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Synchronising the engine shafts: the timing belt pulley
The timing belt pulley is an important component for valve control. It can be found in engines using a timing belt to operate the valves. These engines always have two timing belt pulleys installed.
Timing belt and timing belt pulley: position and function
The timing belt connects the crankshaft with the camshaft. Both shafts have a disc on the same side. The timing belt pulley is screwed onto the protruding brackets of the shafts with three to five bolts. The oil rings behind the discs prevent oil from leaking from the engine. The timing belt pulley is not a gear wheel, but a broad disc with trapezoidal transversal grooves. These grooves fit exactly in the belt's toothing. The power of the piston moving downward drives the crankshaft and causes it to rotate. This rotation is transmitted via the timing belt to the camshaft. The diameter of the timing belt pulley is calculated to transmit in a 2:1 ratio. The crankshaft rotates twice while the camshaft rotates around its axis only once, enabling the four strokes
- compression and ignition
via piston and valves.
Belt pulley defects
Generally, no replacement of the belt pulley is necessary when the maintenance interval for the timing belt is due, except when the manufacturer dictates pulley replacement. A defect of the timing belt pulley occurs when errors are made on occasion of camshaft or crankshaft replacement. The timing belt pulley sits firmly at its position. This is necessary, as the position of crankshaft and camshaft must remain unchanged. Once loosened, the pulleys can easily be removed from the shafts.
Timing belt pulley replacement
Replacing the belt pulley requires two special tools:
First, the pullers are positioned on both pulleys in such a way that the markings on the crankshaft pulley correspond with the marking on the engine block. This secures the TDC (top dead centre) on the first cylinder and gives a reference value for the entire engine. The counter-holder enables unscrewing the central bolt without turning the shaft along. This cannot be done with the puller alone. The necessary force applied to unscrew the central bolt could damage the toothing of the pulley. The belt pulley is fixed to the shaft pins with a central bolt and several securing bolts. First, the central bolt is removed, subsequently the securing bolts. The central bolt has a 18 mm head. Securing bolts are usually 13 mm hexagonal bolts. Using the available timing belt again is not recommended. As a rule it is disposed of and replaced by a new component when the belt pulley requires maintenance. This renews the entire unit and makes it run smoothly until the next maintenance interval.
What to observe on occasion of a belt replacement
A timing belt is always replaced as a set. Working on the pulley implies replacement of the belt. The set usually contains:
- tensioner pulley
- water pump
The most important thing when replacing a timing belt is ensuring that the position of crankshaft and camshaft remains unchanged. They must be precisely aligned, otherwise the engine's performance is seriously impaired. It might even break down totally. The markings on crankshaft and camshaft must always correspond exactly with the markings on the engine block. To be certain, before starting the engine the freewheel should be tested. This is done as follows: Remove all spark plugs. This prevents the engine from building up compression and makes rotation easier. Now the engine is rotated at the crankshaft or the camshaft with help of an 18 mm socket wrench. Alternatively the fourth gear is engaged, the car jacked up and the drive wheel rotated. Front wheel drive cars are jacked up at the front, rear wheel drive cars at the rear. When the engine is not blocked, at least the piston hitting the valve has been prevented. If the engine is blocked, an error has been made. This means: removing the timing belt and realigning both shafts. This test also applies to diesel cars. It makes no sense to remove the glow plugs, as these break off easily which causes high extra costs. At best the compression pressure is taken for granted and additional force is applied for the test.
Costs for a timing belt pulley
A new timing belt pulley costs ca. 10 EUR (±£9).