Valve timing marks — general description and use (Audi 100 C3)

Table of contents: General description ↓ Setting piston #1 to TDC ↓

General description

1. The camshaft and water pump are driven by a toothed belt from the crankshaft. The camshaft and crankshaft sprockets rotate in unison, which ensures precise valve timing.

2. The design of the engines described in this Section is such that if the crankshaft rotates while the timing belt is removed, the valves may strike the pistons. For this reason, it is important to maintain the correct relative position of the camshaft and crankshaft at all times while the timing belt is removed from the engine. To achieve this, it is necessary to set piston #1 to TDC before removing the timing belt (top dead center) and lock the shafts. If the engine has been removed for overhaul, piston #1 should also be set to TDC during assembly to ensure proper shaft alignment.

3. TDC is the highest point in the cylinder that the piston reaches. In a 4-stroke engine, each piston reaches TDC twice per cycle - on the compression stroke and on the exhaust stroke. The expression "TDC position" means that cylinder No.1 (is located at the front end of the engine) stands at TDC on the compression stroke.

Setting piston #1 to TDC

4. Remove the glow plugs to make it easier to turn the crankshaft.

Note: Some engines have a 32mm hex bolted to the power steering pulley to allow the engine to be turned without the aid of the crankshaft sprocket bolt.

5. Fit the socket onto the crankshaft sprocket bolt and slowly rotate the crankshaft clockwise until the timing mark on the edge of the flywheel/drive plate aligns with the timing rib on the clutch housing and the TDC mark on the fuel pump sprocket aligns with the mark on the pump housing. The moment when piston No.1 begins to move in the cylinder can be determined by the sound of air escaping from the hole for glow plug No.1. Alternatively, remove the camshaft cover to expose the lobes of cylinder No.1. When they both point upward, piston No.1 is at TDC.

Note: Due to the dynamic injection timing setting method (the pump rotates on the mount to make fine adjustments) the TDC mark on the fuel pump pulley may not align exactly with the mark on the pump housing.

6. If the engine is installed at TDC, in order to remove the timing belt, lock the camshaft in this position. To do this, remove its rear sprocket and install a special locking rod in the shaft groove. Unscrew the bolts of the cover installed on the rear of the engine, then remove the sprocket (see chapter 6). On the 3D engine, also lock the fuel pump sprocket using the special locking pin.

7. It is recommended to use a special U-shaped tool to lock the camshaft, but a replacement can be made from a metal rod that fits tightly into the groove of the camshaft (see illustration).

7. It is recommended to use a special U-shaped tool to lock the camshaft, but a replacement can be…

8. Remove the rear sprocket and insert the end of the locking rod into the slot on the end of the camshaft (see illustrations).

9. If there is even the slightest play in the locking rod, turn the camshaft slightly (rotating the crankshaft clockwise), so that one end of the rod rests on the cylinder head. Using a feeler gauge, measure the gap between the other end of the rod and the cylinder head. Now return the camshaft to its original position and remove the feeler gauge. Align the locking rod in the grooves by inserting two feeler gauges (each one is twice as thin as the measured gap) under its ends (see illustration).