Copyright: Autodata Ltd. 1997


Timing Belt Information index

Timing belt replacement intervals

How to use the illustrations

Precautions

Toothed timing belts

Auxiliary drive belts

Troubleshooter

Belt tension measurement


Timing belt replacement intervals

The information relating to timing belt replacement intervals is additional to the main purpose of this manual but is included to provide guidance to garages and for customer advice.

Where possible the recommended intervals have been compiled from vehicle manufacturers' information. In a few instances no recommendation has been made by the manufacturer and the decision to replace the belt must be made from the evidence of a thorough examination of the condition of the existing belt.

Apart from the visible condition of the belt, which is explained fully later in this section, there are several other factors which must be considered when checking a timing belt:

1.	Is the belt an original or a replacement.

2.	When was the belt last replaced and was it at the correct mileage.

3.	Is the service history of the vehicle known.

4.	Has the vehicle been operated under arduous conditions which might warrant a shorter replacement interval.

5.	Is the general condition of other components in the camshaft drive, such as the tensioner, pulleys, and other ancillary components driven by the timing belt, typically the water pump, sound enough to ensure that the life of the replacement belt will not be affected.

6.	If the condition of the existing belt appears good, can you be satisfied that the belt will not fail before the next check or service is due.

7.	If the belt does fail, have you considered the consequences. If the engine is an INTERFERENCE type then considerable expensive damage may well be the result.

8.	The cost of replacing a belt as part of a routine service could be a little as 5 to 10% of the repair cost following a belt failure. Make sure your customer is aware of the consequences.

9.	If in doubt about the condition of the belt - RENEW it.


Precautions

♦	Engine damage - where it is stated that engine damage will result from belt failure it is still possible that by chance no damage will have occurred, therefore before removing the cylinder head check the compressions.

♦	Before disconnecting the battery earth lead, ascertain if the vehicle is fitted with a coded radio. If it is, ensure that the owner has a record of the code or that an auxiliary power supply is connected to the set.

♦	Always disconnect the battery earth lead before starting work.

♦	Remove spark plugs (petrol engines) or glow plugs (diesel engines), to ease turning the engine.

♦	Always turn engine in normal direction of rotation (clockwise - unless otherwise stated).

♦	Do NOT turn the camshaft, crankshaft or diesel injection pump, once the toothed belt has been removed (unless otherwise stated).

♦	Do NOT use timing pins to lock engine when slackening or tightening crankshaft pulley bolt(s).

♦	Do NOT turn crankshaft from camshaft or other drive sprockets via the drive belt.

♦	Do NOT use cleaning fluids on belts, sprockets or rollers.

♦	Ensure that replacement belt has correct tooth profile. Trapezoidal, Curvilinear and modified Curvilinear types are NOT interchangeable.

♦	Do NOT forcibly twist belt, turn inside out, or bend through a radius of less than 25 mm.

♦	Check pulley alignment.

♦	Check free running of auxiliary drives, such as water pump, oil pump and balancer shaft.

♦	Check free running of tensioner and guide rollers.

♦	Always mark the belt with the direction of running, before removal.

♦	Always refit a used belt so that its original direction of running is maintained.

♦	Do NOT lever or force the belt onto its sprockets.

♦	Always check diesel injection pump timing, after replacing drive belt - refer to AUTODATA Diesel Injection Manual.

♦	Observe all tightening torques.Check ignition timing after belt replacement - see AUTODATA Technical Data.


Toothed timing belts

 Since its introduction about 20 years ago the toothed (or synchronous) belt has been increasingly used for driving camshafts, balancer shafts and diesel injection pumps, instead of the traditional roller chain or gears.

1.Construction

These belts are of complex construction (1), being manufactured from a fibre-glass, Kevlar or steel braided laminated inner core, coated with synthetic rubber, neoprene or highly saturated nitrile (HSN) which is wear and heat resistant.

The teeth, which may be curvilinear (2), modified curvilinear (with a more rounded form between teeth) or trapezoidal (3), are moulded integrally, to close tolerances and have a durable fabric facing for long life.

2. 	Rounded teeth

3. Trapezoidal teeth

This combination of design and construction results in a belt that will stretch little in use, requires no lubrication, is relatively inexpensive to manufacture, is almost silent in use and has a very high working efficiency.

Service replacement

Contamination

Inspection

Cleaning

Fitting

Tension


Service replacement

When a replacement interval is recommended by the vehicle manufacturer, this appears as a mileage or time interval in the Recommended Replacement Interval box of each model-related page. These intervals should be strictly observed to avoid the possibility of belt failure and consequential expensive engine damage.

NOTE:	Reference to a special, lower, recommended replacement interval for vehicles used under arduous or  adverse conditions, relates to the following types of use:

•	Taxi operations

•	Permanent door-to-door use.

