Setting the ignition timing with a degree wheel and test lamp

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

The ignition timing determines when the spark jumps in relation to the piston position. A few degrees too early and the engine pinks and runs hot. A few degrees too late and it will not pull and uses more fuel. With a degree wheel and a test lamp you measure the opening point of the contact breaker exactly in degrees before top dead centre, without a dial gauge and without any conversion.

This guide shows you how to find top dead centre reliably, how to fit the degree wheel, how to determine the opening point with the test lamp and how the relationship between degrees and millimetres works in principle.

Interactive degree wheel: drag it, detents every 5 degrees.

What ignition timing means

On a contact breaker ignition the spark is produced at the moment the breaker contact opens. The current flow through the primary winding is cut, the magnetic field collapses, and the high voltage is generated in the secondary winding. The ignition timing is therefore the opening point of the contact, given as a crank angle before top dead centre (TDC) or as piston travel in millimetres before TDC.

Two quantities influence the opening point: the position of the backing plate and the contact gap. A larger gap makes the contact open earlier, a smaller one later. That is why you always set the contact gap first and only then the ignition timing. How to measure the gap with a feeler gauge is covered in the guide Using a feeler gauge properly.

Finding top dead centre

Top dead centre is the reference point for everything that follows. If you only find it roughly, the whole measurement is rough. The problem: at the reversal point the piston barely moves over several degrees of crank angle. If you feel for the highest point with a screwdriver in the plug hole, you can easily be several degrees out.

The reliable method uses two symmetrical points. You choose a piston position well before TDC, for example with a dial gauge in the plug hole or a piston stop, and read off the value on the degree wheel. Then you turn the crankshaft the other way until the piston reaches the same position on the other side, and read off again. TDC lies exactly halfway between the two values. This method does not depend on how flat the piston movement is at the reversal point.

If you have a dial gauge, also look for the highest point with it and compare that with the midpoint from the two point method. If the two agree, the wheel is set correctly.

Fitting the degree wheel

The degree wheel with a 120 mm diameter has a 6 mm bore and therefore fits many shaft ends or a suitable auxiliary bolt in the flywheel nut. The large diameter is no accident: at 120 mm the degree marks at the rim are far enough apart to read off a single degree cleanly. A smaller wheel would be less accurate.

The wheel has to turn with the crankshaft without slipping. Depending on the engine you clamp it under the flywheel nut, fix it with a bolt in the thread of the shaft end or secure it with a suitable sleeve. All that matters is that it runs true and does not shift as you turn it.

The pointer is a piece of wire about 2 mm thick. Clamp it under a casing screw and bend the tip close to the rim of the wheel. The nearer the pointer sits to the scale, the more accurately you read it. The German version of the degree wheel has the same dimensions and the same graduation.

The opening point with the test lamp

The 6 to 24 volt test lamp shows you when the contact opens. How it is wired depends on the ignition system.

• Battery ignition: Switch the ignition on, put the test probe on the contact breaker terminal and the earth clamp on the casing. While the contact is closed the lamp is shorted out and stays dark. When the contact opens, it lights up.
• Flywheel magneto ignition without a battery: Disconnect the cable from the ignition coil to the contact breaker. Connect a small battery in series with the lamp between the contact breaker terminal and earth. With the contact closed, current flows and the lamp lights. When the contact opens, it goes out.

In both cases the moment the lamp changes state is the opening point. Turn the crankshaft slowly in the normal direction of rotation and watch the lamp. Never turn back to find the point again. The play in the bearings and the wheel would otherwise falsify the reading. Instead go a full turn further and approach it again in the direction of rotation.

Step by step: setting the ignition timing

1. Check the contact gap

   Turn the crankshaft until the contact breaker is fully open and measure the gap with the feeler gauge. Set it to the value given in the workshop manual for your vehicle. Only then do you move on to the ignition timing.

2. Fit the degree wheel and the pointer

   Put the wheel centrally on the shaft end, clamp the pointer under a casing screw and bend it close to the scale. Check that the wheel does not move as you turn it.

3. Determine TDC and zero the wheel

   Use the two point method from the section above. Turn the wheel so that the pointer reads 0 at TDC. From now on the wheel must not move on the shaft.

4. Connect the test lamp

   Connect the lamp to suit your ignition system. Check that it works by turning the shaft through one full revolution. The lamp has to change state exactly once.

5. Read off the opening point

   Turn the shaft slowly in the direction of rotation until the lamp changes state. Read the value at the pointer. That is your current ignition timing in degrees before TDC.

6. Adjust the backing plate

   If the value differs from the specified figure, slacken the backing plate screws slightly and turn the plate. Turning it against the flywheel direction of rotation advances the ignition, turning it with the direction of rotation retards it. Tighten the screws and repeat the measurement.

7. Check it again

   Measure once more after tightening. The backing plate likes to shift a little as the screws are tightened. Only when two measurements in a row give the same value are you finished.

Degrees and millimetres: the principle behind the conversion

Many manuals give the ignition timing in millimetres before TDC, others in degrees. Both describe the same point, just on different scales. The relationship depends on two engine dimensions: the stroke and the connecting rod length. That is why there is no general conversion table that holds good for all engines.

The principle: the crank radius r is half the stroke. At a crank angle a before TDC the piston sits a distance s below TDC. That distance is made up of two parts. The first is the crank component, r times (1 minus cos a). The second is the component from the connecting rod, which tilts with the angle: rod length l minus the square root of (l squared minus r squared times sin a squared). Adding the two parts gives the piston travel s.

The formula shows why figures in millimetres close to TDC are awkward. Around dead centre the piston travel changes only minimally per degree. A measuring error of a tenth of a millimetre can amount to several degrees there. The degree wheel measures the angle directly and is independent of the piston movement. That is why it is the more accurate method in the workshop. You will find the stroke and connecting rod length of your engine in the workshop manual.

Common mistakes and how to avoid them

• Wheel has slipped: After zeroing, turn the shaft through one revolution and check that the pointer reads 0 again.
• Pointer too far away: If the pointer stands a few millimetres above the scale, you read a different value depending on your viewing angle. Bend the tip close to the rim.
• Turned backwards: Always approach the point in the direction of rotation only.
• Contact gap changed afterwards: Every change at the contact shifts the ignition timing. Keep to the order of work.
• Worn cam or dry felt pad: A dry lubricating felt lets the rubbing block of the contact breaker wear quickly. The contact gap drops and the ignition timing wanders towards retard.