|Forged pistons are favourite for motor sport: here, the piston is stamped out of a billet of alloy (one of the highest grades available, RR-58, was developed by Rolls-Royce during WW2 to withstand the supercharging of Merlin aero engines)|
Generally pistons are replaced only when an engine is rebored – a larger cylinder bore requires bigger pistons – otherwise they can go on almost indefinitely. Failure is rarely down to manufacturing faults, though that isn’t unknown: stress cracks can be derived from use of inferior materials or poor control of temperature during manufacture, and these will inevitably lead to breakdown before long.
Breakdown can also be caused by operating outside usual conditions, such as incorrect engine timing causing severe pre-ignition (pinking), which in extreme cases can burn a hole in a piston. Also, inadequate lubrication can cause a component to seize, crack or even melt; over-fuelling can wash the lubrication oil away, leading to rapid ring/groove wear and even seizure due to piston expansion; and engine overheating (even without boiling) can cause excessive piston expansion, leading to scored bores and seizure.
Other problems include debris getting into a motor, such as a nut from a carburettor falling into the manifold then slipping past a valve. This can severely damage a piston as it is hammered into the crown. Dirt and dust can wear the ring grooves, leading to increased oil consumption; water contamination can erode the piston (as can an incorrect air/fuel mixture); and valves getting out of synchronisation, either by cambelt failure or even by over-revving, can do the same.
The gudgeon pin, normally a high-grade (case-hardened or nitrided) steel tube retained by circlips, links the piston to the ‘little end’ of the connecting rod. Gudgeon pin failure will also destroy components, as
the unrestrained rod flails around knocking chunks off the piston and often out of the cylinder walls. Due to improved modern-day oil formulations, a build-up of carbon deposits on the piston crown is usually a thing of the past, but removing the cylinder head for decarbonising used to be an annual event in the 1950s. When this problem does occur, it can lead to piston damage if allowed to build up excessively.
‘Slap’ is where the piston rocks on the gudgeon pin between compression and ignition strokes, as a result of offset thrust in the combustion chamber; some pistons have offset gudgeon pins to balance this, so must be fitted the correct way round.
If pistons for your old car cannot be found on the shelf, talk to specialists and fellow owners. Surprisingly often there is a modern car variant that can be adapted to fit: we’ve even heard of Honda saloon pistons in a Grand Prix Amilcar. Gudgeon pin type and location relative to the piston crown is, of course, just as important as the size and shape of the crown.
If there is no modern substitute, low-volume manufacture is possible. Modern technology can come to the rescue for high-value engines. CAD (computer-aided design) technology makes it possible for a specialised machine to manufacture new pistons from a billet of solid aluminium as exact replicas of a sample supplied – or indeed, in any specified oversize. But the process is inevitably expensive in terms of materials, tooling and machinery.
Forged pistons are favourite for motor sport: here, the piston is stamped out of a billet of alloy (one of the highest grades available, RR-58, was developed by Rolls-Royce during WW2 to withstand the supercharging of Merlin aero engines) before being heat-treated to relieve stresses and milled to precise dimensions. Tooling up for forging pistons is extremely expensive, especially when they have complex slipper designs (where the piston does not have a full skirt).
There are still places where pistons are made in the traditional ways. First, there are the manufacturers, usually in the Third World, who still make components for classic engines because the tooling for those vehicles was sold there decades ago: Triumph TR pistons produced in India and Rootes pistons made in Iran are just two examples.
Then there are the extremely rare firms who still have the moulds and dies for classic pistons and can manufacture small batches as required, supplying specialists the world over with new components made almost entirely in the traditional way. Foremost among these is JP Pistons in Australia.
Here, pistons are machined from castings as they were when our cars were new. Five- or three-piece dies, made of cast iron with steel inserts for the gudgeon pin holes and cores, are heated before the molten aluminium alloy is poured in. This contains 10% silicon and is heated to 700Â°C to ensure complete filling of the die. What happens next is crucial to the durability of the piston: it must be cooled and tempered under strict control to ensure the integrity of the casting, without stress cracks. First, it is cooled, then removed from the die and placed in hot water. Then it is gently heated and cooled overnight in a heat-treatment plant.
The gudgeon pin hole is machined undersize so it can be used as the datum for the next stage of the process. Here modern technology steps in to save time and labour: a CNC (computer numeric control) lathe machines the piston to exceptionally tight tolerances very quickly, while mounted on a rod through the gudgeon pin hole.
Once machined, the piston is stamped with its part number before finishing. Final processes vary according to the design. Some pistons have holes drilled to ensure good oil supply to the bores and gudgeon pins. Some have slots in the skirt or the ring groove. Some require relieving of the top surface to ensure that they do not hit the valves. Some require material to be scalloped from the piston skirt to avoid it hitting the crankshaft at the bottom of the stroke.
Pistons are usually round – and made that way. But they don’t necessarily leave the factory like that. They grow in use and, if perfectly round, will do so unevenly and may cause the piston rings to jam and score the cylinder bore when the engine is working particularly hard. This is due to the uneven thickness of the piston walls, which are much thicker where the gudgeon pin bosses are cast in, and it’s one reason why you shouldn’t rev an engine hard until it’s warmed up. The piston is mounted on a cam grinder which ensures a tiny amount of material is ground away from the areas which will expand most.
Finally the pin holes are reamed out in an oil bath to the exact size and surface finish required, before the piston is fitted with the correct gudgeon pin and stamped with any oversize data.