Top dead centre (TDC) is the point where the piston is at the very top of the cylinder. On a four-stroke the piston reaches the top twice for every power stroke: once at the end of the compression stroke (both valves closed) and once at the end of the exhaust stroke (valves rocking open). Valve clearance, cam timing and many ignition checks need TDC on the compression stroke. This page shows how to find it with the timing marks, and how to find it when there are no marks.
Safety
- Engine cold, key off, kill switch on OFF. On electric start bikes, disconnect the battery negative lead so the starter cannot turn the engine.
- Turn the engine by hand only, never with the starter or kickstarter while a rod or tool is in the plug hole.
- If you lift the rear wheel to turn the engine, support the bike firmly on a stand.
You need
- A spanner or socket that fits the crankshaft or flywheel nut, or a way to turn the rear wheel in top gear.
- A torch to read the marks through the inspection hole.
- A thin plastic rod, a wooden skewer or a drinking straw for engines without marks. A soft rod cannot scratch the piston or jam.
- The manual, to know which cover or plug gives access to the marks and which way the engine turns.
Steps
- Remove the spark plug. The engine then turns easily and you can feel compression with a finger.
- Remove the inspection plug or cover over the flywheel marks, and the cap over the crankshaft nut if there is one.
- Find which way the engine turns. If you do not know, push the kickstarter gently by hand and watch the flywheel. Always turn the engine in this direction.
- If you cannot reach the crankshaft nut, lift the rear wheel, select top gear and turn the rear wheel forward slowly.
- Four-stroke: watch the inlet valve (or its rocker) through the valve cap, or hold a finger over the plug hole. When the inlet valve opens and closes again, or when you feel air pushing out against your finger, the piston is rising on the compression stroke.
- Keep turning slowly until the T mark on the flywheel lines up with the fixed mark on the case. Do not confuse it with the F mark, which is the firing point a few degrees before TDC.
- Check: both valves now have a little free play (rock each rocker gently). If one valve is tight, you are at TDC on the exhaust stroke. Turn one more full turn and line the T mark up again.
- No marks, or marks you cannot trust: after step 5, slide the soft rod down the plug hole until it rests on the piston. Keep turning slowly. The rod rises with the piston, stops, then starts to fall. The point where it stops is TDC. Do not push the rod hard or let it bind in the hole.
- Two-stroke: there are no valves, so any top position is the right one. Use the T mark or the rod method only.
- If you want permanent marks for later, make a small paint mark on the flywheel and a matching one on the case at this position.
Check your work
- The T mark and the fixed mark line up, and both valves have free play (four-stroke).
- Turning one full turn further and back brings you to the same point each time.
If it did not fix the problem
- Next job: Check and adjust valve clearance, Set cam timing after head or cam work or Check ignition timing.
- The T mark lines up but the cam marks do not: the cam chain may have jumped a tooth. Stop and use Check the cam chain and tensioner.
- To stop the engine moving once you are at TDC, see Hold the engine still to undo a crankshaft, flywheel or clutch nut.