Sep 262013

Don’t know what a camshaft does? Not sure what the difference is between a brake shoe and a brake pad? You’ll find what you need to know here.

(Please note: the terms and definitions listed here are fairly automotive specific. Some of these terms have other meanings outside of the car world; I’ve ignored those definitions to keep things simple.)


Term Also Known As Description See Also
ABS Anti Lock Brakes A system fitted to more modern cars (generally 1990 or so onwards), ABS monitors and controls the braking system on each wheel, releasing brake pressure when it senses a wheel is about to lock. The upshot of this is that wheels do not lock under heavy braking, allowing for better stopping distances and improved control under heavy braking.
Alternator Fitted to the engine, an alternator serves to charge the vehicle battery. Usually found on more modern cars in place of a dynamo due to better efficiency and lighter weight.


A standard alternator as fitted to many classic and modern cars.

Antifreeze Coolant A solution added to liquid cooling systems in engines, this has the effect of lowering the freezing point of water (preventing engine damage such as a split hose or even a cracked cylinder head during winter months), as well as preventing boiling in hotter temperatures (Which can also cause engine damage). The additives also help to prevent corrosion.
Axle A shaft on which a wheel, or pair of wheels, turns. Commonly found at the rear of vehicles connecting the two rear wheels together (either driven wheels or not), although a short axle holding a single wheel (commonly called a ‘stub axle’) is usually found at the front of the car.
Balljoint A spherical joint, usually used for connecting the front suspension assembly to the chassis. A balljoint is used due to the fact that it can rotate as well as having movement in multiple directions, making it ideal for steering assemblies.

Balljoints on suspension

The balljoints can be seen at the top and bottom of this suspension setup.

BHP Brake Horsepower Horsepower is a unit of power measurement; originally made to compare the work output of early steam engines to the draft horses commonly used at the time. Brake Horsepower is determined from the force exerted on a friction brake or dynamometer. Note, however, that the figures quoted are usually for the engine alone – as much as 30% of this power may be lost in the drivetrain before it actually gets to the wheels!
Big End This refers to the end of the connecting rod that connects to the crankshaft.
Boot Trunk The compartment at the rear of the car (or the front of the car in the case of rear engined cars!). In Britain it’s called the boot, in the USA it’s called the trunk. Same thing though.
Brake Booster A system attached to the master cylinder of the brake system that is used to reduce the amount of pedal pressure needed to apply the brakes. The brake booster usually uses vacuum from the engine to boost the force applied to the master cylinder. If you have problems with a hard, heavy brake pedal, this is a good place to look for problems.
Brake Caliper Part of a disc brake setup, the caliper bolts to the axle and houses the brake pads, which sit on each side of the (spinning) brake disc.It employs a piston to squeeze the pads together and ‘clamp’ the disc when the brakes are applied.

Brake Caliper

A typical brake caliper as fitted on a disc brake setup. The caliper has been removed to show the brake pads fitted against the disc.

Brake Disc A metal disc that spins along with the road wheel, this brake disc is clamped by the brake pads to slow or stop the vehicle. Usually seen on the front of cars, although sometimes on all four wheels in sportier cars, or not at all in older cars (where drum brakes all round are more common).
Brake Drum A metal drum used in braking (usually seen on the rear of cars, although sometimes on all four wheels in older cars), this drum spins with the wheels and is clamped by the brake shoes to slow or stop the vehicle when the brakes are engaged.
Brake Pad Part of a disc brake setup, brake pads sit on each side of the brake disc and are squeezed against the disc by the caliper. They are made from a high friction material that grips against the metal disc to slow the car down.
Brake Shoe Part of a drum brake setup. The brake shoes are pushed against the inside of the drum by the wheel cylinder. Made from a high friction material to grip against the metal drum to slow the car down.
Breather Crankcase Breather; Crankcase Vent A method of allowing gases to escape the inside of an engine; pressure tend to build up as a result of combustion gases leaking through to the lower crank area (a process known as blow-by). The vent, or breather, allows this to escape without allowing dust or foreign objects inside the engine.
Bush A Favourite for MOT failures! A bush is a hard rubber object that fits between two components (such as suspension components) that allows a small degree of movement between them, reducing noise and vibration. These tend to perish with age and use; bushes are now available in polyurethane (‘polybushes’) which while expensive do last a lot longer.

