What You Need to Adjust Valves
Valve adjustment requires a small set of hand tools, most of which you may already own. The core tools are a socket wrench or open-end wrench (sized to fit your valve's locknut), a feeler gauge (a thin metal blade that measures the gap between parts), and sometimes a screwdriver — either flathead or Phillips depending on your valve design. Some valves also need an adjustable wrench to hold the valve body steady while you turn the adjustment nut.
The specific sizes depend on your equipment. A car engine valve typically uses a 10mm or 13mm wrench, while a small engine (lawn mower, generator) might use 8mm or 10mm. Industrial valves vary widely. Before you buy anything, check your equipment's manual or look up the exact model — this tells you the precise wrench size and the correct gap measurement you're aiming for.
You'll also need a clean, well-lit workspace and a way to find the equipment so it doesn't move while you work. A vise, clamp, or even a sturdy table with the engine block held down works. Never try to adjust a valve on something that can shift or tip.
Key Takeaways
- Valve adjustment requires a feeler gauge, a wrench that fits your locknut, and often a second wrench to hold the valve body still.
- The correct gap size is specific to your engine or equipment model and is listed in the manual or online documentation.
- You must measure the gap with the valve in a specific position — usually fully closed — so the measurement is accurate.
- Loosening the locknut, adjusting the screw or nut underneath, and retightening in the correct order prevents the adjustment from shifting.
Understanding Valve Gaps and Why They Matter
A valve gap is the space between the valve stem (the rod that moves up and down) and the rocker arm or cam follower that pushes it. This gap must be precise. If it's too small, the valve won't close completely and compression leaks out. If it's too large, the valve won't open fully and the engine runs poorly.
The gap is measured in thousandths of an inch (0.001 to 0.015 inches, depending on the engine). This is why a feeler gauge is essential — a regular ruler cannot measure something this small. The feeler gauge is a set of thin metal blades, each marked with its thickness. You slide the correct blade into the gap; it should fit snugly with slight resistance, not loose and not forced.
Different engines have different gap requirements. A small lawn mower engine might need 0.010 inches, while a car engine might need 0.012 inches. Always check your manual first. Running an engine with an incorrect gap will cause rough idling, poor fuel economy, hard starting, or valve damage over time.
The Basic Steps to Adjust a Valve
Start by positioning the valve so it is fully closed. For most engines, this means rotating the crankshaft (the main shaft that turns) until the valve you're adjusting is at the bottom of its stroke. You can do this by hand on small engines or with a wrench on larger ones. Check your manual for the exact position — some engines require the piston to be at top dead center (TDC), others at a different point.
Once the valve is in the correct position, insert the feeler gauge blade (the thickness specified in your manual) into the gap between the valve stem and the rocker arm. The blade should slide in with light resistance — not fall through, not require force. If the gap is wrong, you'll adjust it next.
Locate the locknut (a nut that holds the adjustment screw in place) and the adjustment screw or adjustment nut underneath it. Use one wrench to hold the adjustment screw steady while you loosen the locknut with a second wrench. Turn the locknut counterclockwise until it is loose but still threaded on.
Now turn the adjustment screw or nut to change the gap. Turning it clockwise (in) makes the gap smaller; counterclockwise (out) makes it larger. Make small turns — a quarter turn at a time — and recheck the gap with your feeler gauge after each adjustment. Once the gap is correct, hold the adjustment screw steady with one wrench and tighten the locknut with the other. Recheck the gap one final time to confirm it didn't shift when you tightened.
Tools for Different Valve Types
Overhead cam engines (where the camshaft sits directly above the valves) often use a shim or bucket adjuster instead of a screw. These require a different approach: you remove a small metal disc (the shim) and replace it with a thicker or thinner one to change the gap. This requires a shim removal tool, which looks like a small lever or hook, and a set of replacement shims in different thicknesses. Your manual will tell you which shim size you need based on your current gap measurement.
Hydraulic valve adjusters (common in newer engines) adjust themselves automatically and usually don't require manual adjustment. If your manual says "hydraulic lifters — no adjustment needed," you can skip this step entirely.
Industrial or large equipment valves may use a different design altogether — perhaps a ball valve with a handle, a gate valve with a wheel, or a needle valve with a fine screw. These don't have gaps to measure; instead, you turn them until they stop (gently, without forcing) or to a marked position. Always consult the equipment manual for the correct procedure.
Common Mistakes to Avoid
The most common error is adjusting the valve while it's in the wrong position. If the valve is partially open, your gap measurement will be wrong, and you'll adjust it incorrectly. Always rotate the engine to the exact position specified in the manual before you measure or adjust anything.
Another frequent mistake is forgetting to retighten the locknut or tightening it too much. If you don't tighten it, the adjustment screw will vibrate loose and the gap will change. If you tighten it too hard, you can strip the threads or bend the adjustment screw. Tighten firmly but not with all your strength — the locknut should be snug, not crushed.
Using the wrong size wrench is also common and can strip the locknut or adjustment nut, making it impossible to turn. Measure the nut first or check your manual for the exact size. A wrench that fits properly should slide on with no wiggle and no forcing.
Finally, don't skip the final recheck. The gap can shift slightly when you tighten the locknut, especially if you're new to this. Checking one more time takes 30 seconds and prevents a wasted trip back to the engine.
When to Call a Professional
If your engine has a timing belt or chain that must be removed to access the valves, this job is beyond basic adjustment and requires professional service. Timing belts are complex and expensive to replace if damaged.
If your feeler gauge doesn't fit into the gap at all, or if the adjustment screw won't turn, stop and consult a mechanic. The valve may be stuck, damaged, or the engine may have a different design than you expected. Forcing it can cause expensive damage.
If you're working on equipment you've never serviced before and the manual is unclear, it's worth paying a professional for the first adjustment so you can watch and learn the correct procedure for that specific model.
Frequently Asked Questions
What size feeler gauge do I need?
The size depends entirely on your engine or equipment model. Check the manual or the manufacturer's website for the exact gap measurement. Most car engines use 0.010 to 0.015 inches; small engines use 0.008 to 0.012 inches. Buy a feeler gauge set (usually 26 blades for $10 to $20) so you have the right size on hand.
Can I adjust valves while the engine is running?
No. The engine must be completely cold and off. A running engine is dangerous, and you cannot measure the gap accurately on a moving valve. Wait at least 30 minutes after the engine shuts down so it cools completely.
How often do valves need adjustment?
This varies by engine type and design. Some engines need adjustment every 10,000 to 20,000 miles; others never need it. Check your manual for the recommended interval. Many modern engines have hydraulic adjusters that don't require manual adjustment at all.
What happens if I adjust the gap wrong?
A gap that's too small causes the valve to stay slightly open, leading to compression loss, rough idling, and poor fuel economy. A gap that's too large prevents the valve from opening fully, reducing power and causing hard starting. Either way, the engine will run poorly until you correct it. Recheck your measurement and adjust again if needed.
Do I need two wrenches to adjust a valve?
Usually yes. One wrench holds the adjustment screw or nut steady while the other loosens or tightens the locknut. This prevents the adjustment from shifting. Some designs use a single wrench if the adjustment screw has a flat head you can hold with a screwdriver, but two wrenches is the standard and most reliable method.