Setting the ignition coil and lighting coil: air gap and centring

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

A weak spark, flickering lights, a flywheel that rubs on the pole shoes: all three faults often have the same cause, a wrong air gap between coil and flywheel. On the Bosch flywheel magneto ignition fitted to Zündapp, Kreidler, Hercules and Puch, the ignition coil and the lighting coil sit on a backing plate and are aligned to the flywheel through slotted holes.

This guide explains how the system works, why the air gap matters so much, how to spot a wrong gap and how to set the coils cleanly with a centring ring and a feeler gauge, even with the flywheel off.

How the Bosch flywheel magneto ignition works

Permanent magnets sit in the flywheel. They run close past the pole shoes of the coils, which are fixed to the backing plate on the engine casing. Each coil has an iron core, the armature, with pole shoes at both ends. When a magnetic pole passes over the pole shoe, the magnetic field in the core changes and induces a voltage in the winding.

The ignition coil feeds the spark plug through the contact breaker. The lighting coil feeds the headlamp, the tail lamp and, depending on the system, the horn. Both work on the same principle and both share the same weak point: the air gap between pole shoe and flywheel. It is the only part of the magnetic circuit that is not iron, and it holds back the magnetic flux more than anything else.

Why the air gap counts

The magnetic resistance of an air gap grows with its length. A gap twice as large as intended lets far less flux through the armature. Less flux means less voltage in the winding. The ignition coil then delivers a weak spark, especially at low revs, when the field changes slowly. The lighting coil delivers too little power for the lamps.

A gap that is too small is just as harmful. The crankshaft has bearing clearance, the flywheel has a small amount of runout, and everything expands in use. With a gap close to zero the flywheel touches the pole shoes. You can tell by the bright rub marks on the pole shoes and by a scraping noise when you turn the engine over.

Bosch specifies at least 0.25 to 0.35 mm for the distance between armature and pole shoe. In the workshop the 0.40 mm blade has become the established setting dimension for Hercules, Zündapp and Kreidler. That keeps you safely above the minimum and leaves a reserve for bearing clearance and heat.

Symptoms of a wrong gap

Symptom	Possible cause	Check
Weak spark, engine hard to start	Ignition coil air gap too large	Measure the gap with the 0.40 mm blade, check the contact breaker gap
Lights weak, barely brighter with revs	Lighting coil air gap too large	Measure the gap at the lighting coil
Scraping noise when turning the engine over	Gap too small, flywheel rubbing	Check the pole shoes for rub marks
Spark cuts out when the engine is warm	Gap at the limit, expansion closes it	Reset the gap, check bearing clearance
Ignition timing cannot be set	Backing plate twisted or coil slipped	Centre the backing plate, realign the coils

Not every weak spark comes from the air gap. A burnt contact breaker point, a faulty capacitor or a loose connection in the plug cap give exactly the same picture. Check the gap first, because it is the quickest thing to measure, then work through the other points.

Centring ring 79.8 mm: aligning the coils without the flywheel

With the flywheel fitted you can only reach the coils through the cutouts in the flywheel. That is enough for measuring, but far too tight for moving the coils. To set them, take the flywheel off and fit the centring and setting ring 79.8 mm. The aluminium ring takes over the job of the flywheel: it sets the inner diameter the pole shoes are aligned to.

The ring rests against the pole shoes with the 0.40 mm blade in between. You press each coil against the ring with the blade, tighten the screws and pull the blade out. All the pole shoes then sit on the same circle, concentric with the crankshaft. When you refit the flywheel, it has the same clearance to the pole shoes all the way round.

Why aluminium? Because it is not magnetic. A steel ring would be pulled in by the pole shoes and could not be positioned cleanly. With the aluminium ring the coils rest against it without any force, and only the blade sets the gap. How to take the flywheel off is covered in the guide pulling the flywheel.

Step by step: setting the air gap

1. Pull the flywheel

   Spark plug out, undo the flywheel nut and pull the flywheel with the flywheel puller M22 / M26. Secure the Woodruff key. Mark the position of the backing plate and casing beforehand.

2. Check the backing plate

   The backing plate must sit tight and flat in the casing. Check the screws and the mark. A backing plate that turns with the flywheel nut as you tighten it will shift the ignition timing.

3. Clean the coils

   Clean the pole shoes with brake cleaner. Rust and dirt on the pole shoes act like an extra air gap. Take light rust off with fine abrasive fleece, not with a file.

4. Slacken the coil screws

   Slacken the screws in the slotted holes of the coils only far enough for the coil to slide under light pressure. Coils that are completely loose will tilt as you tighten them.

5. Fit the centring ring

   Place the ring against the pole shoes. It must sit concentric with the crankshaft and must not tilt. Check all the way round that it beds evenly everywhere.

6. Set the gap

   Slide the 0.40 mm blade between pole shoe and ring, press the coil against the ring with the blade, tighten the screws. Pull the blade out. It must come out with a slight drag. Repeat at every pole shoe.

7. Fit the flywheel and check

   Take the ring off, flywheel onto the taper, tighten the nut to the torque given in the workshop manual. Turn the crankshaft over and listen for rubbing noises. Check again through the cutouts with the blade.

Checking with a test lamp and a test run

Once the gap is set, check the ignition timing. Moving the coils does not change it directly, but something may have shifted while the backing plate was loose or while the flywheel nut was tightened. With the test lamp 6 to 24 volts you find the opening point of the contact breaker, and the degree wheel shows the angle. The procedure is described in setting the ignition timing with a degree wheel.

Then comes the test run. The engine must fire on the first or second kick, and the lights must be on at idle and get brighter with the revs. A spark that is strong on a cold engine and cuts out on a warm one points to a gap at the lower limit. In that case reset it to the full dimension.

Backing plate and ignition timing

The backing plate carries the coils, the contact breaker and the capacitor. Its position in the casing fixes the ignition timing. The slotted holes at the fixing screws allow a few degrees of rotation. Turn the plate against the direction of flywheel rotation and the ignition gets earlier, turn it with the rotation and it gets later.

The coils themselves also have slotted holes. Those are only for radial alignment to the flywheel and do not change the ignition timing. Do not confuse the two: for the air gap you move the coils on the backing plate, for the ignition timing you move the backing plate in the casing.