Suspension bushes

Two suspension bushes – old and new. The one on the right was completely shot and causing some obvious knocking noises!

Camshaft A shaft that is connected (via belt or chain) to the crankshaft of an engine, this shaft controls the opening and closing of the inlet and exhaust valves at the correct time in the combustion cycle.
Carburettor Carb A device that mixes fuel and air into a ‘mist’ before it goes into the engine for combustion. This also controls the throttle and choke operations. Made obsolete in more modern cars by fuel injection systems.
Castle Nut A particular type of nut that has crenellations (indents at regular intervals) along one edge. This allows for a split pin or wire to go through a hole in the bolt or shaft to prevent the nut loosening. Commonly seen on wheel hubs.
Catalytic Converter Cat Part of the exhaust system in more modern cars, usually situated before the silencers, that chemically reacts with the exhaust gases to reduce harmful emissions. Made from some fairly exotic materials such as platinum so can be quite expensive to replace.
Chassis The frame of a vehicle. In older cars, this consisted of a ladder type frame that the suspension, drivetrain and body was bolted to. In more modern cars a ‘unibody’ construction is used where the body and chassis are all part of the same unit and as such are inseperable.

Chassis with Suspension

A bare chassis with suspension fitted.
Source: Wikipedia

Choke Seen with older cars with carburettors (rather than fuel injected cars), the choke acts to partly close off the airflow into the carburettor, making the fuel/air mixture richer than normal. This allows for easier starting and running when the engine is cold.
Clutch A mechanical device that attaches to the flywheel and acts to separate the engine and gearbox when engaged. Operated by either a cable or a hydraulic system similar to the brake system.
Coil The first part of the HT (High Tension) ignition system of the engine, this device changes the 12v battery current to a higher current of thousands of volts. This is then fed via the distributor into the spark plugs.
Compression Part of the combustion cycle, this is when the piston moves upwards in a cylinder to compress the fuel/air mixture before ignition by the spark plug.
Condenser (Ignition) Capacitor A part of the ignition system, this is used to remove power spikes in the HT ignition system that could potentially damage the car’s circuitry.
Conrod Connecting Rod The component in an engine that connects the pistons to the crankshaft.
Crankshaft Crank The part of the engine that all the pistons are connected to (as well as the camshaft), this consists of several ‘throws’ (parts of the shaft offset from the centre) that the pistons connect to; this causes the up and down movement of the pistons.


A typical crankshaft for a 4 cylinder engine.

CV Joint Constant Velocity Joint A joint that allows power to be transmitted through it no matter what the angle of the joint, without any loss of power or increase in friction. Commonly used in driveshafts, especially those in front wheel drive cars.
Cylinder In engine terms, a cylinder is what the piston travels up and down in. For hydraulics such as brake and clutch systems, a cylinder can be either ‘master’ or ‘slave’ – a master cylinder acts on a pedal to push fluid down to the slave cylinder, which then pushes a piston out to activate the device (clutch, brake caliper etc) at the far end.
Cylinder Head The upper part of an engine, this sits on top of the crankcase. It usually contains the inlet and exhaust valves and in the case of ‘OHC’ engines also contains the camshaft. Typically this is where the inlet and exaust manifolds attach.
Differential A device that makes up part of the vehicle powertrain, this has an input from the gearbox and an output to each side of an axle. The differential allows each wheel on an axle to rotate at different speeds – this is essential for cornering where the wheel on the inside of the turn needs to travel a shorter distance (and hence slower) than the outside wheel.
Distributor Part of the ignition system, this device allows HT current from the coil to connect to each of the spark plugs in turn, and at the correct time in the ignition cycle.


A typical distributor unit. This one is for an eight cylinder engine, as can be seen from the eight lead connections.

