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

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A science Master's Degree.

Some typical values:

  • Diagram of bolt with explanation of measurements.M7 x 1.25
  • M8 x 1.25
  • M8 x 1.5
  • M14 x 1.5

These all refer to dimensions of metric fixings - nuts and bolts.

The M means metric, and the number refers to the diameter of the fixing in mm.

The numbers 1.25 and 1.5 refer to the pitch of the thread on the fixing. Fine threads have a pitch of 1 or 1.25, and coarse threads have a pitch of 1.5.


As a piston is pushed down the cylinder on its power stroke, the piston will meet resistance as it tries to turn the crankshaft.

The greater the load on an engine, the greater this resistance will be. The resistance will cause the piston to be pushed to one side with considerable force, as it is also pushed down the bore by the hot expanding air/fuel gasses above it.

This side force is known as thrust, and it causes one side of the cylinder walls to wear at a faster rate than the rest of the cylinder wall. The side of the engine where this thrust acts is known as the thrust side of the engine.

Because there is extra wear on this side of the engine, the connecting rod will often have an oil squirt hole in it, so that oil under pressure from the crankshaft can be squirted on to the cylinder wall on the thrust side, in order to minimise wear and remove excess heat.

That is why it is so important to note any mark on the crown of the piston. Such marks will generally point toward the front of the engine, or one particular side of the engine. For example, in the case of the Lifan 5 HP engine, the mark on the crown of the piston points toward the camshaft side of the engine.

It is also important to note that as well as a major thrust side, each engine also has a minor thrust side, which is opposite to the major thrust side. As the piston moves up on its compression stroke, and it meets the resistance of the fresh air/fuel mixture, this resistance will tend to force the piston off too one side as it moves upward on its compression stroke.

See also the glossary explanation for minor thrust side for more information.

Major thrust side of an engine

Materials under compressive loads must have malleability. This means the material can undergo significant deformation without breaking.

This is when talking about metal or other materials when they are able to be hammered or pressed into shape without breaking or cracking.

Copper hammerFor example, copper is malleable. That is why we use copper hammers when we need to apply large forces to components without damaging them. For example, say we have a roller bearing that needs to be hammered into a casing or housing. If we use a steel hammer against the steel bearing, we run the risk of damaging the bearing. In order to avoid damage, we use a copper faced hammer, because the copper is malleable.

Other words that mean the same thing:

  • pliable
  • ductile
  • pliant
  • soft 
  • workable
  • shapable
  • mouldable

Mark on the crown of the piston to ensure correct installation.Pistons generally have some sort of mark on their crown, to indicate which way the piston should be reassembled in an engine.

Depending on whether the engine is 2-stroke or 4-stroke, the marks serve slightly different purposes.

In 4-stroke engines, generally speaking, the mark will point toward the front of the engine, or toward one particular side.

For example, in the Lifan 5 HP engine, the mark on the crown of the piston points toward the camshaft side of the engine.

The purpose of the mark is to ensure the piston is installed the correct way around, thus ensuring that the oil squirt hole in the connecting rod is pointing toward, and lubricating the major thrust side of the engine.

In 2-stroke engines there is a slight difference. Again, generally speaking, the mark on the piston points toward the exhaust port. This ensures that the piston ring end gaps never align with a port. Because the rings exert great radial forces (in order to achieve a gas-tight seal), if they were to uncover a port, the ring would be inclined to spring into that port. As the piston continues its movement, the end of the ring would be snapped off.

For that reason, 2-stroke pistons have small pins which positively locate the piston ring end gaps. Then, once we observe the mark on the crown of the piston, and reassemble the piston accordingly, the end gaps will never be in line with a port, and the rings will never be damaged.


Mark time, space time and duty cycle.In a digital circuit, Mark Time (Tm) refers to the length of time the circuit is switched on (high), and Space Time (Ts) refers to the length of time the circuit is switched off (low).

The relationship between these two times is referred to as the 'duty cycle'. For example, as in the diagram shown, an astable circuit produces a square wave. If that circuit is switched on for as long as it is switched off, the duty cycle is said to be 50%.


Training Adviser, Dundalk.

  • Mobile: 087 211 7489


  1. Eamonn Boylan, New Boliden Tara Mines Ltd., (g2cp12b)
  2. Eoin Lambe, QME, (g2cp12b)
  3. John McGovern, New Boliden Tara Mines Ltd., (g1cp12b)
  4. Jason Sheeran, New Boliden Tara Mines Ltd., (g1cp12b)
  5. Matthew Walsh, New Boliden Tara Mines Ltd., (g1cp12b)
  6. Fergal Weldon, Navan Hire and Hardware Ltd., (g2cp12b)


Company Contact for ECI JCB.

Telephone: 01 5008700


The measurement of the amount of matter in a body. It is measured in kilogrammes.

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