In vehicle performance upgrades, there’s the tendency to first upgrade parts that bring in the most power. These are the engine internals, with pistons, connecting rods and crankshafts making up the majority of changes. What is often overlooked are the finer details. Parts that are necessary and contribute to the gains from bigger components. This includes items like piston rings, gaskets, bearings, gaskets, girdle plates, and valve springs. Each has a crucial role in the production of cars. For performance vehicles and any intended upgrades more so. They may not be responsible for overall power, but are the links in getting there.
What are Piston Pins?
Piston pins go by a few names -wrist or gudgeon pins. They’re the engine parts consisting of hardened metal connecting the pistons with the connecting rods. They resemble hollowed-out rods (to keep the weight down) that slot into the top end of the conrod and the opening in the piston skirt. They provide bearing support for the connecting rods, as the pistons make their way through each engine cycle. In production cars, wrist pins are sourced in adequate thickness, strength and hardness to effectively cancel out the forces and temperatures acting on them as the piston moves through the cylinder bore. A damaged or cracked piston pin can cause engine seizure and catastrophic engine failure.
In production engines, when you start tampering with horsepower figures, and in effect increase forces throughout, there’s a greater chance of piston pin failure. Performance piston pins are therefore made of tougher materials, often coated and in greater thickness. They’ll also have different methods by which they connect the conrod and piston. Piston pins can be sold on their own, or packaged with pistons and engine rebuild kits.
Types of Piston Pins
There are three types of piston pins -fixed, semi-floating and full-floating. These differ in design and how they connect the pistons to the connecting rods.
Fixed or anchored piston pins pivot freely in the top end of the connecting rod, and are connected to the piston skirts by way of a bushing. The piston locks the pin in place with screws that go through the pin bosses. These types of pins are generally found in industrial engines and rarely in any performance upgrades because of the likelihood of increased wear in the contact points.
Semi-floating pins are those that are secured in the midsection of the connecting rod. The ends of the pins are chamfered to allow for free movement in the bearing in the piston bosses. Securing the pin to the connecting rod is often done by press-fitting, or heating the conrod top-end, during which it expands and the pin is pressed into place. This is the type of pin found mostly in production engines.
Full-floating piston pins float freely in the conrod top ends and the piston bosses. Different types of spring-loaded locks (circlips) lock the pin in place. This may also be achieved by buttons placed on either end of the piston bosses, and in effect centre, the pin in the cylinder bore, and provide for even loads in all cycles. This is what is found in newer engines and is the preferred choice in engine upgrades. as they allow for easy removal and installation.
When it comes to transferring the energy from the pistons to the crankshaft, piston pins have a pivotal role. Performance pins differ in materials, designs, weight and overall thickness from those found in production engines. The choice of materials, and how these are treated determine crucial factors in the performance of every gudgeon pin. Generally, pins are made from steel or steel alloys in what is known as carburising or case hardening. This improves stiffness and reduces wear by adding trace amounts of manganese, nickel and chromium. Further hardening can be carried out with nitride treatment, where the addition of chromium, aluminium or titanium, increases the stresses the pin can take before galling sets in. The same treatment is often applied to crankshafts where inherent strength and rigidity determine rotational speed and stiffness. This is in extreme loads, and higher RPMs. Treated pins will always outperform those that are left without.
Varying levels of stiffness can be achieved through experimentation with wall thicknesses, especially in motorsport where combinations of other metals and alloys, besides steel, have produced interesting results. Wear from excessive friction can be minimised with DLC or Diamond-like Coatings both to the piston bosses and bushings and conrod top ends.
What Makes a Quality Piston Pin?
Any damage to piston pins is a recipe for costly and irrevocable repairs, usually requiring the replacement of the whole engine. Piston pins, therefore need to have enough strength and flexibility to carry the loads at all engine speeds. Those that have been heat-treated and coated serve this purpose better. Pins also need to be precisely machined to the dimensions of the piston bosses, to reduce the likelihood of play and allow for optimal levels of lubrication. For performance pins, lighter is always better (without sacrificing strength). And where there are higher loads, as in upgraded or performance engines, pins that are easily serviced would prove beneficial and save time.