Tight and Loose Valve Clearance

Tight and Loose Valve Clearance

Correct valve clearance is also important to maintain an appropriate film of lubrication on the reciprocating valve

Valve clearances must be

train components. Excessive clearance

correct

allows the working faces to separate in operation and the consequential ‘hammering’ action of these components will destroy the protective oil film. Insufficient clearance prevents the formation of the oil film altogether, causing lack of lubrication. Both scenarios will cause excessive wear and premature failure.

Valve Clearance ฀฀ The characteristic ‘tapping’ noise from the rocker cover and associated loss of performance is a clear indicator of excessive valve clearances. Insufficient clearances causes misfire, unevenness of running and loss of performance.

Valve Clearance Dimensions Manufacturer specific information must be used when checking valve clearances. There are variations in clearance values for exhaust and inlet valves, in addition, the engine temperature at which the measurements should be made can be critical factor.

Checking Valve Clearance Generally, it is also important that the engine is correctly positioned in the working cycle for checking the valve clearances, often individually. This is specified by the manufacturer.

Cam Position ฀฀ Often, the valve clearance check is made when the follower is on the back of the cam lobe. To set this position, if it cannot be seen directly, requires positioning of the opposite valve such that it is fully open.

Adjusting Valve Clearance ฀฀ Remove the rocker cover and observe the sequence of the valves opening and closing as the engine is rotated by hand. Check a valve by setting the opposite valve into the fully open position, then check the clearance with a feeler gauge. If adjustment is required, release the lock nut and turn the adjuster slightly, re-check the clearance with the feeler blade which should show some resistance to being moved back-and-forth whilst in the gap, but should also be relatively free to move.

Tappet Noise Rotate the engine into the correct position to check each valve and adjust where necessary. After all the valves have been checked, repeat the procedure

Valves and rockers checking all the individual valves again note the correct values for exhaust and inlet valves have been maintained. Refit the rocker cover and run the engine, listen for tappet noise or any signs of uneven running. If these are no apparent, then the adjustment procedure has been carried out correctly.

Dial Test Indicator Under certain circumstances, wear between the rocker and valve stem prevents accurate determination of the valve clearances using a feeler blade. In

DTI in use this case it is possible to use a DTI (Dial Test Indicator) to measure movement of the rocker

immediately above the valve clearance measurement position.

Replace Camshaft Toothed Drive Belt It is essential to follow the manufacturer procedure when replacing a camshaft drive or timing belt.

Timing Marks The position of the camshaft and crankshaft, relative to each other, is essential to maintain correct timing of the valve opening and closing sequences. Manufacturers usually provide marks for the

valve train pulley system to facilitate this. The

Cam belt

engine must be aligned correctly with respect to these marks before the belt tension is released and the belt removed. The engine must be rotated in the normal direction of rotation when aligning these marks to avoid incorrect adjustment due to 'one tooth out' on the slack side of the belt.

Cam Belt Replacement Certain engines use dowels or plugs to correctly align the camshaft pulleys. These are inserted through holes in the pulleys, into the engine block to

Timing belt ‘lock’ the pulleys in the correct position during

a belt change. Note that oil leaking onto camshaft drive belts will cause failure. Lip seals are generally employed on the crankshaft and camshaft at the non-drive end of the engine where the cam belt is normally situated. At high mileages they can fail and close inspection is advisable during a cam belt change.

Drive Belts The direction of rotation of the belt must be noted and maintained when removed or replaced. The belt structure will settle into a specific rotation direction after Direction arrow use and reversing this can cause fracture of belt teeth and failure of the belt. Most belts are marked with the correct direction for use shown.

Adjusting Belt Tension Adjusting the belt tension is a procedure specified by the manufacturer and should always be observed. This can involve the use of specific tools, for Tensioner example, a belt tension gauge. Always observe the correct procedures and conditions for this task but, a general rule, is that, when correctly tensioned, the belt must not be

twisted more than 90 º at the mid-point of its longest run between 2 pulleys. In addition, it is

important to visually inspect the belt for signs of cracking or perishing.

 Look back over the previous section and write out a list of the key bullet points here:

ENGINES – CUSTOMER CARE Customer Care It is worthy to note that, generally, engine

mechanical problems do not occur frequently during the life of a motor vehicle. Customers can be reassured that as long as they carry out basic routine checks, are observant of their mechanical problems do not occur frequently during the life of a motor vehicle. Customers can be reassured that as long as they carry out basic routine checks, are observant of their

Service Records Premature failure of the camshaft drive belt is a common problem that causes severe damage to the engine. This failure can be avoided if the camshaft drive belt is replaced at the correct interval, in addition, regular inspection and adjustment of the belt will generally highlight any problems so that the belt can be replaced before it fails. Customers will appreciate appropriate tracking of the requirement for this work so that they can be contacted and reminded when it is due.

Belt condition

ENGINE LUBRICATION FRICTION AND LUBRICATION

Introduction ฀฀ All types of vehicle engines incorporate metal parts that have to rub against each other, thus causing friction which creates stress, wear and heat, e.g., cylinders in cylinder liners. These parts require lubrication to prevent the wear, keep the surfaces clean, and help to remove the heat.

Achieving Lubrication ฀฀ Lubrication is achieved by separating the metal surfaces with a film of oil or grease. Thus the lubricating oil in the engine has traditionally been seen to have these three functions: separation, cleaning and cooling.

Oil circulates throughout the parts of an engine

under pressure produced by a mechanical pump. Oiliness and Lubrication ฀฀ Another important property of a

lubricant is oiliness, which can be described as the ability to adhere to the surface of materials and maintain separation of the rubbing surfaces without breaking down. This type of lubrication is called boundary lubrication and occurs in all engines during starting and before the pumped oil feed is established.

Environmental Regulations Modern engines must conform to environmental regulations and modern engine oils are an important component in helping to achieve this.

Engine oil producers are responding with new blends, additives and synthetic oils to enable extended service intervals, improved wear protection, greater engine cleanliness, sludge inhibition, higher speeds and temperatures and lower oil consumption. These oils also contribute to improved performance, economy and environmental concerns about hydrocarbon emissions into the atmosphere. They are compatible with oil-seal materials so that leakage is reduced. They also have strict limits on volatility so that vapours do not escape into the atmosphere.

LUBRICATING OILS

Oil Composition ฀฀ Most engine oils are refined from crude oil to which are added viscosity index enhancers, reduced-friction enhancers, anti-oxygenates, sludge, lacquer and corrosion inhibitors and cleaning agents for carbon, acids and water.

Oil Types ฀฀ Early specifications for engine oils defined just the physical data. New oils, which have to meet environmental and engine-performance requirements, are given specification code letters to indicate the performance level.

Synthetic and Semi-Synthetic Oils Synthetic and semi- synthetic oils have improved performance for environmental or special purposes.

Multigrade Oils Multigrade oils have been developed in order to modify the viscosity index and give thin oils at low temperatures that do not become excessively thin at higher temperatures.

Viscosity Viscosity is a measure of an oil's resistance to flow, i.e., if thin, the oil will flow more easily than thicker oil. A

viscosity index is the measure of a change in an oil's flow rate with a rise in temperature. The higher the viscosity index, the smaller the change in viscosity.

Viscosity and Temperature ฀฀ Manufacturer’s recommended viscosity ratings generally reflect the lowest temperature at which the vehicle is being used and may be different for summer and winter use. The viscosity rating is not an indicator of oil quality but of oil flow under particular conditions. There are some low-grade oils that carry recommendations that limit the use of the vehicle, particularly for high engine speeds, loads and long journeys. Good quality oils will be labelled with at least the API and ACEA service ratings.