bottom dead center

Bottom dead center (BDC) is the position where a piston reaches its lowest point in the cylinder bore, furthest from the cylinder head. At this moment, the connecting rod and crankshaft form the largest angle possible, and the crankshaft has rotated 180 degrees from top dead center (TDC). The volume of the cylinder is at its maximum, which for a four-stroke engine means this occurs at the end of the intake stroke or the power stroke depending on the cycle phase.

Every reciprocating engine has a BDC position for each cylinder. In a single-cylinder engine, there is one BDC per complete rotation; in a four-cylinder engine, there are four separate BDC positions occurring in sequence as the crankshaft turns. The exact location is marked on the flywheel or crankshaft damper with a scribed line or cast notch that aligns with a stationary pointer on the engine block. This alignment serves as a critical timing reference for technicians performing ignition timing checks, valve adjustment, and bearing inspection.

To locate BDC, the engine must be positioned so the piston moves down until it stops at maximum extension. Mechanical resistance prevents the piston from traveling further; continued rotation of the crankshaft reverses the direction. For engines with accessible timing marks, rotating the crankshaft backward and forward slightly around BDC while watching the pointer against the flywheel mark confirms the position. Modern diagnostic equipment uses crankshaft angle sensors to determine BDC electronically, but the physical datum remains essential for manual work.

BDC matters because many engine specifications are measured relative to it. Compression ratio calculations use the volume swept from BDC to TDC. Valve timing is expressed in degrees before or after BDC on the intake and exhaust strokes. Bearing clearance inspection often requires locking the crankshaft at BDC so a rod or main journal lies in a predictable orientation. Incorrect BDC identification leads to wrong ignition advance, misaligned valve events, and measurement errors that compound into poor engine performance or damage.

The term "dead center" refers to the two points in each piston stroke where the connecting rod is momentarily perpendicular to the crankshaft motion, creating an instant where piston velocity is zero. At these geometric positions, the crankshaft cannot transmit motion to the piston or vice versa, so a flywheel or stored energy is needed to push through them during cranking. This is why dead centers are critical check points and are marked on every piston engine ever built.