•	Frequent short trips with a cold engine in low temperatures.

•	Use in hot countries with temperatures often over +30 degrees C.

•	Use in cold countries with temperatures often below -15 degrees C.

•	Use in countries with a dusty atmosphere.

•	Towing a trailer or caravan.

•	Sustained high speed driving.

•	Using poor quality fuel or oil.

If there is no manufacturer's recommended replacement interval, this does not mean that the belt can be ignored, or that it will last forever.

Belts should be inspected at regular intervals and always replaced if their condition is in any way suspect.

Toothed timing belts

Contamination

Inspection

Cleaning

Fitting

Tension


Contamination

In use the belt is protected from oil or water contamination by a cover, but should a seal or hose fail it is possible that damage to the belt could result, and in such cases the belt should be replaced.

The belt should not be allowed to come into contact with petrol, water, or oil and under no circumstances should any solvents be used to clean it.

If any doubt exists about the condition of the belt it should be replaced as the comparatively low cost, in comparison with the engine damage that can result from belt failure, makes this a sensible precaution.

Toothed timing belts

Service replacement

Inspection

Cleaning

Fitting

Tension


Inspection

At the recommended service intervals, and whenever the timing belt is removed, it should be carefully inspected for wear or damage, however minor, which could lead to failure - with possibly expensive results.

WARNING:	In the majority of cases, failure of the drive belt will result in piston and valve contact with resulting engine damage.

Timing belt damage may be visible as cracking or scuffing on the outer surface (4), possibly caused by deposits on the tensioner roller, which often runs on the back of the belt, or by the tensioner binding at some time.  Any such damage should be investigated further, to ascertain possible causes before fitting a new belt.

4.Cracking and scuffing

5. Damaged teeth

The teeth should be checked for signs of cracking or other damage (5), and the sides of the belt also inspected for wear or damage (6) which could indicate that the sprockets on which it runs, may be out of alignment.

Cracking or damage to teeth may indicate that the camshaft or one of the ancillaries, such as the water pump, which may be driven by the belt, has locked up, if only briefly.  Therefore these should be checked before replacing the belt.

6.Side wear and damage

The teeth on the sprockets should also be checked for damage, and cleaned only with a soft brush.

A wire brush must not be used, nor any form of metal scraper. If there is a build up of dirt or dust in the corners of the teeth, this may be gently removed with a soft wooden scraper.

Toothed timing belts

Service replacement

Contamination

Cleaning

Fitting

Tension


Cleaning

Solvents should never be used to clean oil deposits from the surface of the belt, and if in any doubt, the belt should be replaced.

Any cleaning of the belt must be undertaken carefully using a dry soft bristle brush, such as a toothbrush. The belt should be laid on a flat surface and care taken not to twist or crush the belt.

Under no circumstances should the belt be turned inside out for cleaning or inspection. Any maltreatment of the belt could result in premature failure.

Toothed timing belts

Service replacement

Contamination

Inspection

Fitting

Tension


Fitting

When fitting a belt, the tensioner must be released, and the belt slid into place.

It may be necessary to stretch the belt slightly over the first sprocket, ensuring that the timing marks remain aligned.

Under no circumstances should any form of lever be applied to the belt to force it into place. Once fitted, the engine should always be turned in the direction of normal rotation (except in special cases, where instructed in the text), never backwards as this could cause the belt to slip and the timing to 'jump'.

At each stage of belt fitting, carefully check the timing marks are aligned correctly.

Some belts have timing marks identified which will relate to marks on the sprockets (7).  These may be used in conjunction with other timing marks on the engine castings and sprockets, or may be used solely as the timing reference marks.  Again, the specific fitting instructions must be followed.

7. Belt timing marks

Never use the belt to lock the camshaft sprockets when undoing the sprocket retaining bolts, as this will damage the belt teeth.

Use a sprocket holding tool, or the hexagons or flats which are provided on some camshafts for this purpose.

8. Direction arrows

 Certain manufacturers specify the direction of use for the belt, this being identified by arrows on the top surface (8), and should be strictly observed.  If removing a belt for any reason which may be re-used, the direction of rotation should be marked on it, in chalk, for reference when re-fitting.

Any recommended belt checking or replacement intervals should be followed.

Toothed timing belts

Service replacement

Contamination

Inspection

Cleaning

Tension


Tension

The tension of the belt is important and can be set in a number of different ways.

Engine manufacturers issue specific instructions for each application, which are described in each section. These should always be followed, as incorrect tension can lead to premature failure.

In many cases the tensioner operates automatically, but the installation procedure must be carefully followed to ensure that the correct tension is achieved.

Manually tensioned belts normally require the use of a gauge to measure the tension and these are listed under special tools, in each section.

