The term “valve lash” is often used when discussing valve train adjustment.
A valve lifter (sometimes referred to as a tappet or cam follower) follows the contour of the camshaft lobe and transfers that motion to open and close the valves.
Let’s start with understanding the basics, Hydraulic and Mechanical.(valve lash)
Valve lash setting and adjustment. A domestic V-8 engine from the muscle car era will have either solid (mechanical) or hydraulic valve lifters. A solid lifter is a rigid component.
In contrast, a hydraulic lifter is designed to accommodate variations in valve train clearance in order to automatically maintain zero lash in the valvetrain under all operating conditions.
It does this by filling and emptying the lifter with engine oil through a metering orifice and a check valve. When the lifter is filling with oil, it is pumping up. When the oil is exiting the lifter, it is bleeding or leaking down.
Solid lifter have a predetermined lash or clearance. An engine equipped with a hydraulic cam does not require a feeler gauge.
When adjusting the valve lash on an engine with hydraulic lifters, you are not really setting the lash or clearance. You’re actually setting the preload on the lifter through the pushrod and rocker arm.
The traditional adjustment on a hydraulic lifter is zero lash. This is followed by a predetermined amount of turns on the hold-down nut.
Not all engines with hydraulic lifters have a preload adjustment.(valve lash)
A (valve lash)
If the rocker arm is stud-mounted, such as on a Chevy, then proper procedure involves attaining zero lash plus the specified amount of preload. Some engine designs, such as the Pontiac V-8, use a stud, but are considered non adjustable in factory form. On these engines, the rocker stud needs to be changed to make the valvetrain adjustable with a hydraulic camshaft.
Other designs use a pedestal mount for the rocker and may require shims. Some engines, including Buick V-6s and V-8s and many Chrysler V-8s, use shaft-mounted rocker arms. With this arrangement, the shaft mounting bolts are torqued to a designated specification. If the valve train is noisy, there is a wear issue or mechanical problem.
A hydraulic valve lifter offers several advantages,
- No more maintenance valve adjustments
- Quieter operation due to having no lash
- Longer valve and cam life due to eliminating the pounding of a solid lifter
- Smoother engine operation due to precise valve control and automatic compensation for expansion and contraction of the valvetrain parts caused by temperature change
To understand how the hydraulic lifter is able to accommodate valvetrain slack while maintaining zero lash, we should look at its inner workings. When the valve is closed, the plunger spring in the hydraulic lifter takes up all clearance in the valvetrain. Oil enters the lifter body through feed holes and flows inside to the plunger. The oil continues to flow down through the hole in the bottom of the plunger, around the check valve and through the holes in the check-valve retainer to completely fill the cavity below.
As the lifter begins to ride up the cam lobe, the oil below the plunger tries to escape past the check valve. This sudden flow of oil forces the check valve to seat, which seals the hole at the bottom of the plunger. The full load of the valvetrain is on the lifter. The lifter now acts almost as if it were a solid design.
A predetermined and closely held clearance between the lifter’s plunger and its body permits a minute amount of oil to escape from below, moving past the plunger. This movement of the plunger with respect to the lifter body after the check valve is seated is called leak down or bleed-down; it consists of the oil draining out.
As the lifter returns to the base circle of the camshaft, oil fills the high-pressure cavity and the cycle begins again. When the engine temperature change requires shortening the lifter’s effective length, a hydraulic lifter automatically compensates: the valve spring forces the plunger down. The return spring raises the plunger when lengthening of the lifter is required . This causes more oil to flow into the spring cavity.
One of the disadvantages of a hydraulic valve lifter is that it cannot follow as aggressive a cam profile as a mechanical design, thus limiting the engine’s power and operating speed. In addition to the cam profile being milder, a hydraulic valve lifter requires a certain amount of time to respond to changes in the engine; this limits engine power when compared to a mechanical design.
Another potential drawback of hydraulic lifters is that at excessively high engine speeds, valvetrain inertia may open the valves further than intended. This results in additional valve train clearance. A hydraulic lifter senses this clearance; the plunger begins to lengthen, and may actually extend far enough to prevent the valve from closing. Valve-to-piston collision and a ruined engine can result.
Understanding the Noise.(valve lash)
Valvetrain mechanism noise is similar to a clicking sound of a sewing machine. The sound frequency of the valve train noise is one-half the crankshaft speed. A clicking lifter is one very common valvetrain noise. This is because the amount of lash or free play is excessive. If the engine has hydraulic valve lifters, however, a number of things could be wrong.
The problem with diagnosing and correcting valve train noise is that there are many components that can cause undesirable sounds. Not every noisy valve train is suffering from improper adjustment. The engine may still be noisy after adjusting valves. If this is the case, then the problem is not with the lash; instead, a mechanical failure due to wear is probably causing the sound. Though there are many possible causes, the valve lifter is often the source of the noise.
Other things that can cause valve train noise are,
- Sticky valves
- Weak springs
- Excessive revving of the engine
In any hydraulic application, another cause of valvetrain noise can be the result of machining of the cylinder head and valve seats, which changes the rocker-arm geometry.
Many mechanics think that this is caused by faulty hydraulic lifters; that is incorrect.
The lifter is only responding to the conditions and trying to adjust for them. That’s why it is critical to make sure that the valvetrain is the culprit when chasing any excessive noise before it becomes a major issue.
