, UTV, lawn mower, etc.) have a mechanical valvetrain due to the reduced cost and the requirement of a pressurized oiling system feed the hydraulic lifter. Over the years there have been great strides made in metallurgy and valvetrain design that allow a mechanical tappet to stay in adjustment much longer and work well with less lash. Often these are referred to as a tight lash design.

NOISE AND WEAR

An inherent problem with lash in a mechanical valvetrain is the noise it creates when the engine is cold and the clearances are expanded, and the inherent wear as the parts move around. In addition, the lash setting means that the effective valve lift is less than the height of the cam lobe working with the multiplicative effect of the rocker arm ratio (this is the offset of the fulcrum from the rocker mounting).

For example, if the cam lobe is 0.350 inch and the rocker arms have a ratio of 1.6:1, the valve lift would be 0.350×1.6 = 0.560 inch (if the engine used a hydraulic lifter since there is no lash).

If it is a mechanical design with a 0.020-inch lash, then the valve lift would be 0.540 inch. This decrease may not sound like much difference as you read the numbers, but it is approximately 6% less valve travel and a corresponding impact on airflow into and out of the cylinder. As the parts wear from constantly colliding as the lash is taken up, the performance of the engine degrades and in today’s world, the emissions output is altered.

You may be under the false assumption that a solid lifter camshaft makes more power than a hydraulic design. That is not true in a pure sense. A solid lifter has the potential to follow a more aggressive camshaft lobe and also to work effectively at higher engine speeds. Other than a racing engine or one in a pulling tractor, it is not relevant.

DIFFERENCES IN LIFTER DESIGNS

For this discussion, a solid lifter is as its name implies: one piece of metal. It can be considered just a means to transfer camshaft lobe action to the pushrod.

In contrast, a hydraulic lifter is hollow and has an internal piston and spring, and it allows oil to enter and exit. In many ways, it has similarity to a hydraulic piston on a tractor bucket. Oil from the engine’s lubricating system is fed 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 the lifter cavity fills with oil. The internal piston is now at its maximum travel upward since the oil is below it.

As the camshaft transitions through rotation into opening the valve, the piston is forced down and a check ball usually is employed to close the oil inlet orifice.

Since oil is considered incompressible, the piston can no longer move since the oil is trapped below it and the bottom of the cavity. This now makes the tappet work as a solid lifter and transfers the motion from the camshaft lobe to the pushrod. Over the lift of the camshaft due to valve spring pressure, the oil is pushed out of 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 at rest position. Fresh oil now enters the cavity.

DIAGNOSTICS

If an engine with hydraulic lifters is noisy, then either the internal spring has lost some tension or the check ball is not sealing or allowing the oil to fill the cavity. For all practical purposes, the tappet needs to be replaced.

If you are good with oil changes and don’t constantly over rev the engine, then the hydraulic lifter will work as designed indefinitely. Most hydraulic lifters fail due to poor maintenance.

If you want to try to determine which lifter is making noise, pull off the valve cover, start the engine, and let it idle. Keep in mind that oil will be spraying, so take the proper precautions.

Using a 3∕8-inch drive long extension, push down gently on the rocker arm where it connects to the pushrod. This will take up some of the internal piston slap in the lifter and should change the sound.

Due to the effort to get to the lifters, I recommend replacing all of them. If one is worn now, the rest will be soon. Always coat the bottom of the lifter with engine assembly lubricant before installing it so it doesn’t start dry against the lobe of the camshaft.

Some engines use a threaded nut on the rocker stud to adjust preload; others place a shim under the rocker stand. In some designs that employ a rocker shaft, if the valve fits properly (height is correct) and the pushrod is the proper length, that is the adjustment. Regardless of the design, a good rule is to rotate the pushrod between your fingers. When you can no longer do that, the preload is set properly.

If a stud-mounted rocker is used, then you should add one quarter rotation to the nut once the pushrod preload is created.

KNOW THE DIFFERENCE BETWEEN MECHANICAL AND HYDRAULIC VALVE LIFTERS