Drive Train The system of components that generates power and transmits it to a vehicles wheels. This includes the engine, gearbox, driveshafts and/or propshafts and the differential.
Drivebelt A belt (some cars may have more than one) that connects the engine crankshaft (via a pulley) to ancillaries such as the alternator, water pump, power steering pump, aircon pump etc.
Driveshaft A shaft used to connect a driving wheel to the gearbox (or more correctly, the differential). Commonly has a CV joint at each end.
Dynamo Replaced in most newer cars by the more efficient alternator, a dynamo attaches to the engine (usually by a drivebelt) to charge the vehicle battery.
Earth Ground Not just a posh name for mud! In electrical terms, earth is basically a return path for electrical current (often electrical circuits are literally earthed to the ground, hence the name). In automotive terms, in most metal-bodied cars, the car body itself is the earth point. Rather than have a wire going all the way back to the battery on each circuit, the earth (or negative) wire is connected to the vehicle body, and a cable is then connected from the body to the battery. Beware though; some older cars work the other way around and the positive connections go to the body (known as ‘positive earth’).
Exhaust Muffler Generically, this is the term for the system that expels used gasses from the engine after combustion. Exhaust valves open inside the engine to release these gasses at the correct moment; these are then forced through a manifold and into the tubular exhaust pipe. The silencers (or mufflers, in the USA) serve to reduce the noise made from the engine; modern systems will also include a catalytic converter to reduce harmful emissions.
Fender Wing The body panels at each corner of the car, where the wheelarches are located. Known as wings in the UK, fenders in the USA.
Filter A device used to filter foreign particles (such as dirt and metal particles) from various fluid and air systems. In an average car you’ll usually find; air filter (to remove dirt etc from outside air before entering the engine), fuel filter (to remove dirt and rust flakes from fuel between the tank and the engine) and an oil filter (to remove metal and other harmful particles from engine oil as it circulates through the engine). All these filters need replacing (although some can be cleaned) at regular intervals.
Flasher Unit Flasher Relay An electrical device that regulates the flashing of your indicators/hazard lights. If your indicators don’t flash or behave weirdly, this is probably the place to look.
Flywheel A large and heavy metal disc that is connected to the engine crankshaft. The clutch is attached to the flywheel and the flywheel also incorporates a ‘ring gear'; a toothed ring around the outer edge that is engaged by the starter motor to turn the engine over.


An engine flywheel. The shiny circle in the middle is where the clutch plate connects. Not the ring gear around the outside.

Fuel Injection A system used to regulate, and deliver, the fuel/air mixture into the engine. This has largely replaced the carburettor in all modern cars due to better performance and efficiency. Most systems now are electronic, however there are some cars around (such as the Lucas PI system fitted to many Triumphs and Jaguars) that use a mechanical injection system.
Fuel Pump A device for pulling (or pushing) fuel from the tank to the carburettor/injection system. Usually engine-driven and located in the engine bay in older, carburettored cars, while more modern fuel injected systems are operated electrically and are usually located near (if not actually in) the fuel tank.
Gaiter Boot A rubber sleeve that fits over a joint, this serves two purposes: It keeps grease in the joint, and keeps dirt and foreign matter out. Another common MOT failure when these split! Commonly found on driveshafts and balljoints (if a balljoint gaiter is split it’s usually easier to replace the whole joint).
Gasket A sheet of thin material that sits between two mating faces (eg a water pump bolted to an engine block) to ensure a seal that may not be possible by just putting two metal surfaces together. Depending on the use, these may be made from rubber, metal, cork or paper – or sometimes a composite of two or more materials.
Gearbox Transmission An assembly of gears fitted to the engine (sometimes via a propshaft) that change the speed of the roadwheels in relation to the engine. Can be either manual or automatic. Called a gearbox in the UK, transmission in the USA.
Gudgeon Pin Wrist Pin A sliding pin that connects the conrod to the piston.
Hardtop This has two meanings; in terms of car bodies, a hardtop simply describes a car that has a solid metal roof (rather than a soft removable one). Additionally, hardtop can also describe a solid (often glassfibre or similar), detachable roof fitted to a convertible car in place of the usual soft roof.

Triumph Spitfire Hardtop

A hardtop fitted to a Mk1 Triumph Spitfire.

