Changing the piston on a two-stroke: pressing the gudgeon pin out, rings, fitted direction

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

Changing the piston is one of those jobs every two-stroke mechanic does sooner or later: after a seizure, with worn rings or when moving up to the next oversize. The work is manageable as long as you press the gudgeon pin out without a hammer, watch the fitted direction and use new circlips.

This guide takes you from removing the cylinder to sliding it back on. It applies to the single cylinder two-strokes from Zündapp, Kreidler, Hercules and Sachs, Puch, Simson and Piaggio. Model specific figures such as piston clearance, ring end gap or tightening torques come from the workshop manual for your machine.

When the piston has to come out

A piston shows its condition in three places: on the skirt, in the ring grooves and on the crown. Vertical scoring on the skirt, especially on the exhaust side, is the mark of a seizure. You can spot worn ring grooves by a new ring rocking noticeably in the groove. A holed or partly melted crown points to pre-ignition or too weak a mixture.

Before you order a new piston, measure the cylinder bore. A cylinder with scoring or wear beyond the tolerance needs the next oversize and therefore a piston of larger diameter. A standard size piston in a worn cylinder has too much clearance, it rattles and does not last long.

A piston change always includes new piston rings, new circlips and a new gudgeon pin if the old one shows wear marks or discolouration. Check the needle roller bearing in the small end for play and discolouration.

Tools

• Gudgeon pin extractor: presses the pin out of the piston with thread force, with no blow to the connecting rod.
• Piston support block: holds the piston and crankshaft while the cylinder head nuts are undone and protects the piston.
• Needle nose pliers or circlip pliers for the gudgeon pin circlips.
• Feeler gauge for the ring end gap, a clean rag for the crankcase.
• Torque wrench for the cylinder head and the cylinder base.
• Two-stroke oil or engine oil for oiling the piston, rings and pin.

A hammer does not belong on this list. Drive the gudgeon pin out with a hammer and a drift and you feed the force through the connecting rod into the crankshaft. The webs go out of true, the crank runs out of round, and a few hundred kilometres later the crankshaft bearings are due. What happens then is described in the guide changing the crankshaft bearings.

Removing the cylinder and taking the piston out: step by step

1. Get at the engine

   Take the spark plug out, remove the exhaust and the carburettor, and on fan cooled engines take the shrouding off. Clean the area around the cylinder base thoroughly. Dirt that falls into the crankcase now ends up in a bearing later.

2. Take the cylinder head off

   Undo the head nuts crosswise in several stages. A head that is only released on one side will distort. Take the head off and put the head gasket aside, it gets replaced.

3. Pull the cylinder off

   Pull the cylinder straight upwards. If it is stuck, tap the cooling fins lightly with a rubber mallet, never lever at the cylinder base. Hold the piston for the last part so that it does not hit the casing.

4. Cover the crankcase

   Stuff a clean rag into the casing opening around the connecting rod. A circlip that falls into the casing means splitting the engine.

5. Remove the circlips

   Pull both gudgeon pin circlips out with needle nose pliers. Use the cutout in the pin bore. The old clips go in the bin, not back into the piston.

6. Press the gudgeon pin out

   Set up the gudgeon pin extractor. The pressure pin sits on the gudgeon pin, the abutment braces against the piston. Turn the spindle evenly until the pin is clear of the opposite pin boss. A tight pin can be freed by warming the piston with a hot air gun.

7. Take the piston off

   Lift the piston off the connecting rod and keep the needle roller bearing safe. If you are refitting a used piston, mark its fitted position with a felt tip pen.

Fitted direction: arrow towards the exhaust

A two-stroke piston is not symmetrical. On the crown you will find an arrow, sometimes the letter A or a mark on the skirt. The arrow points towards the exhaust. On some pistons the pin is set slightly off centre, on others the windows in the skirt are arranged asymmetrically. A piston fitted the wrong way round covers transfer ports or puts the ring gap in the exhaust port.

At the same time, check the ring locating pegs in the ring grooves. Each ring has a small peg in its groove that fixes the ring gap. Without that peg the ring would travel in use, and its open end could catch in a port and break off. The pegs always sit away from the exhaust, because that is where the ring ends are under the least load.

Before fitting the rings, check the ring end gap: lay the ring in the cylinder, square it up with the piston, measure the gap with a feeler gauge. The specified figure is in the workshop manual for your machine. Too small a gap makes the ring bind at operating temperature.

Fitting the new piston: step by step

1. Fit the piston rings

   Spread the rings only as far as you need to get them over the piston. On a piston with two rings, the lower one goes on first. Turn the rings so that the ring gap sits on the locating peg. Some rings have a TOP mark, and that faces upwards.

2. Fit the first circlip

   Fit a new circlip into the groove on the side you are not pushing the pin in from. The opening of the clip faces up or down, never towards the cutout in the pin bore. That way it cannot work its way out through the cutout as the piston accelerates and decelerates.

3. Oil the needle roller bearing and the pin

   Fit the needle roller bearing into the small end, and do not forget to oil it. Coat the pin and the pin bosses thinly with two-stroke oil.

4. Fit the piston

   Fit the piston onto the connecting rod with the arrow towards the exhaust. Push the pin in with your thumb or with the extractor working the other way round. If the piston is tight, warm it with the hot air gun and the pin will slide in easily.

5. Fit the second circlip

   Fit the second new clip and check all the way round with a screwdriver that it is fully seated in the groove. It must turn in the groove and must not spring.

6. Remove the rag

   Take the rag out of the crankcase and lay a new base gasket on. Check that no remains of the old gasket are stuck to the casing.

Sliding the cylinder on and fitting the head

Oil the cylinder bore and the piston thinly. Squeeze the rings together with thumb and forefinger and slide the cylinder squarely over the piston. The lead-in chamfer at the cylinder base helps. If it catches, do not push, lift it off and line the rings up again. A ring whose gap has jumped off the peg will break as the cylinder goes on.

This is where the piston support block comes in. It goes into the crankcase underneath the piston, locks the crankshaft and holds the piston in position. That lets you guide the cylinder with both hands. Later, when you tighten the clutch nut or the flywheel nut, it locks the crankshaft without a screwdriver stuck in a piston window.

Once the cylinder is down, take the block out, turn the engine over several times and listen for an even running noise. Then fit the head gasket and the cylinder head. Tighten the head nuts crosswise in three stages to the figure given in the workshop manual for your machine. For studs in an aluminium casing the rule is: torque wrench, not feel. What a tightening torque means and how an extension changes it is covered in the article understanding tightening torque, and the figures for each thread size are in the table of tightening torques.

Running in and checking

A new piston needs running in. The rings and the cylinder wall have to bed to each other. For the first few kilometres, ride with changing engine speeds, no full throttle and no long steady running. A two-stroke with oil injection gets a little extra oil in the tank during this period. With premix, mix richer during the running in phase; the ratio is in the workshop manual for your machine.

After the running in phase, check the plug reading. How to read it is explained in changing the spark plug and reading the plug. A white insulator after a piston change means an air leak, often at the new base gasket or at the inlet manifold.

Which special tools your engine needs for the piston change and the jobs around it is shown by the tool finder.