Changing a spark plug and reading the plug

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

The spark plug is the cheapest diagnostic tool on a two-stroke. Its appearance tells you whether the mixture and the heat range are right, whether the engine is drawing oil or running too lean. Changing it takes five minutes if you have the right spark plug spanner and know the tightening rule.

This guide explains the difference between a 21 mm and a 16 mm hexagon, the principle behind the heat range, how to tighten correctly without a torque wrench and how to read a plug. On top of that comes looking after the electrodes with a brass brush and a feeler gauge.

Spark plug spanner: 21 mm or 16 mm?

Spark plugs come with two hexagon sizes that you meet on mopeds and motorcycles. In the classic design, plugs with an M14 thread have a 21 mm hexagon, as is usual on Zündapp, Kreidler, Hercules, Puch and Simson. Modern M14 plugs with a slim insulator and M12 plugs mostly have 16 mm. Plugs with an 18 mm thread, as fitted to very old engines, have larger hexagons still.

The Spark Plug Spanner 21 mm is a tube spanner with a tommy bar. The tube guides the plug cleanly out of the deep plug well as you unscrew it and holds it fast. An ordinary socket is often too short or too thick for the well between the cooling fins. For 16 mm plugs you need a correspondingly smaller spanner. If in doubt, measure the hexagon of your plug with a vernier caliper.

What matters is that the spanner takes the hexagon completely. A spanner that is too large rounds off the corners, and the plug is then stuck in the cylinder head.

Heat range: the principle

The heat range describes how quickly a plug carries the heat away from the insulator nose into the cylinder head. A cold plug has a short insulator nose and conducts heat away quickly. It suits hot, heavily loaded engines. A hot plug has a long insulator nose, holds the heat longer and burns itself clean better at low load.

In service the insulator nose should reach a temperature at which deposits burn off by themselves, but it must not get so hot that it ignites the mixture on its own. Too cold: the plug soots up. Too hot: pre-ignition and, in the worst case, a hole in the piston.

Manufacturers give the heat range as a number in the plug type, and the scales run in different directions depending on the brand. Which plug belongs in your engine is given in the workshop manual for your vehicle. On a tuned engine with more compression, one step colder can make sense. You check that on the plug reading.

Changing the spark plug: step by step

1. Let the engine cool down

   Only unscrew the plug with the engine cold or barely warm. An aluminium cylinder head expands more in the heat than the steel body of the plug. While it is hot the thread can be damaged as you unscrew it.

2. Clean the plug well

   Pull the plug cap off straight, do not pull on the lead. Blow or brush the dirt away from around the plug. Anything left lying there will otherwise drop into the combustion chamber as you unscrew the plug.

3. Unscrew the plug

   Put the spark plug spanner fully onto the hexagon and turn anticlockwise. If it is stiff, put a few drops of penetrating oil on the thread and wait. Never force it, the thread in the aluminium head is delicate.

4. Read the plug

   Look at the insulator nose and the electrodes straight away, while they still show the picture from the last run. How to interpret that picture is covered in the next section.

5. Check the thread and the sealing face

   Look into the plug well. The sealing face has to be clean and flat. Remains of an old sealing washer or soot on the face stop the plug sealing properly and carrying heat away.

6. Set the electrode gap

   Check the gap on the new plug with the feeler gauge. The specified value is in the workshop manual for your vehicle. To adjust it you bend only the earth electrode, never the centre electrode.

7. Screw the plug in by hand

   Wind the plug in with your fingers or with the spanner without the tommy bar until the sealing washer seats. If it binds after one turn, the plug has gone in crooked. Unscrew it and start again.

8. Tighten with a part turn

   Now fit the tommy bar and tighten to the rule in the section after next. Push the plug cap on until it audibly clicks into place.

Reading the plug: fawn brown, black, wet, white

The plug reading shows the state of the engine under the load it last ran at. For a meaningful reading, ride a few kilometres in the upper rev range, pull the clutch, stop the engine with the kill switch and coast to a halt. Then unscrew the plug straight away.

Plug reading	Meaning	Possible cause
Fawn brown to grey brown, dry	All in order	Mixture, heat range and ignition are right
Black, dry, velvety	Too rich	Air filter blocked, jet too large, choke open, plug too cold
Black, wet, oily	Oil in the combustion chamber	Too much two-stroke oil, faulty shaft seal, plug too cold for short runs
White to light grey, electrodes eroded	Too lean or too hot	Air leak, jet too small, plug too hot, ignition too far advanced
Glassy, shiny deposits	Overheating with glazing	Plug too hot, engine overloaded
Insulator cracked or electrode melted	Pre-ignition or detonation	Ignition too far advanced, wrong heat range, plug mechanically damaged

A slightly wet reading straight after a cold start is normal. What counts is the reading after a run under load. On a two-stroke with premix lubrication the plug is also never as clean as on a four-stroke. A thin, even deposit is not a fault.

Tightening rule: hand tight plus a part turn

You do not need a torque wrench to tighten a spark plug if you apply the plug manufacturers' rule. It holds for plugs with a flat sealing face and a sealing washer, as fitted to almost all moped engines:

• New plug with a new sealing washer: Wind it in by hand until it seats, then about half a turn with the spanner. The sealing washer is deformed plastically in the process and seals.
• Used plug with an already crushed washer: Seat it by hand, then only about 1/12 of a turn, that is 30 degrees. The washer is already flat and only needs preload.
• Plugs with a taper seat and no sealing washer: Seat it by hand, then about 1/16 of a turn. They are rare on moped engines.

Plugs tightened too loosely carry the heat away poorly and overheat. Plugs tightened too hard stretch the thread in the aluminium head or tear the plug body off. If you want to work with a torque wrench, the manufacturers' figures for each thread size are in the tightening torque table. What lies behind the figure is explained in the article Understanding tightening torque.

Cleaning the electrodes and checking the gap

A sooted up but otherwise sound plug can be cleaned. Use a brass brush, not a steel brush. Brass is softer than the electrodes and the insulator and leaves no conductive traces of metal. Brush the insulator nose and the electrodes until the soot is gone. Then rinse with brake cleaner and blow dry.

Then check the electrode gap with the feeler gauge. The right blade has to slide between the centre and earth electrode with a light resistance. If the gap is too large, tap the earth electrode down carefully on a hard surface. If it is too small, lift it slightly with a thin blade or with the blade of the feeler gauge.

A plug with a rounded centre electrode, a clearly enlarged gap or cracks in the insulator is replaced. Cleaning no longer helps here: the spark needs sharp edges to jump reliably. On a two-stroke with premix lubrication you reckon with a shorter plug life than on a four-stroke anyway.

When the plug is seized

A plug that will not come loose has usually seized through corrosion between the steel thread and the aluminium head. Let the engine cool right down, put penetrating oil on the thread and wait an hour. Then work it loose with the spark plug spanner in small movements, tightening it again slightly each time, until it turns freely. Jerking at the tommy bar will tear the thread out of the head.

If the thread in the head is damaged, a thread repair with an insert will help. The thread dimensions of the plug are in the thread table. Which other tools you need for the ignition on your model, such as a flywheel puller or a feeler gauge for the contact breaker, is shown by the tool finder.