DIRTY STUFF

PISTONS. I guess these vital pieces began their evolution in the beginnings of the steam age, and got adapted to the new benzol-burning internal combustion engines real fast. Aluminium pistons didn’t get much of a look-in, due to problems machining the stuff, along with their tendency to seize in the cylinder bores, so cast iron was the first weapon of choice.

Until new aluminium alloys were developed and the engineers found different ways to control these materials’ rate of expansion, heavyweight cast-iron pistons were still in production engines right up until the middle 1950s, as with Chevrolet’s Blue Flame sixes. Long life, no piston slap, and as they weren’t revving the ring out of these mills, that extra weight didn’t matter all that much.

But the engineers got smarter, and the increasing demands for higher revolutions brought about stronger, lighter conrods carrying even lighter pistons. Y alloy was developed, and full-skirt pistons made from this material – often carrying two oil rings with one below the piston pin – managed to rush up and down the cylinder bores without seizing.

The penalty was increased space between the new alloy pistons and the cylinder walls, which led to cold-engine piston slap. This often sounded like the slugs were about to exit through the sides of the block. So some manufacturers cut a split into their full-skirt piston faces, opposite the thrust face, and closed up the running clearance. The idea was that as these pistons reached their running temperatures, the piston would expand and close this split to compensate, maintaining a close running clearance and minimising noise. Great idea, but with sustained revolutions over 5000, these thin-walled alloy slugs simply fell apart.

So where to from here? Smart people invented the ‘slipper’ piston, which didn’t look anything like bedroom slippers. It was realised that if only three rings above the piston pin were allowed, the upper ring land section could be smaller in diameter to compensate for the faster expansion rate, because of the thicker mass of metal in that area. They could then take away half the piston material from the pistonpin sides of the skirt, and suddenly they had a better mousetrap. Years before, engineers had discovered that if they machined the lower section of their pistons to an oval shape, giving more room at the pin bosses for extra expansion, this worked really well. But that involved more expensive machining, so somebody figured if they made their piston skirts shorter, and took away a flat slab of alloy from each end of the pin bosses, this left them with a T-shaped piston where there were only two working faces and the round bit of the crown that carried the piston rings. That’s how they arrived at the slipper piston design.

But there was still a problem, mostly with higher-revving competition engines. Initially, they were casting alloy pistons in slowproduction sand moulds, and machining from there. Along came steel piston dies, as seen in the film The World’s Fastest Indian, where actor Anthony Hopkins carried a piston die with a newly cast piston out to his water drum and dunked this still-hot tong-held mass, ruining the afternoon tea.

That made for stronger pistons, but the damn things still fell apart. The search was on for a better design, for when the revs were rising past 10,000. The answer was to use a forging process where molten aluminium billets are physically pressed into modified steel dies, which produced pistons far denser and stronger than a porous cast piston, and that’s about where the technology is today.

Apart from blower pistons having thicker crowns, new designs are arriving with almost no skirts – just the piston bosses and enough skirt to carry the ring lands. Even piston rings are being deleted. From the old four rings per piston, three became common, then two, until the small two-stroke racing engines in bikes and karts have only one short-life thin ring.

We make top-quality pistons right here in Australia. JP has been creating a huge range of both die-cast and one-off sand-cast pistons for decades in Adelaide. They saved a 1955 Formula One Ferrari Super Squalo engine from being a home for spiders, by magically producing four peak-top 14:1 pistons for me, using sand casting from a single sample of an original Italian Borgo piston that I sent them. Colin at Special Piston Services in Dandenong, Victoria, is heavily involved in making piston sets from the business’s own forgings for many of the Supercars, but still finds time for people like me who want bulletproof stuff for the rare and unusual.

There are always innovative ways to help competition pistons survive. The main enemy is incredible combustion chamber heat; internal combustion mills are, after all, simply heat engines. The latent heat of evaporation you get with alcohol fuels helps piston crowns to a huge extent (unless the fuel/air mix is way too lean), and underside oil cooling assists with survival. Some racing turbocharged Sierras were having problems, until smart engineers plumbed oilsquirting jets into the main internal oil gallery, directed at underside super-hot piston crowns. Sierra pistons didn’t melt anymore!

THERE ARE ALWAYS INNOVATIVE WAYS TO HELP COMPETITION PISTONS SURVIVE. THE MAIN ENEMY IS COMBUSTION CHAMBER HEAT; INTERNAL COMBUSTION MILLS ARE, AFTER ALL, SIMPLY HEAT ENGINES