In some cases an alternative, such as the Burroughs tension gauge can be used and a comparison table, between this and some other gauges, is included in this manual.

Toothed timing belts

Service replacement

Contamination

Inspection

Cleaning

Fitting


Auxiliary drive belts

V-Belts

V-belts used for driving auxiliary components, such as alternator, water pump, air conditioning compressor and cooling fan are normally of 'raw-edged' construction.

9. V-Belt construction

 In this type of belt, sub-surface fibres are laid perpendicular to the direction of belt rotation (9), providing a high degree of flexibility, but having extreme transverse stiffness and high wear resistance.

10. V-belt and pulley contact

Drive is transmitted from the crankshaft pulley through the side wall contact between the belt and the pulley 'V' groove (10).

Several belts may be used, where the loads are high and there are several auxiliary components to be driven.

Poly V-Belts

 Introduced in 1979, Poly V (or Micro-V) belts are now fitted to an increasing number of engines.

 These belts are considerably wider and thinner than their V-belt counterpart and usually have between three and six ribs (11).

Additionally the back of the belt can be used to drive components or for tensioning purposes.

 Due to its reduced thickness, compared to the traditional V-belt, it can drive smaller diameter pulleys, resulting in the so-called 'serpentine drive' layout (12), in which all the auxiliaries are driven by one long belt.

11. 	Poly V-belt construction

12. Typical serpentine drive belt layout


Troubleshooter

Symptom	Probable Cause	Remedy
Broken belt	Foreign body in drive Excessive tension Belt crimped during installation	Ensure cover is fitted correctly Set tension correctly Avoid mishandling belt
Sheared teeth	Insufficient tension Seized drive sprocket Sprockets misaligned	Set tension correctly Remedy seizure Align correctly
Worn teeth	Incorrect tension Worn sprockets	Set tension correctly Replace sprockets
Hollow teeth	Pre-set tension very low Running tension too low	Set tension correctly Ensure tensioner operating correctly
Top surface cracks	Excessive heat Tensioner/guide binding	Check cause Free tensioner/guide
Land wear	Excessive tension Rough sprocket surface	Set tension correctly Replace sprockets
Oil contamination	Engine oil leak	Remedy oil leak
Edge wear	Sprocket flange damaged Sprockets misaligned	Replace sprocket Align correctly
Noisy operation	Excessive tension Insufficient tension Sprockets misaligned Sprocket flange damaged	Set tension correctly Set tension correctly Align correctly Replace sprocket


Belt tension measurement

Vehicle manufacturers specify a wide range of different gauges for checking the tension in timing belts.

It is not possible to directly measure the tension in an installed belt so most tension gauges measure belt deflection for a known load or, in some cases, the load for a known deflection. A few manufacturers specify special tools which are essentially a torque arm to load the belt tensioner by a predetermined amount.

To assist workshops offering all-makes service Autodata has prepared a comparison chart for a range of the gauges specified by different car manufacturers. Using this chart it is possible to convert readings from one gauge to another, enabling one gauge to be used for a variety of makes.

The tests were carried out by comparing known belt tensions with a Burroughs gauge, calibrated in Newtons and other gauges calibrated in arbitrary units.

The restricted ranges on some gauges has reduced the readings available for comparison.

With any tension gauge it is important to take care when using the gauge, following the manufacturer's instructions.

It is often difficult to obtain consistent readings with these gauges and it is good practice to take several readings to establish the correct setting.

Tension gauge comparison

Burroughs (BT3373F/ BT33-86J)	Sykes- Pickavant 316690	SEEM C.Tronic G2 105.5	SEEM C.Tronic 87 (Pin S)	Lowener Ford 21- 113 & Vauxhall 510-2	Peiseler VAG 210, Volvo 5197 & SEAT U.10.028	BMW 11.2.080* & Volvo 9988500*	Burroughs/ Rover KM 4088AR
Newtons	Kg	Units	Units	Units	Units	Units	Units
50			13
100			22,5
150		33	31,5			28
200	16	40	38,5		11,7	35	1,7
250	20	47	46	4	12,5	41,5	2,7
300	25	53	52	7,4	13,1	46,5	4
350	29	61	58	9,6	13,4	51	5
400	35	67	63	11	13,6	53	6
450	40	72	68,5	12			7
500	46	78	73,5				7,5
550	51						8
600	58						8,5
650	63						9
670	65

*NOTE: This gauge is sensitive to belt thickness and should only be used as recommended by the vehicle manufacturer.


How to use the illustrations

1  Illustration identification number

2  Abbreviated sprocket identification

3  Tightening torque - Newton metres

4  Detail of timing marks

5  Timing belt tension checking point

6  Number of teeth between sprockets (where applicable)