by removing a plug from the crankcase. In every case, the  engine manufacturer’s instructions give the location of built- in timing reference marks. In using built-in timing marks to position the crankshaft, be  sure to sight straight across the stationary pointer or mark on  the nose section, the propeller shaft, crankshaft flange, or bell  gear.  [Figure 4-38]  Sighting at an angle results in an error  in positioning the crankshaft. Normally, the No. 1 cylinder  is used to time or check the timing of the magnetos. When  installing magnetos, the timing marks must be lined up and  the No. 1 cylinder must be on the compression stroke.  The amount of gear backlash in any system of gears varies  between installations because there is clearance between  the gear teeth. Always take timing when reading, or stop movement of the engine for timing set up, in the direction  of rotation. Another unfavorable aspect in the use of timing  marks on the reduction gear is the small error that exists when  sighting down the reference mark to the timing mark inside  the housing on the reduction gear. This can occur because  there is depth between the two reference marks. Most timing disk devices are mounted to the crankshaft  flange and use a timing plate.  [Figure 4-39]  The markings  vary according to the specifications of the engine. This plate  is temporarily installed on the crankshaft flange with a scale  numbered in crankshaft degrees and the pointer attached to  the timing disk. Any given piston position, whether it is to be used for ignition,  valve, or injection pump timing, is referenced to a piston  position called top dead center. This piston position is not to be  confused with a piston position called top center. A piston in  top center has little value from a timing standpoint because the  corresponding crankshaft position may vary from 1° to 5° for  this piston position. This is illustrated in  Figure 4-40,  which  is exaggerated to emphasize the no-travel zone of the piston.  Notice that the piston does not move while the crankshaft  describes the small arc from position A to position B. This  no-travel zone occurs between the time the crankshaft and  connecting rod stop pushing the piston upward, and continues  until the crankshaft has swung the lower end of the connecting  rod into a position where the crankshaft can start pulling the  piston downward. Top dead center is a piston and crankshaft  position from which all other piston and crankshaft locations  are referenced. When a piston is in the top dead center position  of the crankshaft, it is also in the center of the no-travel zone.  The piston is in a position where a straight line can be drawn  through the center of the crankshaft journal, the crankpin,  and the piston pin. This is shown on the right of  Figure 4-40 .  With such an alignment, a force applied to the piston could  not move the crankshaft. Timing lights are used to help determine the exact instant  that the magneto points open. There are two general types  of timing lights in common use. Both have two lights and  three external wire connections. Although both have internal  circuits that are somewhat different, their function is very  much the same.  [Figures 4-41 and 4-42] 4-24 Figure 4-38.  Typical built-in timing mark on propeller reduction gear. Timing Lights Figure 4-39  A timing plate and pointer. Piston Position Indicators Timing Disks 20 Indexing groove T C 25°