What tappet clearances are and why they matter

Tappet clearance is the small gap between the tip of your valve lifter (the tappet) and the top of the valve stem. This gap lets your valves open and close fully as your engine runs. If the clearance is too tight, your valves won't close completely and you'll lose compression and power. If it's too loose, your valves won't open far enough and your engine will run rough, especially when cold.

Most engines need this clearance reset every 20,000 to 40,000 miles, though some modern engines with hydraulic lifters adjust themselves. Older engines with solid lifters — common in cars built before the 1980s and in many small engines — need manual adjustment. The job takes 30 minutes to two hours depending on how many cylinders you have and how accessible your valve cover is.

Key Takeaways

  • Tappet clearance is the gap between the valve lifter and valve stem, and it must be set to your engine's exact specification or your valves won't open and close properly.
  • You need your engine's service manual to find the correct clearance measurement for each valve, because different engines and different valve positions require different gaps.
  • The engine must be cold and at a specific position in its rotation cycle (usually with one piston at top dead center) before you measure or adjust any clearance.
  • You adjust clearance by loosening the locknut on the valve adjuster screw, turning the screw to open or close the gap, then tightening the locknut while holding the screw steady.
  • After adjustment, you must rotate the engine through a full cycle and recheck every clearance, because tightening one adjuster can shift others slightly.

Gather your tools and the correct specification

Before you touch anything, you need two things: the exact clearance measurement for your engine, and the right tools to measure and adjust it. Open your engine's service manual and find the valve clearance specification — it will list a measurement in thousandths of an inch (like 0.010") or millimeters (like 0.25mm), and it may differ between intake and exhaust valves. Write this down. If you don't have the manual, search online for "[your engine model] valve clearance specification" — most manufacturers publish this data.

Gather a feeler gauge set (a set of thin metal blades in graduated thicknesses that measure gaps), a wrench or socket to fit your valve adjuster locknut, a screwdriver to turn the adjuster screw, and a socket wrench to rotate your engine by the crankshaft bolt. You'll also need a clean rag and possibly a gasket scraper to remove old gasket material from the valve cover. Have the engine cold — let it sit for at least an hour after running, because metal expands when hot and will throw off your measurements.

Remove the valve cover and locate the valves

Unbolt the valve cover — usually four to eight bolts around the perimeter. Lift it straight up; if it sticks, tap gently with a rubber mallet rather than prying, which can bend the cover. Scrape away old gasket material from the cylinder head surface with a gasket scraper or old credit card, then wipe the surface clean with a rag. Do not let debris fall into the engine.

Look at the top of the cylinder head. You'll see a row of valve stems (the thin metal rods sticking up) with adjusters on top — each adjuster is a locknut with a screw through it. Count the valves and identify which are intake (usually on one side) and which are exhaust (usually on the other), using your service manual as a guide. Some engines label them; others don't. Write down the order so you don't lose track.

Position the engine and identify which valves to adjust

Valve clearance must be checked and adjusted with the engine at a specific point in its rotation. Most engines require you to set clearance when one piston is at top dead center (TDC) on the compression stroke — the highest point of its travel. At this position, both valves on that cylinder are fully closed and the lifters are at rest.

Use your socket wrench on the crankshaft bolt (at the front of the engine) to rotate the engine slowly by hand. Watch the valves as you turn: when both valves on a cylinder are fully closed (the rocker arms or lifters are not pushing down on either stem), that cylinder is at or near TDC. Your service manual will show you exactly which valves to adjust at this position — usually you adjust one cylinder, then rotate the engine 90 or 180 degrees and adjust the next, working around in a specific sequence. This is called the "firing order" sequence and it matters because it keeps you from adjusting a valve that's about to open.

Measure and adjust each clearance

With the engine positioned correctly, slide your feeler gauge blade of the correct thickness between the valve stem tip and the adjuster screw. The blade should slide in with light resistance — not so tight it bends, not so loose it falls through. If the gap is too tight, the blade won't fit. If it's too loose, the blade rattles.

If the clearance is wrong, loosen the locknut on the adjuster with your wrench — turn it counterclockwise about half a turn. Hold the locknut loose and turn the adjuster screw with your screwdriver: turn it clockwise to tighten the gap (make it smaller) or counterclockwise to loosen the gap (make it larger). Make small turns, a quarter turn at a time. After each turn, test with the feeler gauge again. Once the blade slides in with the right resistance, hold the adjuster screw steady with your screwdriver and tighten the locknut firmly with your wrench. The screw will want to turn as you tighten the nut, so keep pressure on the screwdriver to hold it in place.

After you tighten the locknut, test the clearance one more time with the feeler gauge. Sometimes tightening the nut shifts the screw slightly. If it's now too tight, loosen the nut again and adjust.

Work through all cylinders in the correct sequence

Once you've adjusted the first cylinder, rotate the engine to the next position in the firing order sequence (your manual will tell you how many degrees — usually 90 or 180). Adjust those valves, then rotate again. Continue until you've adjusted every valve on every cylinder. This usually means four rotations for a four-cylinder engine, three for a three-cylinder, and so on.

After you finish the last cylinder, rotate the engine one full turn (360 degrees) to return to your starting position. Now go back and recheck every single clearance you adjusted. Tightening one locknut can sometimes shift the adjuster screw slightly, and you want to catch any that have drifted. If any are out of spec, loosen the nut and adjust again.

Reinstall the valve cover and test

Once all clearances are correct and rechecked, clean the valve cover gasket surface one more time with a rag. Install a new gasket (reusing old gaskets often causes leaks) and bolt the valve cover back down. Tighten the bolts in a crisscross pattern — if there are four bolts, tighten top-left, then bottom-right, then top-right, then bottom-left — so the cover seats evenly. Don't overtighten; you only need enough pressure to compress the gasket.

Start the engine and listen. It should run smoothly without ticking or knocking from the valve area. If you hear a loud ticking that wasn't there before, stop the engine when ready and recheck your clearances — you may have set one too loose. If everything sounds normal, let the engine idle for a minute, then shut it off and check for oil leaks around the valve cover. If you see seeping, the gasket may not be seated; let the engine cool and retighten the bolts slightly.

Frequently Asked Questions

What happens if I set the clearance too tight?

The valve won't close completely, so compression leaks out and the cylinder loses power. You'll also hear a hissing sound from that cylinder and the engine may overheat because the valve stays partially open and doesn't seal heat in the combustion chamber. Over time, the valve will burn and fail.

What happens if I set the clearance too loose?

The valve won't open far enough, so less fuel and air enters the cylinder and less exhaust leaves. The engine will run rough, especially when cold, and you'll hear a loud ticking or knocking from the valve area. Power will drop noticeably.

Do I need to do this on a modern car?

Most cars built after 1990 have hydraulic valve lifters that adjust themselves automatically, so you don't need to set clearance manually. Older cars, motorcycles, small engines like lawnmowers and generators, and some trucks still use solid lifters and need manual adjustment. Check your service manual to be sure.

Can I adjust clearance with the engine warm?

No. Metal expands when hot, so a warm engine will give you false measurements. You must wait until the engine is completely cold — at least an hour after it last ran — or your clearances will be too tight once the engine cools down and the metal contracts.

What if I can't find the firing order sequence for my engine?

Search online for "[your engine model] firing order" — this information is published for nearly every engine ever made. If you can't find it, you can also rotate the engine slowly and adjust each valve as you come to it, but you risk adjusting a valve that's about to open, which gives you a false reading. The firing order sequence prevents this.