'Clunk' / rattling noise on a SYM Symphony ST 125 2026

Loose or worn drive chain

Diagnosis confirmed: a drive chain that's either too loose or worn out, slapping against the swingarm and rear sprocket (especially when you roll on the throttle or ride at low revs), wearing down the sprockets prematurely, and in the worst case, capable of jumping off or snapping. What you do next depends on its condition. If the chain is just loose but otherwise in good shape, you adjust its slack to the manufacturer's spec (the vertical play at the middle of the lower run, often somewhere around 2 to 4 cm depending on the model -- check your manual), keeping the wheel perfectly aligned using the marks on the chain adjusters. This is a basic skill worth knowing, and it goes hand in hand with regular lubrication. If the chain is worn (stiff links, tight spots, a section that lifts noticeably off the sprocket, hooked teeth on the sprockets), you'll need to replace the whole set -- chain, rear sprocket, and front sprocket together (a "chain kit") -- since these three parts wear as a unit: fitting a new chain onto old sprockets wrecks it fast. A full kit replacement is an intermediate-level job (quick-link that clips on, or a rivet link that needs a dedicated riveting tool). Risk level: low for adjusting tension, moderate for a full replacement. On cost: adjusting and lubing is close to free (chain grease runs about 10 euros); a good-quality full chain kit costs somewhere around 80 to 250 euros depending on the model. A chain that's kept properly tensioned and lubricated will last a lot longer. This tool helps you understand what's going on -- it doesn't replace a certified mechanic.

How to make the repair yourself

🟢 Within reach of any rider: no advanced mechanical skill or specialised tooling needed.

1. 1Put the bike on its center stand (or a paddock stand) so the rear wheel spins freely, and turn it slowly to find the tightest point in the chain (wear is rarely even) -- that's where you'll measure and adjust.
2. 2Check the slack: press up on the middle of the lower chain run, between the two sprockets, and gauge how much it moves. Compare it to your manual's spec (often around 2 to 4 cm depending on the model). Too tight wears out the bearings; too loose and the chain slaps and can jump.
3. 3Check for wear: if you can pull the chain away from the back of the sprocket enough to see a full tooth, or if links feel stiff or stuck even after lubing, the chain is worn out and the whole kit needs replacing.
4. 4To adjust tension: loosen the rear axle nut, then turn the adjuster bolts/eccentrics on each side of the swingarm by the same number of marks, using the alignment marks to keep the wheel straight.
5. 5Check wheel alignment (matching marks on both sides, or a straightedge/string along the tire): a misaligned wheel wears the chain and tires at an angle and hurts handling.
6. 6Retighten the axle nut to the manufacturer's torque spec (with a torque wrench if you have one), then recheck the slack -- tightening the axle can shift it slightly. Refit the cotter pin or lock nut if there is one.
7. 7Clean and lubricate the chain with a proper chain grease (ideally when the chain is warm after a short ride, applied to the inner side while turning the wheel), and wipe off any excess that would otherwise fling off.
8. 8If you're replacing the full kit, use a matching chain + sprocket set of the correct part number and pitch, and rivet the master link properly (or clip the quick-link the right way round) -- if you're unsure about riveting, leave that step to a professional.