Using a feeler gauge properly: contact gap, air gap, valve clearance

Workshop guideUpdated 2026-09-13ELDI / Racing Planet editorial team

The feeler gauge is the simplest measuring tool in the workshop and still the one that produces the most mistakes. A thin steel blade with the size stamped on it, pushed into a gap. Whether the result is right is decided not by the blade, but by your hand.

Here you will learn how the gauge is built, the right feel when measuring, how to combine blades and the four classic jobs on a moped or motorcycle: electrode gap, contact breaker points, air gap at the coils and valve clearance on a four-stroke.

Construction and versions

A feeler gauge consists of hardened, ground steel blades of a defined thickness. The size is stamped on every blade. There are two designs, and in the workshop they complement each other.

The single 0.40 mm blade is fixed to one value. It fits into the tight space under the flywheel and into the backing plate, where a fan of blades would be in the way. For ignition and lighting coils on Hercules, Zündapp and Kreidler, 0.40 mm is the setting dimension, and for the contact breaker gap it is the figure the manual gives on many ignition systems as well.

The fan with 6 blades covers a range. The blades are held at one end in a holder and can be swung out individually. With it you measure electrode gaps, valve clearances and every gap whose specified value you do not already know. Both versions belong in the toolbox.

The right feel when measuring

A blade of the right thickness slides through the gap with a light, even resistance. Workshop people call that a sliding fit. If the blade slips through with no resistance at all, the gap is larger than the size. If you have to push, or the blade bends, the gap is smaller.

Hold the blade flat and parallel to the two faces. If you tilt it, you measure diagonally and read too small. On spring loaded gaps such as the contact breaker points, the blade must not push the spring open. So feed it in without pressure and draw it slowly back out. The resistance as you pull it out is the measurement.

If you are unsure, work with the go / no-go method. The specified size has to be a sliding fit and the next blade up must no longer go in. Only when both are true is the gap correct.

Combining blades

If an intermediate size is missing, you lay two blades together. The thicknesses add up. Two blades of 0.20 and 0.15 mm give 0.35 mm. Three rules apply when combining them.

• Do not stack more than two blades. Every blade has a tolerance, and the tolerances add up. Dirt and a film of oil between the blades come on top of that.
• Put both blades together clean and dry and guide them flat as one. If they shift against each other, at the edge you are only measuring one blade.
• Put the thinner blade on the outside, where the face is smaller. That way it is less likely to catch as you feed it in.

For the air gap on Bosch systems the single 0.40 mm blade is the quickest. You then have nothing to combine and have a stiff blade of a defined size in your hand in the tight space under the flywheel.

Electrode gap at the spark plug

The electrode gap is given in the workshop manual for your vehicle or on the plug packaging. Push the blade between the centre and earth electrode. On a new plug with a flat earth electrode the flat gauge is accurate enough. On a used plug the centre electrode has a rounded edge, and the flat blade then measures too small. For used plugs a wire gauge is better; for the workshop the flat gauge with new plugs is usually enough.

To adjust it you always bend the earth electrode, never the centre electrode. The centre electrode sits in the insulator, and that breaks under sideways load. Bend in small steps and measure after each one. To open the gap, lever carefully under the earth electrode with the body of the feeler gauge; to close it, tap the electrode lightly on a hard surface.

Contact breaker points

The contact gap is measured with the contact breaker fully open. Turn the crankshaft until the rubbing block of the contact breaker sits on the highest point of the cam. Only then do you push the blade between the contacts.

The contact faces have to be clean and flat. Burnt or pitted contacts measure wrongly because the blade rests on the rim of the crater. Clean the faces with a brass brush or a contact file and finally pull a piece of dry paper through the closed contacts to remove dust. Oily fingers on the blade leave a film on the contact that upsets the ignition.

The contact gap changes the ignition timing. A larger gap makes the contact open earlier. So always set the gap first and then check the ignition timing with a degree wheel and test lamp.

Air gap at the ignition and lighting coil

On a flywheel magneto ignition there is a narrow air gap between the pole shoes of the coils and the rotating flywheel. Bosch quotes at least 0.25 to 0.35 mm for the gap between armature and pole shoe. In practice, Hercules, Zündapp and Kreidler are set with the 0.40 mm blade. Too large and the coil delivers too little voltage. Too small and the flywheel rubs on the pole shoes when there is bearing play or heat.

You measure between the pole shoe and the flywheel with a magnet pole standing exactly over the pole shoe. Without the flywheel you use the 79.8 mm centring ring. It replaces the flywheel as a stop, and you align the coils against the ring with the 0.40 mm blade. The full procedure is described in the guide Setting the ignition coil and lighting coil.

Measuring valve clearance on a four-stroke

1. Engine cold

   Valve clearance is measured with the engine cold, unless the workshop manual says otherwise. Warm components expand and change the clearance.

2. Cylinder at TDC on the compression stroke

   Unscrew the spark plug and turn the crankshaft until the piston is at TDC and both valves are closed. On the compression stroke both rocker arms have clearance and move easily.

3. Insert the blade

   Push the blade with the size specified in the manual between the end of the valve stem and the adjusting screw or rocker arm. It has to be a sliding fit. Check against the next blade up.

4. Adjust

   Slacken the locknut, turn the adjusting screw until the blade slides with a light drag, tighten the locknut. The screw often moves as it is locked. Measure again after tightening.

5. Second valve

   Repeat the same procedure on the second valve. Inlet and exhaust can have different values. Both are given in the manual.

Care and sources of error

A feeler gauge only measures as accurately as its blades are flat. A kinked blade is scrap. Rust makes the blade thicker, a burr on the edges catches in the gap. Wipe the blades after use, oil them very lightly and wipe the oil off again before the next measurement. A film of oil between blade and workpiece falsifies the reading.

Keep the fan closed so that the blades do not bend. The single blade belongs in a sleeve or in the compartment of the onboard toolkit. Suitable tightening figures for the locknuts and screws around the valve gear and the backing plate are in the tightening torque table, and the across flats sizes in the across flats table.