Heater Matrix A radiator fitted to the cooling system of a car; instead of serving to cool the engine, however, this is fitted in the passenger compartment (usually behind the dashboard) and serves to heat the passenger compartment, usually with a fan blowing air through it.
Hood Bonnet The body panel that covers the engine bay. Called a bonnet in the UK, a hood in the USA.
Idle The engine speed that an engine should run at when in neutral and with no throttle applied – in most cars this is around 700 – 1000RPM (Revs Per Minute)
Ignition This can refer to three things; 1) the key switch used to turn on and start the car, 2) the act of the spark plug igniting and burning the fuel/air mixture in the engine, 3) the complete electrical system involved in creating the spark at the spark plug.
Judder Shudder Shaking and/or rapid vibration. Often felt through the steering wheel at certain speeds (pointing to unbalanced wheels or incorrectly adjusted steering) or while braking (pointing to warped brake discs).
Kingpin An older type of steering pivot fitted to the front hubs of vehicles. The disadvantage of this type of pivot is that it blocked off the centre of the wheel hub, making it impossible to have front wheel drive on a car with this system. Balljoints solved this, along with other problems, and kingpin type suspension is now pretty much obsolete (except in some commercial/heavy vehicles where it still has some advantages).


A Kingpin setup seen here on a Ford Model T. Note how this arrangement passes through the centre of a wheel, making it impossible to fit a driveshaft to this wheel.

Manifold Header An exhaust part, fixed to the engine, that collects exhaust gasses from the multiple cylinders and outputs it into one, or sometimes two, pipes. The name comes from the Anglo-Saxon manigfeald (Many Fold).
Master Cylinder Found in hydraulic systems such as the brake or clutch, the master cylinder acts on an input (such as a pedal) to force hydraulic fluid down a pipe, causing the slave cylinder to push a piston out.
MIG Mig Welding Stands for Metal Inert Gas. This is a type of welding, common to the hobby and aftermarket/vehicle repair industry. The name comes from the type of gas used (usually a mix of carbon dioxide and argon) to shield the weld from contaminants in the air.
NOS Can stand for Nitrous Oxide, a gas injected into an engine to increase performance, or (more usually in the classic car world) New Old Stock – basically, this refers to components and spare parts that are unused and often still packaged (hence ‘new’) but were manufactured many years ago (often these are as old as the cars they’re for!). While some items such as rubber seals can degrade over time, many NOS items are preferable to modern ‘pattern’ copies due to superior manufacturing quality.
OEM Original Equipment Manufacturer. This refers to a part or accessory that has been manufactured by the the same company that made the original part as fitted to the vehicle. This is in contrast to a ‘pattern’ part, which is one that has been manufactured by a different company – while it may fit and operate in the same manner as the original part it may be done to a different (often inferior) standard.
OHC Overhead Camshaft. This is a type of engine design where the camshaft is located at the top of the engine (in the cylinder head, or sometimes in a separate housing above the head) and connects directly to the inlet and exhaust valves.
OHV Overhead Valve. This is a type of engine design where the inlet and exhaust valves are located at the top of the cylinders, facing the piston. This is different to the OHC design due to the location of the camshaft, which in OHV engines is normally located lower down in the engine and connects to the valves by means of pushrods.
Oversteer Oversteer occurs when a vehicle turns more than the amount expected by the driver. This is more common in rear wheel drive cars. Taken to the extreme, oversteer can result in the rear of the car spinning out.
Percussive Maintenance A fancy term for hitting something with a hammer!
Pillar The pillars between the car windows that hold the roof up. The ‘A’ Pillars are at the front at each side of the windscreen, the ‘B’ Pillars are behind the front doors and the ‘C’ pillars (if present) are behind the rear doors.
Pinking Pinging, Knocking, Detonation Pinking occurs when the temperature or pressure inside a cylinder is high enough that ignition can occur without the spark plug. Typically identified by a metallic pinging noise or a knocking noise, if left to continue it can cause sometimes catastrophic damage to the engine.
Piston Connected to the crankshaft, the piston moves up and down within the engine cylinder, either pushed by the crank itself or forced down in response to combustion, causing the crankshaft to move.
Points (Ignition) Contact Breaker Located within the distributor and driven by the cam inside, the points serve to interrupt the current in the low tension ignition circuit; this in turn causes a high voltage pulse in the ignition coil, creating the spark at the spark plug.

Contact Breaker Points

A set of contact breaker ‘points’ as fitted into a distributor.

