Tech guide – BMW con rods examined and explained

2014 / 2015 Drive-My

Following on from last month’s look at pistons, we thought it would make sense to look at con rods, as their design, weight and size depends heavily on the piston’s design. Con rod is, in fact, short for connecting rod, which pretty much explains what the component’s job is. The con rod sits between the piston and crank, connecting the two and transferring the forces exerted on the piston, through combustion, to the crank. Just like with piston design, a lot of focus is given to reducing mass when designing con rods. Reduced mass means reduced rotational inertia, which means the crank doesn’t need to be as strong and the whole system operates more efficiently.

Manufacture and material

A major factor in con rod design, for road cars at least, is cost. As with anything that’s fitted to a road car, it has to be approved by the accountants, and con rods are no different. The majority of road cars will use cast con rods, which are okay for daily commuting but not so well suited to performance engines. For high performance engines, forged or ‘billet’ rods can be used. Forged and billet rods offer far more strength but are a lot more time intensive to make. For small batches, billet rods are the most cost effective. Billet rods involve machining out the entire rod from a solid lump of metal which is a laborious process! Forged rods are quicker to make and offer similar properties to billet rods but have larger set up costs due to the requirement of a forge tool.

Once the process has been decided, the next criteria to look at is material. Three materials are widely used for con rods: steel, aluminium, and titanium. For longevity, steel is the best choice, as it’s got a longer fatigue life – i.e it can be used for longer without failing due to continuous loading. Titanium has the shortest fatigue life, but it also has the best strength-to-weight ratio of the three materials. Therefore, titanium is generally only really used in motorsport, where regular servicing and cost isn’t so much of a problem. Aluminium offers a middle ground, with a decent fatigue life and a better strength-to weight ratio than steel whilst also being considerably cheaper than titanium. It’s still not ideally suited for low maintenance road engines, but aluminium is the best bet for a performance engine.


Con rods have three distinct sections: the ‘big end’, which connects the rod to the crank; the ‘little end’, which connects the rod to the piston; and the actual rod section in the middle. The little end utilises a gudgeon pin which slots through the piston and rod to hold the two together. It’s not uncommon for the little end to be sleeved in a hard wearing material, such as bronze, to increase its longevity and also add an element of serviceability to the component.

The big end is slightly more interesting, and is where a couple of different design approaches can be taken. Most rods are designed with the big end comprising two parts, which allows you to fit it over the crank during assembly. A few, mostly historic, designs have a single piece rod and instead utilise a multi-piece crank to make assembly possible. For obvious reasons, a multi-piece crank is incredibly expensive, which is why it’s very uncommon. The reasoning behind it, though, is very logical – by having the rod as a single piece rather than two, strength is greatly increased. Having the rod as a two-piece assembly brings about a number of headaches. First, it’s a weak spot – fastening the two halves together with bolts is never going to be as strong as having the rods as a single piece. It’s also difficult to machine the main bore perfectly, as it has to be finished with the rod in an assembled state (so the two halves can be cut as one), making the method for clamping/holding the rod during this process critical.

Unfortunately, there’s not a lot that can be done about the requirement for fasteners. Material advancements have helped greatly but it still remains a weakness. For machining the two halves, though, there’s a fairly cunning solution that a number of manufacturers use. They’re called ‘fracture split rods’, which is where the rod is made as though it were one solid piece which is then broken it into two halves when it’s finished. It’s impossible to get a perfect break, so fracture split rods must always be kept as their pair but it means that the bore concentricity is perfect, and that the two halves line-up perfectly – the imperfection of the broken surface acts as a register for alignment.

Finally, the design of the mid-section. Thanks to technological advancements such as computational FEA (Finite Element Analysis), con rods can be tested and refined in a virtual environment, making it easier for manufacturers to innovate and improve their designs. The mid-section is usually based on a ‘I’ of ‘H’ beam shape, with thicknesses and widths all dependant on the engine design and power output.


Con rods of old used to run on roller bearings but these took up a lot of room and weren’t a very efficient way of operating. So this all changed and now the universal method is to use shells. These are thin plates of metal that sit between the big end and the crank journal. When the engine is operating, oil is squirted into the gap between the journal and the crank, creating a thin film which gives the system its lubricity. This thin film of oil is an incredibly efficient way of allowing the rod to move rotationally and is a superior method to using bearings. It only takes up a fraction of the space, too. As they’re separate components from the rods, it’s possible to replace/service them – although this is probably unlikely on a road engine. If you do plan to undertake this work, it’s important that you buy a set of nuts and bolts for reassembling the rods. This is because the bolt will have stretched when it was initially done up, and undoing it then retightening it will stretch it further, creating a point of potential failure.

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