It is important to remember that several different conditions can produce a noise identical to that caused by a failed or failing valve lifter. Some of these are,
- Lack of lubrication between the rocker arm and the pushrod
- Lack of lubrication between the rocker arm and the end of the valve stem
- Excessive valve guide-to-valve stem clearance
- Badly worn valve tips and/or rocker arm pads
- Worn camshaft lobes
- A loose valve seat on heads with inserts
- A warped valve
- Excessive clearance between the lifter and the lifter bore hole in the block
- A loose rocker-arm shaft
- A broken valve spring
If the engine noise is not caused by one of the listed issues then it is coming from a failed lifter.
You will need to find the bad lifter. A simple method is to use a piece of garden hose. Remove the valve covers. With the engine running, place one end of the hose near the spring retainer of each intake and exhaust valve; put the other end of the hose to your ear.
It will be very apparent which is the offending valve lifter.
Another method is with the engine off push down on each of the rocker arms on the pushrod side. If the rocker arm is free to move or there is a spongy feeling, it is a good indicator that the valve lifter is leaking down too fast or not retaining oil from the engine.
Once you locate the lifter that is causing the noise, you will need to determine the reason for its malfunction.
There are four types of noise that can be caused by an inoperative or failing valve lifter,(valve lash)
Loud, rapping sound.
This can be caused by the plunger being stuck in the body, usually due to excessive varnish between the plunger and body, or by dirt or other foreign materials wedged between the plunger and body. Another cause of a loud rapping sound is an excessively worn base or foot on the lifter itself.
Moderate clicking noise.
This can be the result of varnish or a worn lifter bottom. The noise level depends on the amount of varnish and the degree of wear. Two other causes of a moderate clicking noise are excessively fast or slow leakdown. Slow leakdown generally will cause the engine to be noisy only when cold and the oil is thick. With fast leak down, the valvetrain will be noisy when the engine is warm. Fast leakdown will also occur if the ball-check in the lifter fails to seal.
This type of noise is hard to locate by its very nature: There will be a few clicks, and then it will be quiet, but the noise will reappear after a short period of time. The usual cause of intermittent clicking is a very minute piece of dirt that holds the ball check off the seat for a few seconds and then passes through.
General valvetrain noise.
When the sound is throughout the entire valvetrain, the cause is usually the weight of the oil or the oil supply itself. Too much oil in the crankcase will cause foaming and aeration. When air gets into the lifters, they will fail to operate properly.
Insufficient oil supply to the lifters can also cause general valvetrain noise. This could be the result of too little oil in the crankcase, an oil pump not operating properly, or clogged main oil gallery lines.
Excessively thick engine oil will cause entire valvetrain noise when cold-starting an engine. The noise will diminish when the oil warms up and starts to flow properly. An engine with thick oil will be more prone to noise in colder weather than during the summer months.
Steps to making it Right.(valve lash)
The first step in fixing valvetrain noise and creating the correct valve events is recognizing that valvetrain parts are consumables. All too often, I hear of someone rebuilding an engine but reusing all of the rocker arms, lock nuts, valve springs, pushrods and other components. Even if you’re only trying to quiet the valvetrain, not rebuild your engine, you need to examine every component. You can only properly adjust the valve train after removing and closely scrutinizing each of the rocker arms, pushrods and valve stems (the cylinder heads do not need to come off at this point). If the lash on one valve seems excessively loose or tight, there is something wrong.
A very loose valve may indicate an excessively worn rocker arm, bent pushrod, mushroomed valve stem or the like. In contrast, an excessively tight valve on one cylinder may mean that valve recession is occurring–the valve is starting to pull through the cylinder head. Service the heads or you may drop a valve. The best approach when adjusting the valves on any engine is to remove the valve train parts to study them while identifying their location. You want each part going back on where it came from. This is due to wear patterns.
My suggestion is that if the engine has been in service for many years, it is best to install new lifters, rocker arms, pushrods and lock nuts. These are inexpensive parts, and you shouldn’t hesitate to replace them. This way, everything is new when you perform the final adjustment and the valvetrain should be nice and quiet, and not pose a threat to the engine. Some engines use rocker-shafts or a bridge between the valves (like on some Oldsmobiles).
You may be required to install shims to get the preload correct. As always, it is best to familiarize yourself with the proper steps from a factory shop manual before starting the job. There are many different thoughts on how to position the valves when adjusting an engine with rocker studs, such as a Chevrolet. Some manuals tell you to mark the balancer and then jump around to different cylinders that have the lifter on the base circle of the cam.
Personally, I find that method burdensome. It is often hard to see the balancer on newer engines, I do not like moving all my tools and my fender cover from side to side, and it gives the lifters a chance to bleed down more than I would like. The method that I use is to use a helper or a jumper switch to bump the starter until the cylinder I am working on has both valves closed. Then I am sure the lifters are on the base circle. Once the lifters are positioned properly, try to twist the rocker arm from side to side to check for play.
If all looks good, loosen the lock nut slightly while trying to twist the pushrod between your fingers. When the push rod starts to turn, stop loosening the nut. Then start tightening the nut while twisting the pushrod between your fingers. When the pushrod will no longer turn, you’ve reached zero lash. Now turn the nut between one-half and one full turn to set the preload. Follow the same procedure on the other valve, and then move on to the rest of the cylinders.