Power Steering A hydraulic (or sometimes electric) system that augments steering effort from the driver. This is supplementary to the main steering system; in the case of failure of the power steering system, the driver can still operate the steering although this will be much more difficult – the driver will have to force the power steering system to operate as well as moving the roadwheels.
Propshaft Similar in concept to the driveshaft, the propshaft is used in rear wheel (or four wheel) drive vehicles to transmit power from the gearbox or differential to the rear axle. Can be fitted with either Universal Joints or CV joints, or a combination of both.
Radiator Part of the engine cooling system, the radiator is usually located at the front of the vehicle and cools the coolant system by means of air flowing through it. In slow moving or stopped traffic a fan (either electric or engine-driven) is used to force air through the radiator.
Relay An electrical switch. Usually operated with an electromagnet, a relay is used where it is necessary to operate a high power circuit with a low power switch – for example, the high powered headlight circuit is operated by means of a relay to avoid the need to have heavy duty wires and switches in the dashboard. A relay can also be used to operate several circuits at once with the use of a single switch.
Rocker Arm Rocker A lever that connects the pushrod in an overhead valve engine to the top of the inlet or exhaust valve.

Rocker Arms

Rocker arms as fitted to a pushrod (OHV) engine. Valves on the left, pushrods on the right.

Roll Cage Roll Bar A metal frame bolted to the passenger compartment of a car that protects occupants in the event of a collision – particularly when the car rolls over. A Roll Bar is a single bar located behind the driver, often fitted to convertible cars, to provide protection in the event of a rollover. Some more modern convertibles utilise a strengthened windscreen frame to act as a roll bar.
Scuttle Shake Cowl Shake Commonly experienced in convertible cars, this phenomenon occurs when, due to lower structural rigidity, the chassis flexes in the middle, causing the bulkhead to vibrate when the car is moving. This is often felt as a noticeable vibration.
Shim Spacer A thin piece of material that is used to fill in small gaps between objects to make a closer fit. Often used to adjust spacing in tappets on older engines.
Sill The parts of the vehicle body between the wheelarches on the outside, underneath the doors. In most cars with a unibody structure these provide structural integrity and stiffness. Well known for rusting through on most older cars.
Slave Cylinder The secondary part of a hydraulic system (such as brake or clutch systems), this acts on pressure from the master cylinder to operate a piston, pushing on a lever or similar to engage or disengage another component. A brake caliper or wheel cylinder can be considered to be a type of slave cylinder.
Soft Top The roof of a convertible car, usually made from some form of cloth or vinyl. Also used to refer to convertible cars themselves.
Solenoid Part of the vehicle starting system, this can be mounted directly to the starter motor itself or mounted remotely in another part of the engine bay. It has two inputs; a high current input (usually directly from the car battery) and a smaller, low current input from the vehicle ignition switch. When the low current input is activated this causes a connection from the high current input to the (high current) output that then goes to the starter motor itself.

Starter Motor

A typical starter motor: note the solenoid – the smaller cylinder on top of the motor.

Spark Plug The part of the ignition system that actually delivers the spark that combusts the fuel/air mixture in the cylinder.
Split Pin A type of fixing, a split pin goes through a hole in a shaft or bolt and is then bent out so it cannot be removed; this then stops anything behind it from coming off the shaft. Often used in conjunction with a castle nut.
Strut This can refer to 1) a gas strut, usually used to hold boot lids and tailgates open, or 2) a suspension strut, also known as a MacPherson strut, which consists of a shock absorber and spring in a single unit.
Sump The lower part of an engine, this is where engine oil collects when the engine is not running. The pickup for the oil pump is located here.
Suspension A system of springs and shock absorbers that connect the roadwheels to the car body. This serves two purposes; firstly to improve the handling and roadholding of the vehicle, and secondly to insulate the passengers from road noise, bumps and vibration.
Tacho Rev Counter, Tachometer A gauge used to display the engine speed in RPM (Revolutions Per Minute). While fairly common in modern cars, in older vehicles these were generally only fitted to high end or sports cars.
Tappet Cam Follower Part of the engine, the cam follower sits between the cams (on the camshaft) and either the valves or pushrods. While the term ‘tappets’ is commonly used, this is actually incorrect. ‘Adjusting the tappets’ is a term that actually means adjusting the valve clearances between the cam (or rocker) and the valve top – either by shims or with an adjusting bolt. More modern engines use hydraulic lifters that adjust these clearances automatically.

Adjusting Tappets

Adjusting tappets (valve clearances) with the aid of a feeler gauge.

