Changing the front sprocket on a moped: 415 or 420 chain, undoing the nut, gearing

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

The front sprocket is the smallest and hardest working gear in the drivetrain. It turns more often per kilometre than anything else, and it sits exactly where oil, dirt and chain pull all come together. A worn front sprocket eats the chain, and a stretched chain eats the next front sprocket. Anyone who wants to change the gearing has to go through the front sprocket as well.

Here you can read how to tell 415 and 420 chains apart, how to undo the sprocket nut without any tricks, what is different about the taper on Sachs engines and how to check chain alignment and tension at the end.

Chain pitch 415 or 420

The pitch, that is the distance from pin centre to pin centre, is the same on both chains: half an inch, 12.7 mm. The difference lies in the internal width between the inner plates. The 415 chain is narrower, the 420 wider. Front and rear sprockets have correspondingly different tooth widths. A 420 sprocket will not run in a 415 chain, and a 415 sprocket runs in a 420 chain with play and out of line.

Chain	Vehicles	Holding tool
415	Kreidler Florett, Flory	Front sprocket holding tool 415 Kreidler
420	Zündapp KS 50, KS 80, K 80, SX 80, Kreidler Florett RS, Hercules Ultra	Front sprocket holding tool 420 KS 50 / RS / Ultra
420	Zündapp KS 125, KS 175	Front sprocket holding tool 420 Zündapp KS

You will find the designation stamped on the outer plates of the chain. If it is missing, measure the internal width with a vernier caliper and compare it with a new chain. Front and rear sprockets rarely carry the pitch, so there only a comparison with the chain helps.

Gearing: what one tooth on the front sprocket changes

The final drive ratio is the number of teeth on the rear sprocket divided by the number of teeth on the front sprocket. A front sprocket with fewer teeth makes the gearing shorter: more pull on hills, quicker acceleration, lower top speed at the same engine speed. A front sprocket with more teeth makes it longer: less pull, higher speed at the same engine speed, but only if the engine has the power for it.

Each tooth counts for more on the front sprocket than on the rear one, because the front sprocket is small. One tooth more or less at the front is roughly equivalent to three or four teeth at the rear. That is why the front sprocket is the easier way to adjust the gearing. Keep two things in mind: the chain length has to suit the new combination, and a larger front sprocket needs room in the sprocket housing. A change in gearing may also affect your vehicle's type approval. Check that before you rebuild anything.

Undoing the sprocket nut: a holding tool instead of tricks

The sprocket nut is tight, and the front sprocket turns with it as you undo it. The usual tricks are a screwdriver between chain and sprocket, a gear engaged with the rear brake held on, or a rag in the rear sprocket. All three feed the force through the gearbox, the chain or the rear sprocket. The screwdriver between chain and sprocket damages teeth and chain plates, and the engaged gear loads the gearbox gears with the full breakaway torque.

The holding tool engages directly in the teeth of the front sprocket and braces itself against the casing or the swingarm. The force goes from the spanner through the sprocket into the tool, and none of it into the gearbox. You have both hands free, one for the spanner, one for the tool. There is a matching version for 415 and for 420 chains, because the tooth width differs.

Check in the workshop manual for your machine whether the sprocket nut has a right hand or a left hand thread. Some manufacturers secure the nut against the direction of rotation with a left hand thread. Turn it the wrong way and you tighten the nut further until the thread tears out. You will find an overview of the thread types in the thread table.

Changing the front sprocket step by step

1. Remove the sprocket cover

   Unbolt the cover or sprocket guard and expose the chain at the sprocket. Clear dirt and old chain grease from around the sprocket so that nothing falls into the casing.

2. Bend the locking tab back

   If the nut is secured with a locking tab washer, bend the folded tab back with a chisel or a pair of pliers. Never lever with a screwdriver, it slips and damages the casing.

3. Fit the holding tool

   Set the tool into the sprocket teeth so that it braces against the casing or the swingarm as you undo the nut. Check that it sits fully in the teeth and cannot tip out.

4. Undo the nut

   Spanner onto the nut and undo it in the direction given in the manual. A ring spanner or a socket sits more securely than an open ended spanner. Put the nut and the locking tab washer to one side.

5. Take the chain off

   Push the rear wheel forward so that the chain has slack, and lift the chain off the front sprocket. On a chain with a split link you can also go via the link.

6. Pull the front sprocket off

   If the sprocket sits on splines, pull it off by hand or with light levering at two opposite points. If it sits on a taper, you need the puller described in the next section.

7. Fit the new front sprocket

   Clean the shaft, fit the new sprocket with the markings facing the same way as on the old one. New locking tab washer, tighten the nut to the torque given in the workshop manual. The torque calculator helps with the conversion. Fold the tab of the locking washer over.

Sachs engines: front sprocket on a taper

On many Sachs engines the front sprocket does not sit on splines but on a taper on the output shaft. As with the flywheel, it is held by friction, and the nut presses it onto the cone. A sprocket on a taper cannot be levered off. The lever grips at the edge, the sprocket tilts and jams, and the shaft gets bent.

The sprocket puller assembly Sachs 50 fits Hercules K 50, Sachs S and GS 75 and other engines with the Sachs 50 unit. It braces against the sprocket and presses centrally on the shaft with a spindle, exactly like a flywheel puller. For Hercules K 125 BW, GS, MC, K 125 T and others with the Sachs 125 engine there is the sprocket puller assembly Sachs 125. It has one special feature: the speedo drive sits behind the sprocket. The assembly is built so that the force does not go into the speedo drive, only into sprocket and shaft.

After pulling it off, clean the taper and the sprocket hub dry and free of grease. The taper only transmits the torque if both faces bed together cleanly. On reassembly, tighten the nut to the figure given in the manual. The tool finder shows you which assembly belongs to your Sachs engine.

Checking chain alignment and chain tension

Front and rear sprocket have to run in one plane. Hold a long straight edge or a straight batten against the side of the rear sprocket and see whether it is parallel to the front sprocket. Alternatively, sight from behind across the rear sprocket to the front one. Misalignment shows up in the chain running bright on one side of the teeth and the plates being worn on one side.

Misalignment often comes from a sprocket fitted the wrong way round, a missing spacer washer or a rear axle set unevenly. The chain adjusters on the swingarm must be at the same setting on both sides. The marks on the swingarm are a help but not always accurate. If in doubt, measure from the swingarm pivot to the wheel spindle on both sides.

Set the chain tension by the amount of slack given in the workshop manual. It is measured in the middle of the lower chain run, with the machine loaded or however the manual specifies. A chain that is too tight loads the bearings of the sprocket shaft and the wheel hub, one that is too loose whips and jumps the teeth.

Spotting wear

A worn front sprocket has teeth that are flattened on one flank and drawn out to a point on the other, the so called shark's teeth. A stretched chain can be lifted a little way off the teeth at the back of the rear sprocket. The two together show that the whole set is due for replacement.

Front sprocket, chain and rear sprocket bed into each other. A new front sprocket in a stretched chain is ruined again after a short time, because the chain no longer matches the tooth pitch. So always change all three together, unless the other parts are demonstrably new. You will find the right spanner sizes for the sprocket nut and the axle nut in the table of across flats sizes.