Connect four… These vital components
connect the pistons to the crank in our four-stroke engines (and make the title
of this feature make sense). Here’s the low down on them…
Introduction:
The humble connecting rod, or con-rod as it
is more commonly known, has a simple but very vital role to play in an engine.
That role is basically to transfer kinetic energy to and from the piston and
crankshaft. Starting from the beginning of the process, the fuel/air mixture is
burned in the cylinder and expands rapidly, pushing the piston down the bore.
The connecting rod in turn takes the force from the piston and applies it to
the crankshaft, this converts the linear action from the piston to a rotational
force to the flywheel, your gearbox and beyond.
The
humble connecting rod, or con-rod as it is more commonly known, has a simple
but very vital role to play in an engine.
to popular belief, rather than all the
stress being in the power stroke of the engine, the con-rod actually has to
take a large amount of stress in both the form of compression and tension.
Compression stress comes from the compression, power and exhaust engine
strokes, and tension during the induction stroke – when the crank pulls the
con-rod down and in turn the piston, to draw a fresh intake charge into the
cylinder.
Although this all sounds very simple, the
con-rod is actually a precision-made component. This high level of manufacture
is necessary due to the high stress it’s placed under while travelling up and
down between the crank and piston, the very high speed it has to travel at, and
the fact it has to change direction extremely quickly at the top and bottom of
the crank’s throw. The size, weight and balance of these components are all
produced to exacting standards to help reduce the wear as well as prevent early
failure.
Although
this all sounds very simple, the con-rod is actually a precision-made
component. This high level of manufacture is necessary due to the high stress
it’s placed under while travelling up and down between the crank and piston
The con-rod has two ends, these are
referred to as the “big” and “little” ends. The little end goes at the top and
attaches to the piston, it has a precision milled hole that houses a pressin
bronze-type material bushing. This type of bushing is used to reduce friction
and prevent surface-to-surface contact from the gudgeon pin and con-rod. The
gudgeon pin slides through both sides of the piston, and through the con-rod,
locking them securely together. Constructing the little end this way also means
that the connecting rod can be rebuilt with new bushings, and will not require
a new set of con-rods during something like an engine rebuild.
The
gudgeon pin slides through both sides of the piston, and through the con-rod,
locking them securely together.
The opposite end of the rod connects to the
bearing journal on the crankshaft – the big end is constructed slightly
differently to the little end due to the way it is installed, although the
basic layout is the same. It’s still a precision milled hole with a bushing on
the inside to prevent wear, but unlike the gudgeon pin, the crankshaft bearing
journal can’t be slid through the hole because the crank is of a one-piece
construction and it’s the con-rod that has to come apart to enable them to fit
together. The big end is split across the middle so the bottom of it can be
removed, this is called the bearing cap and allows the two halves of the rod to
be installed around the crankshaft bearing journal. The cap is held in place by
two bolts known as the con-rod bolts, these are made of very high grade, high
tensile steel, which is necessary due to the huge strain put on them at high
rpms. Should these bolts snap or fail, the bearing cap can come off, the
con-rod flails around, the crank smashes into it, your engine gets wrecked and
you cry… a lot.
