Stihl BR 550 Technical Guide - Page 14
The valve clearance must be set at the top dead
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Page 14 Technical Information 08.2005 5.4.2 Top dead centre The top dead centre (TDC) is defined as the point at which the piston ends its upward stroke and starts its downward stroke. In a 4-MIX engine, the power stroke is executed once in every two revolutions of the crankshaft. The piston consequently passes through the top dead centre twice in order to execute one power stroke. Refer to the STIHL Service Training System "Carburetor", under the heading four-stroke principle. Top dead centre, valve overlap Top dead centre, ignition 2 2 3 3 1 1 254TI010 KN 254TI009 KN Cam (1) points upwards. The rocker arms (2) and valve stems (3) move up and down (see arrows above) when the crankshaft is turned back and forth a few degrees. This represents the change of cycle between intake and delivery. Cam (1) points downward. The rocker arms (2) and valve stems (3) do not move up and down when the crankshaft is turned back and forth a few degrees. This represents the change of cycle between compression and power stroke. The valve clearance must be set at the top dead centre for ignition. The inlet and exhaust valves are only closed simultaneously in this position. TI_08_2005_11_01_02.fm