Engine rebuilds are a costly, time-consuming process akin to creating a work of art. You pour a lot of passion and patience making sure everything fits just right and to the specified tolerances. Of course, replacing one part often means changes to another. For the engine innards, this means a need for balancing new bhp and torque figures when modifying things like pistons, conrods and cranks. And all the fiddly parts that can make or break an engine, literally. Choosing the right parts is one part of the picture, making them work in sweet sync is another.
One of the engine parts that takes a beating in every combustion cycle is the conrod. This delivers the power of the combustion process acting on the piston to rotate the crankshaft. Conrods need to be built tough, be of the right size for the engine and vehicle application, and get you the power boost you’ve been craving in a newly rebuilt engine.
Conrods – the Basics
Conrods, or connecting rods, are the engine parts that connect the pistons to the crankshaft. They convert the linear motion of the pistons, moving up and down during the combustion of air and fuel, to the rotational forces that turn the crankshaft. Each cycle involves a tremendous amount of pressure exerted on the conrods. First, during the compression stroke, pistons push the rods down with an extreme force which only increases as the engine turns faster. After delivering power to the crankshaft, conrods are pulled pack for another cycle over again. This happens hundreds of times within a second. This constant pushing and pulling, and squashing and stretching of the conrods at extreme speeds can lead to failures. Conrods that snap will completely destroy anything in their path.
Parts of a Conrod
Conrods consist of three major and several smaller parts. From the ground up, we have the ‘big’ or bottom end of the conrod, which is removable and known as the conrod cap. This allows it to be attached to the crank journal and secured in place with two bolts, one at either side. Semi-circular bearings line the inner circumference and have bored holes to provide for better lubrication between the conrod and crankpin. Next is what bears all the rotational forces, or the conrod shaft, comprising the body. And at the ‘small’ or top end, the conrod connects to the piston via the piston pin.
Different engines, including the fuel they use and whether they’re stock production or one-off performance cars, will have conrods made of different materials. Steel conrods are seen in most production cars producing moderate RPMs, and are adequate in power delivery. The high tensile strength of steel also increases longevity. Diesels also tend to gravitate towards steel conrods because of the high pressures exerted on them in the compression stroke. For race cars, the preferred material is aluminium, as this is more flexible during higher revs, and it is lower weight, acting as a spring in the exhaust stroke, where conrods are stretched and warped. Current production processes allow for steel and aluminium conrods to be made in large numbers. However, for the best properties of steel and aluminium, namely strength and flex respectively, look to titanium conrods. These are produced in small numbers and are exorbitantly expensive.
How They’re Made
Two production processes are commonly used for performance conrods. Forging involves using tool dies or moulds to produce what can be considered a negative of the conrod. The metal is heated at a high temperature, allowing it to be forced into the die. The raw conrod is then machined. The caps are sized and holes for the bolts are drilled. The end product is one with high strength in areas where the conrod endures the most stress. Billet conrods are produced out of a single piece of metal using CNC machining. This is a more flexible process than forging, as it allows for different designs and sizes of conrods to be produced with the touch of a button. Custom-made conrods are often of the billet type.
There are two types of conrods currently used in terms of how they are designed. These are I-beam and H-beam conrods. They get their names from the cross-section cutouts of the conrods resembling the capital I and H. I-beams are easier and cheaper to make, so are found in most entry-level and moderately priced cars, whereas H-beams have lighter and thinner shafts making them the choice for performance cars. These are also the type you’ll see in most engine rebuilds and upgrades.
Conrods are sold for specific engines and need to be paired to the right type of pistons to ensure they function as they should and provide for the increase in power. Often, you’ll find conrod and piston combos replaced, along with strengthened or new crankshafts. The first parts to break in high revs are the conrod caps, with low-quality bolts. These are often replaced by cap-style bolts that are screwed directly onto the conrod body. When choosing conrods, pair parts carefully, and when the engine is complete, enjoy the fruits of your work.