TDC stands for top dead center, and BDC stands for bottom dead center. Both terms describe the position of a piston inside an engine cylinder. TDC is the point where the piston sits at its highest position, creating the smallest possible volume inside the cylinder. BDC is the opposite: the piston at its lowest point, where cylinder volume is at its maximum.
These two positions are the bookends of every piston stroke and form the basis for how engines are timed, measured, and diagnosed.
Where TDC and BDC Sit on the Crankshaft
A piston doesn’t move on its own. It’s connected to the crankshaft by a connecting rod, and as the crankshaft spins, the piston travels up and down. TDC corresponds to 0 degrees of crankshaft rotation, while BDC sits at 180 degrees, exactly halfway through one full revolution. A complete crankshaft rotation (360 degrees) takes the piston from TDC down to BDC and back up to TDC again.
The distance the piston travels between TDC and BDC is called the stroke. You can measure it directly, or calculate it by taking the distance between the crankpin center and the main bearing center and multiplying by two. Stroke length is one of the core measurements used to calculate an engine’s displacement.
How TDC and BDC Map to the Four-Stroke Cycle
In a four-stroke engine, the piston passes through TDC and BDC multiple times during a single operating cycle. Each of the four strokes begins at one position and ends at the other.
- Intake stroke: The piston moves from TDC down to BDC with the intake valve open, drawing in the air-fuel mixture.
- Compression stroke: The piston moves from BDC back up to TDC with both valves closed, compressing the mixture into a much smaller space.
- Power stroke: Combustion forces the piston from TDC back down to BDC. By the time the piston reaches BDC, combustion is complete and the cylinder is filled with exhaust gases.
- Exhaust stroke: The piston travels from BDC up to TDC, pushing spent exhaust gases out through the open exhaust valve. When the piston reaches TDC at the end of this stroke, one full operating cycle is complete.
That means a full four-stroke cycle requires two complete crankshaft revolutions (720 degrees), and the piston passes through TDC twice: once at the top of the compression stroke and once at the top of the exhaust stroke. This distinction matters more than you might expect.
Why BTDC and ATDC Matter
You’ll often see the abbreviations BTDC (before top dead center) and ATDC (after top dead center) when people talk about ignition timing. These describe when the spark plug fires relative to TDC, measured in degrees of crankshaft rotation.
In a gasoline engine, the spark plug doesn’t fire at the exact moment the piston reaches TDC. It fires slightly before, while the piston is still moving upward on the compression stroke. This gives the flame front time to spread across the combustion chamber so that peak pressure builds just as the piston passes TDC and starts heading down. Firing the spark earlier (more degrees BTDC) is called advancing the timing. Firing it later (closer to TDC or even slightly ATDC) is called retarding the timing.
Getting this balance right is critical. Fire too early and the expanding gases fight against the piston while it’s still traveling upward, which causes knocking and wastes energy. Fire too late and the piston is already well into its downward travel before pressure peaks, leaving power on the table. Modern engines use electronic control modules to adjust timing continuously based on engine speed, load, and sensor data.
Finding TDC on the Compression Stroke
If you’re working on an engine and need to find TDC, simply rotating the crankshaft until the piston reaches its highest point isn’t quite enough. The piston reaches TDC twice per cycle, and you need to know which one you’re at: compression TDC or exhaust TDC.
One common method is to remove the spark plug from cylinder number one and place your finger over the hole while someone slowly turns the crankshaft. As the piston rises on the compression stroke, you’ll feel air pressure pushing against your finger. If you don’t feel pressure, the piston is on its exhaust stroke instead. You can also slide a long pencil or dowel into the spark plug hole and watch it rise to confirm the piston is approaching its highest point.
Another way to tell the difference is to watch the valves. At the transition between the exhaust and intake strokes (exhaust TDC), you’ll see the valves “rock,” meaning the exhaust valve is closing while the intake valve starts to open. At compression TDC, both valves are fully closed. If both valves are shut and the piston is at its peak, you’re at TDC on the compression stroke, which is the reference point used for setting ignition timing and degreeing a camshaft.