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…
As the
big end is of a two-piece construction, the bearing it houses has to be the
same. Rather than the press-fit 360-degree bearing in the little end, the big
end houses a split bearing in two halves.
Cosworth
pistons with new rings and the PEC conrods for R13,000 neg
Each half normally has some kind of
locating lug on the back of it which sits into a corresponding indentation in
the conrod and bearing cap. This prevents the bearing from “wandering” or
causing excess movement, which could result in failure and/or a greatly
increased wear rate.
Material types:
There is no one type of material that all
manufacturers use for the construction of con-rods, as there are many different
parameters to consider when designing an engine. Things such as cost, intended
rpm and torque, intended life span of the engine etc, all have to be taken into
account. However, the most common type of material used is cast steel, as it is
relatively inexpensive and can be manufactured easily in large numbers. Cast
rods are normally found in mass-produced engines that aren’t destined for high
rpm or high torque. For this purpose they do their job admirably, however, if
the engine is then tuned to give a substantial power increase these may become
unreliable.
A
racing con rod with ‘inverted’ bolts may allow the engine designer to lower the
centre-of-gravity height of the engine
If the engine is going to be a more
powerful model it may well have forged con-rods, which are stronger than cast,
but also more expensive, hence manufacturers only using them where they have
to. Rather than molten metal being poured into a mould like in the production
of cast rods, metal is heated and then pressed into shape. This causes the
metal to be denser and have a more uniform grain structure (yes metal has a
grain), which results in a much stronger finished product. But of course there
is a trade-off and this production process is more intensive, again making
forged rods more expensive.
JASMA
H-Beam Forged 4340 Chrome Moly Steel Con Rod
Once you get in to after-market rods, the
sky is the limit, you can get much thicker and beefier units made from very
high-grade steel such as 4030 and 4040 chrome-moly, able to take huge amounts
of punishment. Almost as much punishment as they will put on your wallet when
you come to pay for them. If you manage to match all six numbers and the bonus
ball you can really splash out and get some billet con-rods, machined from a
solid block of metal, although these might be a touch of over kill unless
you’re running truly crazy amounts of horsepower. Almost all aftermarket rods
are of a forged construction, as using cast wouldn’t be much of an upgrade after
all. Which grade of metal is used will vary between manufacturers, and depend
on the particular application they are being used in.
