Operating the measuring instrument

No. Operation Instructions

1 Prepare measuring Lower pointer into measuring instrument, instrument for testing

2 Pick up measuring instrument Grasp rubber strap between thumb and index finger.

3 Position measuring instrument Position measuring instrument in center between pul leys on the drive belt.

The lateral stop of the instrument must rest on the side of the V-belt.

4 Measure belt tension Press pushbutton to apply uniform pressure in perpen dicular direction on top of V-belt until the detent spring can be heard (or felt) to disengage. After the detent spring has disengaged, the measuring instrument or the pushbutton, respectively, must not be pressed anymore as the reading will otherwise be incorrect.

5 Read off measurement Lift measuring instrument carefully off the V-belt.


Do not push against instrument when lifting off the instrument as sudden impacts may alter the position of the pointer and may give a faulty reading. Read off measurement (tensioning value) at the point of intersection of the pointer with the upper scale (KG scale).

Checking and/or adjusting alternator and fan wheel drive belt Caution: Make sure the ignition key is pulled off whenever measurements are made.

Checking used drive belts:

Use belt tension measuring instrument (Special Tool 9574) to check tension.

A description of how to use the Special Tool is contained in Repair Group 13, page 13-30.

Retensioning the belt is only required if the belt tension displayed at the measuring instrument is less than 15 scale increments when the engine is cold and less than 20 scale increments when the engine is hot.

Retensioning the belt

Retension the belt as described in the General Adjustment Notes section. Before measuring the belt tension, start engine and let engine idle briefly.


Cold engine: 15 to 23 scale increments

Engine at operating temperature: 20 to 28 scale increments

Fitting a new belt

When fitting a new belt, be sure to observe the correct assembly sequence in order to avoid any loss of belt tension during vehicle operation.

1. Fit new belt. Adjust tension by inserting or removing shims as required.

Shims are available for belt tensioning in thicknesses of 0.5 mm and 0.7 mm. The 0.7 mm shim is identified by a 2 mm dia. drill hole.

2. Before measuring the belt tension, start engine once more and run engine at idle briefly.


Cold engine: 23 to 35 scale increments

3. Run engine at idle for approx. 15 minutes or test drive vehicle for approx. 10 miles. Check tension again.

Specification: 28 to 40 scale increments

Retension if required.

Test requirement: Engine at operating temperature

General adjustment notes

Fan wheel and alternator have separate drives.



Rotate engine only at lower belt pulley or by operating the starter. After completing all operations, check to make sure that hexagon head nut has been tightened suffl ciently on alternator shaft.


1 = Alternator drive components

2 = Fan wheel drive components

Adjusting the V-belts

1. Use polygon wrench (999 571 052 02) to lock shaft and undo hexagon head nut. Remove shims and pulley half.

2. If belt tension is below specification, remove one shim from between pulley halves and refit in front of pulley half.

Tighten hexagon head nut to 50 ± 5 Nm.

3. Three additional M 6 screws have to be undone for V-belt and fan wheel.

4. Use only V-belts that have been approved by the manufacturer.

911 Carrera 4 Engine, Cylinder Head, Valve Drive

Engine timing

1 - Left-hand chain tensioner

2 - Seal for chain tensioner

3 - Cover for chain tensioner

4 - Aluminium washer

6 - Seal for chain housing

7 - Tensioner blade

8 - Bearing pin

9 - Distributor drive gear

10 - Timing gear on the crankshaft z = 35

11 - Slide rail, right

12 - Sprung plunger

13 - Bearing bridge

14 - Rollpin

15 - Duplex roller chain

16 - Chain housing, right

18 - Hexagon head screw M 12 x 1.5

19 - Straight pin, 6 mm diameter

20 - Chain tensioner, right

21 - Hexagon head screw M6

24 - Crankcase half, right

25 - Crankcase half, left

26 - Adapter sleeve

27 - Aluminium washer

29 - Slide rail, left

30 - Chain housing, left

Engine timing

Printed in Germany - 1988

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