Contents: General information ⬇ Bringing the engine to TDC with the…⬇ Bringing the engine to TDC position…⬇
General information
Note: The material presented in this subsection is only meaningful if the distributor, high-voltage wires and timing belt are installed correctly.
The timing belt is placed on the toothed wheels of the crankshaft and camshaft, which rotate in phase with each other. When the toothed timing belt is removed during engine maintenance, each shaft can rotate independently, which inevitably leads to a violation of the valve timing.
The design of the engine in question does not exclude the potential possibility of damage to the pistons and valves when they hit each other near the upper limit of the piston stroke due to the absence of recesses in the bottoms of the latter.
For the stated reason, the correct installation of the valve timing is of particular importance and is achieved by setting the engine to a certain initial position, called the top dead center (TDC), before installing the timing belt. After bringing the engine to the TDC position, turning the crankshaft and camshaft individually is by no means allowed before putting on the timing belt.
Note: The timing belt is also used to drive the water pump, but the timing of its rotation to the crankshaft and camshaft rotation phases is not as important.
Top dead center (TDC) is the highest point of the piston stroke in its cylinder. Generally speaking, in 4-stroke engines, during the rotation of the crankshaft, this position is reached by the piston twice: once at the end of the compression stroke and the second time at the end of the exhaust stroke. In order to adjust the valve timing and set the ignition timing angle, in the vast majority of cases, the TDC of the end of the compression stroke of the piston of the first cylinder is used. It is this position that is usually called the TDC position of the engine (the numbering of cylinders from 1 to 4 is carried out in the direction from the timing belt to the flywheel).
[Text taken from this website: skodabook.ru]
In one of the teeth of the crankshaft gear wheel, a groove is machined, the alignment of which with the corresponding mark on the flange of the seal confirms that the pistons of the first and fourth cylinders are brought to the TDC position. However, access to this gear wheel is opened only after removing the lower cover of the timing belt, therefore, on the end surface of the shaft pulley, an additional risk is provided, which in the situation under consideration (TDC of pistons of 1st and 4th cylinders) is aligned with the zero mark on the lower timing belt cover. The timing gear is also equipped with a timing dot mark, the alignment of which with the convex pointer cast on the inner timing belt cover confirms the fact that the engine has been correctly brought to the TDC position, when the piston of the first cylinder is at the top dead center of the end of the compression stroke.
6 Below is a description of the procedures for bringing the engine to the TDC position with the timing belt installed and removed.
Bringing the engine to TDC with the timing belt installed
Before you begin this procedure, disconnect the negative battery cable.
Note: If your vehicle's stereo system is equipped with a security device, make sure you have the correct code to activate the audio system before disconnecting the battery.
Disconnect the ignition system by disconnecting the central high-voltage wire from the cover and grounding it to the cylinder block ground using a jumper wire. To prevent the vehicle from accidentally moving, apply the parking brake and support the rear wheels with wedges. Make sure the manual transmission is in neutral.
Mark the position of the high-voltage wire terminal of the first cylinder on the distributor cap relative to the housing of the latter. On some models, such marking in the form of a special mark is already applied at the manufacturer. Otherwise, trace the wire from the first cylinder to the distributor cap, then use a marker or paint (under no circumstances with a slate pencil!) mark the edge of the distributor housing directly below the corresponding terminal.
Remove the distributor cap (see Chapter Engine electrical equipment).
Disconnect the high-voltage wires from the spark plugs. Try to remember the order in which they were connected.
To bring the piston of any of the cylinders to the TDC position, it is necessary to turn the crankshaft manually. This turning is done using a socket wrench for the central bolt of the shaft gear (see section Removal, condition check and installation of gears and timing belt tensioner).
Turn the crankshaft in the normal direction (see Specifications at the beginning of the Chapter) so that the distributor runner is turned with the tip of the electrode towards the mark of the high-voltage wire terminal of the spark plug of the first cylinder on the rim of the housing.
Note: It will be much easier to turn the crankshaft if you first remove all the spark plugs.
Remove the upper timing belt cover (see section Removal and installation the timing belt and its covers) in order to provide access to the camshaft gear located underneath it.
Locate the timing marks on the camshaft sprocket and the inner section of the belt cover. Continue rotating the crankshaft clockwise until the marks are completely aligned.
Now find the timing marks on the crankshaft pulley and the lower belt cover. The cover has raised marks in the form of the letters Z and 0. When the mark on the pulley rim aligns with the zero mark, the engine will be set to the TDC position.
Alternatively, ensure that the timing marks on the crankshaft gear and its front oil seal flange are correctly aligned.
Note: The crankshaft pulley and lower belt cover must be removed in this case.
Make sure that the distributor rotor is still turned with the electrode tip towards the mark of the first cylinder spark plug terminal on the rim of the assembly housing. If it is not possible to achieve simultaneous alignment of the installation marks of the shafts and the rotor, the correct installation of the distributor should be checked. (See Chapter Engine electrical equipment).
After all the listed conditions have been met, the piston of the first cylinder of the engine can be considered to be brought to the TDC position at the end of the compression stroke.
Note: If it becomes necessary to remove the timing belt now, make sure that the crankshaft and camshaft are securely locked by their gear wheels to prevent the engine from moving out of the TDC position.
Bringing the engine to TDC position with the timing belt removed
The material in this subsection will help an amateur mechanic to return the engine to the TDC position in the event of a misalignment of the timing marks when the timing belt is removed during, for example, a major overhaul of the power unit.
Turning the camshaft with the timing belt removed and the crankshaft set to TDC may cause mechanical damage due to the valves coming into contact with the piston crowns. In view of the above, the following procedure must be strictly followed.
Before installing the cylinder head on the engine, the crankshaft should be turned in the normal direction by the central bolt securing its gear, bringing the pistons of all cylinders to the middle part of their stroke. Moreover, the piston of the first cylinder should be fixed on the compression stroke, approximately 90° before TDC.
After installing the cylinder head and camshaft gear, locate the timing marks on the gear and the inner section of the timing belt cover.
Turn the camshaft gear in the normal direction until the mentioned marks are aligned.
Now find the marks on the crankshaft gear and the flange of its front oil seal. Now turn the crankshaft by the central bolt of the gear in the normal direction by approximately 90° (quarter turn) and achieve alignment of the marks.
Now find the marks on the crankshaft gear and the flange of its front oil seal. Now turn the crankshaft by the central bolt of the gear in the normal direction by approximately 90° (quarter turn) and achieve alignment of the marks check the correct installation of the distributor (see Chapter Engine electrical equipment).
After all the listed conditions have been met, the piston of the first cylinder of the engine can be considered to be brought to the TDC position at the end of the compression stroke.
Note: Before installing the timing belt, ensure that the crankshaft and camshaft are securely locked by their gear wheels to prevent the engine from moving out of the TDC position.