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Glossary of terms you will come across during your Phase 2 training.


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V

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Space void of air.

Atmospheric pressure will always seek to fill this space with air.

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Valve with parts namedThis is the angled area on the valve head, which forms the gas-tight seal with the cylinder head. It is normally cut at an angle slightly different to that of the valve seat. For example, if the land is cut at an angle of 450, then the valve seat would be cut at an angle of, say, 45.50. This ensures narrow valve seats.

Valve land and valve face

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Valve land and valve faceThis is the vertical part of the valve head, immediately below the angled part of the valve that forms the gas-tight seal with the valve seat when the valve is closed.

During valve overhaul and repair, it may become necessary to re-grind the valve face in order to produce a surface suitable for lapping and subsequently producing a gas-tight seal with the valve seat.

You must be careful not to remove too much metal from the valve face, as this reduces the amount of metal in the valve land. There is a danger that the small amount of remaining metal can become incandescent in the combustion chamber and cause pre-ignition, which can cause catastrophic damage to the engine.

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Valve timing chart showing valve lead, valve lag and valve overlapValve lead is where an inlet or exhaust valve in an engine opens early. For example, the inlet valve will open early during the power stroke-valve lead.

Valve lag is where a valve stays open longer. For example, the inlet valve stays open long after the inlet stroke is finished. By doing this, we can use the momentum of the incoming air to continue to fill the cylinder, even when the piston is coming up on compression.

Valve overlap is where both valves are open together, as they are at the top of the exhaust stroke.
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Check valve guide for wearThe valve seat is in the cylinder head of a four stroke engine. There are at least two seats (often, there are more), one for the inlet valve, and one for the exhaust valve. The inlet valve and seat tend to be the bigger of the two. It is vital that the valve face and the valve seat form a gas-tight seal.

Valve seat

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Valve stem oil sealThis is a component that goes over the end of the valve stem. It keeps excess oil from getting between the valve guide and the valve stem.

If the valve stem oil seal leaks or the valve stem wears, lubricating oil can get down the valve stem into the combustion chamber, where it will be burned. This causes smoky emissions from the engine.

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Gear timing marksThe process whereby the camshaft is timed to the crankshaft, so that the inlet and exhaust valves open at the right times. Camshafts are driven by the crankshaft by means of a belt, chain or gears. With each of these systems, there is some sort of marking system on the gears, pulleys or sprockets to assist you in correctly timing the two shafts.

As a general rule of thumb, in a four cylinder engine, you can get the crankshaft at tdc by having the keyway pointing directly upwards, at the '12 o'clock' position. With the crankshaft at this position, if you get number 4 cylinder rocking, that means number 1 cylinder is firing. With both shafts in these positions, they are timed relative to each other.

Note: This is only a general rule of thumb. Many modern engines use specific methods and specific markings both on gears, sprockets and pulleys, as well as on the engine block, to set up valve timing. Always consult the manufacturers specifications, because if you get it wrong, it can result in serious, expensive, damage to the engine.

Once you time an engine, always turn it over two full turns by hand. This is to ensure that pistons and valves are not colliding due to incorrect timing. Doing this will not prove your valve timing is correct, but it will prove the engine can turn over freely. The reason why we turn it twice is that, in a four stroke engine, each stroke and each opening of a valve is going to happen at some stage during those two turns.

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Electronics supplier of components and instruments such as the PPS10 and HPS10 hand held oscilloscopes.

Velleman website

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In physics, velocity is defined as the rate of change of position. In order to properly define it, both speed (the rate at which a body undergoes displacement) and direction are required.
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Venturi

A fitting or device that consists of a tube constricted in the middle. A fluid's velocity will increase and a fluid's pressure will decrease while passing through the constriction. (Bernouli's Principle)


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