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CHAPTER 5 The Two-Stroke Cycle Engine
Copyright Goodheart-Willcox Co., Inc.
the entrance side of the counterbore to hold the
piston pin in place.
The connecting rod is positioned on the
underside of the piston. The piston pin passes
through one side of the piston pin bore, through
the small end of the connecting rod, and into the
piston pin bore on the other side of the piston.
Figure 5-15 shows the location of the connect-
ing rod. Between the piston pin and connecting
rod is a needle bearing. See Figure 5-13. The
needle bearing is a bearing that uses thin cyl-
inder-shaped rollers, called needles, to reduce
friction. The needles are separated and held in
position by a cage. The needle bearing is lubri-
cated by the two-cycle oil premixed in the fuel
and minimizes friction between the connect-
ing rod and piston pin as the connecting rod
changes angles while the engine is running. The
piston pin needle bearing is considered service-
able because it can be replaced when needed.
Connecting Rod and Crankshaft
The following are the basic engine parts that
work together to convert the reciprocating linear
motion of the piston into usable rotary motion:
• Connecting rod.
• Crankshaft.
• Crankpin bearing.
The basic terms related to the crankshaft are:
• Throw.
• Crankpin journal.
• PTO end.
• PTO journal.
• Mag end.
• Mag journal.
• Counterbalance.
• Left-hand threads.
• Half crankshaft.
Needle
bearing
Piston
pin bore
Connecting
rod Crankshaft
Retaining clip Piston pin
Goodheart-Willcox Publisher
Figure 5-13. The piston pin passes through bores
in the piston and connecting rod and is held in
place by a retaining clip. The retaining clip snaps
into the groove at the end of the piston bore.
Bore Bo Bo Bo Bo Bo Bo Bo Bo Bo Bo Bo Bo Bo Bo Bo Bo or Bo Bo Bo Bo Bo Bo Bo Bo Bo Bo B B Bo Bo Bo Bo B B B B Bo Bo B Bo Bo o Bo Bo B B Bo B o o o o o o re re re re re re re e re re re re re re re re re re re e re e re e re re re e e re e r r e
Counterbore C Co Co C C C Co Co Co Co Co Co o o o o o o Co Co Co Co Co o o o o C o o C C C C C C Co Co Co C o o C Co o Co o o o un un un un un un u un un un n n n n un u u n n un u n un un u u u u u u u u u u te t te te te te te e e e e e t t te e te te e e te e e e te te te te e e te e e e e e te e e e te e e t rb rb rb rb rb rb b b rb b rb b rb r b rb rb b rb r rb rb rb rb rb b o or o or or or o o r or or or r or r o o or o o o e e e e e e e e e e e e e e e
Counterbore
shoulder
Retaining
clip groove
Goodheart-Willcox Publisher
Figure 5-14. The counterbore creates a shoul-
der to stop the piston pin from going completely
through the piston pin bore. The retaining clip
holds the pin in place.
Piston Connecting rod Crankshaft
Goodheart-Willcox Publisher
Figure 5-15. The connecting rod is located under
the crown of the piston. Between the connecting
rod end and the piston pin is a needle bearing.