What valve clearance adjustment is and why engines need it

Valve clearance is the small gap between the top of your engine's valve stem and the part that pushes it open. As your engine runs and heats up, metal expands. Without that gap, the expanding valve would stay slightly open even when it should be closed, letting compression escape and burning fuel inefficiently. Adjusting valve clearance means measuring that gap and filing or shimming parts to bring it back to the manufacturer's specification.

Most modern cars with hydraulic valve lifters adjust this gap automatically as the engine warms up, so you may never need to touch them. Older engines, motorcycles, small equipment engines, and some performance vehicles use solid valve lifters that do not self-adjust. If the gap gets too tight, the valve stays open and the engine loses power and runs hot. If the gap gets too wide, the valve train makes noise and the valve does not open fully.

You will typically need a valve clearance check after your engine has been rebuilt, after very high mileage, or if you hear a persistent ticking or tapping from under the hood that gets worse as the engine warms up.

Key Takeaways

  • Valve clearance is the gap between the valve stem and the rocker arm or lifter, and it must stay within the manufacturer's specification or the engine will not run efficiently.
  • Engines with hydraulic lifters adjust this gap on their own; engines with solid lifters require manual adjustment.
  • Adjustment requires the engine to be cold, the valve cover removed, and the engine turned to specific positions so each valve can be measured and corrected in order.
  • You will need a feeler gauge to measure the gap, and either a wrench to turn an adjustment screw or shims to replace, depending on your engine design.
  • If you are not confident working inside an engine, a mechanic can complete this job in one to three hours depending on how many valves need adjustment.

How to tell if your valves need adjustment

The most common sign is a ticking or tapping noise from under the valve cover that gets louder as the engine warms up and the metal expands further. The noise usually comes from the top of the engine and sounds like a pencil tapping on wood. It may disappear briefly when you first start the cold engine, then return as temperature rises.

A second sign is loss of power or rough idle that does not respond to a tune-up. If valves are too tight, compression leaks past them and the cylinder does not fire properly. If valves are too loose, they do not open far enough to let the fuel mixture in or the exhaust out.

The most reliable way to know is to check the service history. If the manufacturer recommends valve clearance inspection at a certain mileage interval and you have passed that point, the job is due regardless of whether you hear noise. Many engines run silently with incorrect clearance until something fails.

What you need before you start

Gather the manufacturer's specification for your engine first. This will be in your owner's manual or service manual, listed as a measurement in thousandths of an inch or millimeters. For example, intake valves might be 0.010 inch and exhaust valves 0.012 inch. You need both numbers because they are usually different.

You will need a feeler gauge set, which is a fan of thin metal blades each marked with its thickness. You slide the correct blade into the gap; it should fit snugly with slight resistance. A set costs ten to twenty dollars at any auto parts store.

You will also need the correct wrench or socket for your engine's adjustment mechanism. Some engines use a locknut and adjustment screw that you turn with a wrench. Others use shims — small metal discs that sit on top of the valve stem — that must be removed and replaced with a different thickness. Your service manual will show which type yours is and what size wrench you need.

Finally, you need the engine cold. Valve clearance must be measured and adjusted when the engine has not run for at least several hours, because heat expands the metal and changes the gap. Many people do this job first thing in the morning or after the engine has sat overnight.

The step-by-step adjustment process

Start by removing the valve cover — the large metal or plastic lid on top of the engine. Unbolt it carefully and set the bolts aside in order so you can reinstall them in the same holes. You may need to disconnect hoses or wires attached to the cover.

Next, turn the engine by hand using a wrench on the crankshaft bolt until the first cylinder is at top dead center on the compression stroke. Your service manual will show you how to find this position; usually you align a mark on the crankshaft pulley with a mark on the engine block. At this position, both valves for that cylinder are closed and you can measure their clearance.

Slide the correct feeler gauge blade into the gap between the valve stem and the rocker arm or lifter. It should slide in with light resistance — not so loose it falls through, not so tight you have to force it. If the gap is too tight, you will need to loosen the locknut and turn the adjustment screw counterclockwise to open the gap. If it is too loose, turn the screw clockwise to close it. After each adjustment, recheck with the feeler gauge.

