1. Pistons and Crankshafts
This presentation will explore:
Pistons and Pins
Connecting Rods
Crankshafts and Bearings
Oil Seals and Balance Shafts

2. Piston Construction
Most pistons are forged or cast from an aluminium alloy for light weight and strength.
Cylinder
Piston
The piston must be able to withstand rapid changes of speed and direction over a very short distance.
Compression height
Gudgeon pin hole
Connecting rods link the pistons to the crankshaft.
Skirt
A connecting rod is attached to its piston by a pin through a precision bore in the piston body.
Connecting
rod
The part of the piston below the pin hole is known as the skirt.

3. Piston Shape The top of the piston is the hottest part.ºC
The piston tapers toward the top to allow for extra expansion at the higher temperatures. ºC
A cam-ground (oval) piston is narrowest across the pin hole where there is more metal. ºC
It expands more across the minor diameter to become round when hot.
Minor
diameter

4. Piston Rings
Compression rings are generally made from cast iron and prevent “Blow-by” (pressure leakage into the crankcase).
There are generally two compression rings, both acting as gas seals. Designs may vary from engine to engine.
Compression
rings
The lower ring is an oil ring which is used to wipe oil from the cylinder wall, limiting oil seepage to the combustion chamber.
Butt joint
Step joint
There is a gap in the compression rings to allow for expansion. Three typical ring joints are shown: butt, step and angle.
Angle joint

5. Oil Ring
As the piston moves down the cylinder, the oil ring wipes crankcase oil off the cylinder wall.
The oil ring is specially designed so that the surplus oil passes through the oil ring, then through holes in the piston under the oil ring.
This oil is then returned to the crankcase and sump.
Oil holes
The hole for the gudgeon pin is just below the oil ring.

6. Press-Fit Gudgeon Pin Construction
The gudgeon pin is typically made of case-hardened steel and precision ground to really tight tolerances.
The diagram shows a press-fit gudgeon pin, which is pressed into the small end of the connecting rod and would remain static in the small end.
It is free to move in the pin hole of the piston, but not in the small end of the connecting rod.
The press-fit type is reliable and cheap to manufacture and generally does not use any retaining rings (circlips) in the gudgeon pin bore.

7. Full-Floating Gudgeon Pin Construction
The full-floating gudgeon pin is free to move in both the pin hole and the connecting rod small end.
There is usually a bronze bushing in the small end of the connecting rod.
Circlips in grooves prevent the floating pin from coming into contact with the cylinder walls.
Gudgeon pin hole offset locates the gudgeon pin and rod toward the major thrust surface of the piston, away from the piston’s centre line.
Gudgeon pin offset is used to equalize thrust friction on the cylinder walls and prevent piston slap (or knocking). The front of the piston is usually marked to ensure correct fitting.

8. Connecting Rod Construction
The connecting rod is typically made of ductile steel to withstand the high loads, and incorporates bearing shells and may have a bush for the gudgeon pin.
Rod and cap
numbering
The big end is connected to the crankshaft and normally has bearing inserts called bearing shells.
Small end
The small end may have a bronze bush for the gudgeon pin in full-floating operation.
Oil hole
Bearing
shells
Some connecting rods may have an oil spurt hole through their length, allowing system oil pressure to be supplied to the gudgeon pin bush.
Big end
Each rod and cap must be numbered for identification when re-fitting. These must be obeyed to pair up rods with caps.

9. Piston Assembly
This diagram shows the components that are typically found in a piston assembly.
Compression rings
Oil ring
Piston
Gudgeon pin
Circlip
Bushing
Connecting rod
Bearing shells
Cap

10. Crankshaft
Journal
The crankshaft offset is the distance between the centre of the crankshaft and the centre of the journals. Twice the offset is the distance that a piston travels in one stroke.
Offset
Oil passages
The journal (or crankpin) is the bearing surface on the crankshaft that the connecting rod is fitted to.
Counterweights
Counterweights are used to counteract the thrust of the pistons and rods, so reducing vibration.
Passages in the crankshaft supply lubricating oil to the connecting rod journals from the main bearings. These oil passages are normally drilled into the crankshaft after forging.

11. Engine Bearings
There may be three uses of plain engine bearings: connecting rod, crankshaft main and sometimes camshaft.
Connecting rod
The body of the bearing is normally steel with softer alloy layers bonded on the surface, such as lead bronze, nickel or babbitt.
The outside diameter of bearing shells are made slightly larger than their mating bore and “crushed” into place as the bolts are tightened. This “interference fit” helps prevent turning and assists with heat dissipation.
Crush height
(nip)
Bearing spread is used to hold the bearing in place for ease of assembly.

12. Undersize Bearings and Locators
To overcome wear of the crankshaft, undersize bearings can be fitted after the journal has been machined (reground) to a smaller diameter. The amount of reduced journal diameter is stamped on the underside of the bearing.
Undersize
marking (.020)
Locating lugs or dowels may be used to position the bearing correctly in the crankcase or cap.
Undersize bearing
Locating
lug
Locating dowel

13. Oil Holes and Grooves
The crankshaft rotates in the bearings between the cylinder block and the caps. Because the crankshaft operates at high load and speed, a large amount of friction and heat is generated that could lead to damage. Therefore, lubrication is necessary to both remove heat energy, and create a hydrodynamic film of oil between the two surfaces to minimize wear.
Cylinder
block
Grooves and holes are cut into the cylinder block and caps to allow oil circulation from the pump to the main bearings.
Bearing cap

14. Thrust Bearings and Washers
Crankshaft
(a)
(b)
Thrust washers, or thrust main bearings, prevent lateral movement of the crankshaft.
There may be thrust flanges on main bearings (a), or separate thrust washers (b).

15. Bearing Components
This diagram shows an exploded view of the bearings and thrust washers in a crankshaft main bearing assembly.
Thrust washer
These bearings assemble into the main bearing caps and are bolted (torqued) up to provide optimum operating conditions. As in the case of the connecting rods, these caps are also non-interchangeable and are numbered to suit.
Bearing
Crankshaft
Bearing cap
Bearing

16. Oil Seals
Main bearing seal
Body
Oil groove
Inner seal lip
Oil seal recess
Spring
Outer seal lip
Oil seals may be made of neoprene or synthetic rubber moulded on to a metal body. Bores are machined into the cylinder block and bearing cap to locate and house the seals.
Some oil seals may have one lip to keep the oil in and another to keep dust and dirt out.

17. Balancer Shafts
Balancer (silencer) shafts are fitted on either side of the cylinder block and are used to reduce vibration forces and noises caused by piston, connecting rod and crankshaft movements.
Guide
Drive chain
Balancer shafts are commonly chain driven at twice the crankshaft speed. The balancer shafts are typically rotated in opposite directions and are timed to the crankshaft.
Balancer shafts
Lubricating oil is pressure-fed to the balancer shaft bearings.