A chainsaw engine can run for hundreds of hours without trouble, but after long use the cylinder and piston wear. Cylinder repair calls for experience and knowledge of the machine, so if you are unsure, a service centre is the better option.
Signs of cylinder trouble include a damaged piston or cylinder wall, engine seizure, sudden stalling under load, loss of power and blue smoke from the exhaust. Noticed one of these symptoms? Then it is time to strip the saw down and find the cause.
What you will need
Important: Only dismantle a cold engine with an empty fuel tank, in a ventilated area away from naked flames. Photograph every stage: it makes reassembly far easier.
Step 1
Remove the engine ancillaries
To reach the cylinder, remove the top cover, air filter, throttle linkage and fuel hose, then the shroud, guards and gaskets, along with the support and control handle. Unscrew the spark plug.
Treat screws that have seized from heat and moisture with WD-40 Multi-Use: it penetrates the thread and helps undo fixings without rounding them off.
Step 2
Remove the cylinder
The finned cylinder is held by studs or bolts, usually four. These seize hardest of all on a hot cylinder. Spray them with WD-40 Specialist Penetrant and wait a few minutes: the penetrating lubricant quickly reaches tight spots and frees stuck joints.
Undo the fixings and pull the cylinder gently upwards without tilting it. Then remove the piston and the cylinder gasket. Take the studs out carefully so as not to damage the threads in the crankcase.
Step 3
Inspect the piston and cylinder
The cylinder bore must be clean and smooth. Run a finger over it: scratches, scoring or smeared aluminium are felt immediately. Check the exhaust port for damage.
The piston skirt must be free of grooves, scratches, burn marks or chips. Clean dust and sawdust from the cooling fins: blocked fins cause overheating.
Step 4
Repair or replace parts
What you do depends on the damage. If the cylinder or piston is deeply scored, replace the cylinder and piston as a set. If the piston only has carbon deposits and a stuck ring, replacing the piston ring is enough.
Remove carbon from the piston gently with a soft wooden or plastic scraper, never a metal one.
Step 5
Reassemble the piston and cylinder
Connect the piston to the connecting rod by inserting the gudgeon pin and securing it with both circlips. The arrow on the piston crown points to the exhaust side, so follow it exactly.
Coat the piston, ring and cylinder bore lightly with two-stroke engine oil so the parts do not scuff on first start. Replace any damaged gaskets.
Step 6
Fit the cylinder and refit parts
Slide the cylinder over the piston while compressing the ring, and tighten the studs in a cross pattern. Pull the starter cord slowly a few times: the piston should move without binding.
Refit all removed parts. Clean the spark plug and treat it with WD-40 Specialist Contact Cleaner before fitting. Check the carburettor and clean it if needed, since it prepares the fuel-air mixture.
Tip: The most common causes of cylinder scoring are a lean mixture and blocked cooling fins. Mix fuel exactly to the manufacturer's ratio and clean the fins after every working day.
What next
After a repair, run the first 2-3 tanks without heavy loads. Clean the filter and chain regularly, and see our separate guide on chain care.
These uses come from the experience of WD-40 users in many countries. Always follow the instructions and warnings on the product label.
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