Once both valves for that cylinder are correct, rotate the engine to the next cylinder's top dead center position and repeat. Your service manual will show the firing order and how many degrees to rotate between cylinders. Continue until all cylinders are done, then reinstall the valve cover with a new gasket if the old one is cracked or flattened.

When shims are used instead of adjustment screws

Some engines, particularly many motorcycles and some imports, do not have an adjustment screw. Instead, they use a shim — a thin disc that sits on top of the valve stem under the rocker arm. To adjust clearance, you must remove the rocker arm, take out the old shim, measure how much thicker or thinner the new shim needs to be, and install a shim of the correct thickness.

This process requires a shim removal tool (a small lever that pops the shim out) and access to a set of replacement shims in various thicknesses. Your service manual will show the calculation: if your clearance is 0.015 inch and it should be 0.010 inch, you need a shim that is 0.005 inch thicker. Shim kits for common engines cost thirty to sixty dollars and include a range of sizes.

Shim adjustment takes longer than screw adjustment because you must remove and reinstall the rocker arm for each valve. If you are not comfortable with this level of disassembly, a mechanic is the better choice.

Common mistakes and how to avoid them

The most common mistake is adjusting valves on a warm engine. Metal expands as it heats, so the gap will be larger than it actually is when cold. You will adjust it too tight, and when the engine cools, the gap will be too small. Always wait until the engine is completely cold.

A second mistake is rotating the engine to the wrong position. If you turn it to the wrong cylinder or do not have both valves closed, you will measure and adjust the wrong valve or get an incorrect reading. Use your service manual's diagram and count carefully.

A third mistake is over-tightening the locknut after adjustment. If you turn it too hard, you will move the adjustment screw and change the clearance you just set. Tighten it snugly, then recheck the clearance one more time before moving to the next valve.

When to do this yourself versus calling a mechanic

Valve clearance adjustment is a job you can do yourself if you are comfortable removing the valve cover, reading a service manual, and working carefully with small measurements. It typically takes two to four hours for a four-cylinder engine and longer for six or eight cylinders. The cost of tools is low — a feeler gauge set and any special wrenches you do not already own will total under fifty dollars for most engines.

You should call a mechanic if your engine uses shims and you do not have the shim kit and removal tool, if you are not confident rotating the engine to the correct positions, or if you have never worked inside an engine before. A mechanic will charge fifty to one hundred fifty dollars in labor depending on how many valves need adjustment and how long it takes to access them. The job usually takes one to three hours at a shop.

If you are unsure whether your engine needs adjustment, a mechanic can inspect the clearance and tell you whether it is within specification. This inspection alone costs twenty to fifty dollars and takes thirty minutes, and it may save you from doing unnecessary work.

Frequently Asked Questions

Can I adjust valves while the engine is warm?

No. Metal expands when hot, so the gap will measure larger than it actually is when the engine cools down. You will adjust it too tight, and the valve will stay slightly open when cold, causing loss of power and rough running. Always wait at least several hours after the engine last ran, or do the job first thing in the morning.

What happens if I adjust the valves too tight?

The valve will not close completely, allowing compression to escape and exhaust to leak back into the cylinder. The engine will lose power, run rough, and overheat. You may also hear a hissing sound from the cylinder head. Loosen the adjustment screw slightly and recheck with the feeler gauge.

What happens if I adjust the valves too loose?

The valve will not open far enough, restricting the fuel mixture entering the cylinder and the exhaust leaving it. The engine will lose power and you will hear a loud ticking or tapping noise from the valve cover. Tighten the adjustment screw and recheck.

Do I need to adjust valves on a newer car?

Most cars built after the 1980s use hydraulic valve lifters that adjust clearance automatically as the engine warms up. Check your owner's manual to see if valve clearance inspection is listed in the maintenance schedule. If it is not mentioned, your car almost certainly does not need manual adjustment.

Can I damage the engine by adjusting valves incorrectly?

Incorrect adjustment will cause poor running, noise, and loss of power, but it will not usually cause when ready engine damage. However, if valves are too tight and stay open, the engine will overheat and run rough for long periods, which can damage the cylinder head or pistons over time. If you are unsure of your adjustment, have a mechanic check it before driving the car.