Tensioner A device that applies force to a chain or belt to maintain tension and keep it to the required tightness. Usually takes the form of a spring but can be a hydraulic unit. Commonly used to keep tension in drivebelts and timing belts.
Thermostat In engine terms, a thermostat serves to control coolant temperature in a cold engine – when the engine is below a certain temperature the thermostat stays closed. This blocks the flow of coolant to the radiator, keeping the heat in the engine area to allow the engine to reach operating temperature as quickly as possible. Once a set temperature has been reached the thermostat opens, allowing coolant to flow through the radiator to cool the engine.
Timing (Engine) Distinct from ignition timing, engine timing is a process of aligning the camshaft with the crankshaft (connected by a belt or chain) so that the valves open and close at the correct time to correspond with the position of the piston. Incorrect timing will cause poor running (or no running at all) and significantly misaligned timing (for example if the belt or chain snaps) can cause damage to the engine.
Timing (Ignition) This is a process where the timing of the ignition spark is set such that the spark happens at a precise time in the combustion cycle – usually just before the piston has reached Top Dead Centre – to allow for the correct combustion. In older engines this is usually set by means of a timing strobe light and twisting the distributor to set the timing correctly.
Timing Belt Timing Chain This belt or chain connects the crankshaft to the camshaft and keeps the camshaft turning in sync with the crankshaft. While the timing belt is more commonly seen in modern cars, the timing chain has been used for many years and is still in use in engines made today. Whether belt or chain, these need to be replaced at regular (albeit long) intervals – in many engines a broken belt or chain can cause catastrophic damage to the engine.

Timing Chain

These may be a lot of work to replace, but if left they can break and wreck your engine so it’s best to replace when needed.

Tonneau Cover A Tonneau cover can refer to either the fabric cover that covers up a folded and stowed soft top roof when opened, or can also refer to a cover that extends over the folded roof and across the passenger compartment of the car, giving weather protection with the roof down. Some of these covers can also cover the passenger side of the car, leaving the driver compartment open for partial protection when driving.
Torque A measure of the turning force on an object (for example an engine or torque wrench), measured in either newton metres (Nm) or pound-foot (lb-ft). Not to be confused with horsepower!
Torque Steer The effect of engine torque on steering, manifested in the steering pulling off to one side under heavy acceleration.
Trafficator A predecessor to the indicator, trafficators are turn signals that physically extend from the bodywork of the vehicle when operated. Pretty much phased out since the 1950s due to legislation requiring modern flashing indicators.
Transaxle A combined gearbox and differential. Almost universally fitted to front engine, front wheel drive cars but also seen on a few front engine, RWD cars – having the gearbox in the rear makes for great weight distribution.
Trunnion Part of the suspension in many older cars that allows for free movement of the wheel hub in relation to the chassis.
Tub Body, Body Tub The actual body of a vehicle, one that is separate and removable from the chassis. Particularly refers to just the passenger compartment in cars such as the Triumph Spitfire which does not have an integrated engine bay.
Turbo Turbocharger A method of forced induction, a turbo uses exhaust gasses to spin a turbine in the air inlet to force air into the engine at higher than atmospheric pressure. This has the effect of increasing engine power over what would normally be possible with normally aspirated engines.


A turbocharger unit. Note the turbine in the middle which acts to push air into the engine.

Understeer Understeer occurs when a vehicle turns less than what was expected by the driver. This is most common in front wheel drive cars.
Unibody A more modern technique of car design, a unibody vehicle is one that has the car body and the chassis integrated into one inseparable unit, rather than the separate chassis and body system seen in older cars. With the exception of some specialist sports cars and kit cars, virtually all modern cars today are unibody.
Universal Joint UJ, Hardy Spicer Joint A type of joint that can transmit rotational power while allowing the shaft to bend. Commonly seen in propshafts, although more modern cars tend to use CV joints instead.
Valve In engines, a valve is a component which, when controlled by the camshaft, opens and closes to allow or block either the fuel/air mixture or burnt exhaust gases at the appropriate stage in the combustion cycle. In general terms, a valve is a device that controls the flow of a liquid or gas (think of a tap – that’s a type of valve!).
Wheel Cylinder Part of a drum brake setup. The wheel cylinder pushes pistons out when the brake pedal is depressed which forces the brake shoes to push against the drum, slowing the car.

This article is a part of the Servicing & Maintenance series of Howto’s.

 September 26, 2013  Posted by at 10:36 pm Servicing & Maintenance Tagged with: , , ,  Add comments

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