No, they are slightly oval, by maybe about a thou per inch of diameter or so. When hot, they expand differently because of the gudgeon pin bosses, so are more round at working temperature. Pistons for small model ic and steam engines are mostly machined round, but the IC ones probably bed in slightly oval.

The Honda suffered from rules essentially designed to prevent four strokes from competing. It was an attempt to get the effect of an eight cylinder while only allowed four by the rules. The shape of the pistons made for difficulty in sealing. Meanwhile, the two strokes were effectively supercharged.

John

The Pistons for my BSA's are slightly tapered, being round at the crown across the piston ring lands and oval towards the skirt. The oval section is narrower across the gudgeon pin axis, and wider from front to back at the bottom af the skirt, which is common on many Alluminium alloy Pistons for internal combustion engines.

I remember the oval pistoned NR750 road bikes when they first appeared, I think they cost around £37,000 back then, with a Titanium ignition key costing about £200 for a replacement, if any owner ever actually used it.

Some digital caliper measurements from a NOS standard bore MZ ETZ 250 piston that lives on a shelf next to my computer at home (air cooled piston port two stroke) :

Nominal standard bore 69 mm

Factory piston size marked on crown 68.95 (used with a "0" marked barrel to give 0.05mm running clearance)

Measurement at base of skirt at right angle to gudgeon pin 68.94 (this is where the factory measurement is taken, so my calipers are close)

90 degrees relative to pin bore just above pin bore 68.88

In line with pin bore just above it 68.81

90 degrees relative to pin bore above top ring 68.71

In line with pin bore above top ring 68.70

No measurement at base of skirt in line with pin bore due to transfer port cutouts. Overall piston length 87.5 mm.

So a taper top to bottom & oval near the gudgeon pin – kind of what you would expect, as the piston head will expand more as it runss hotter & the area around the gudgeon pin has a greater thickness.

Nigel B.

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Normally, the top part with the rings is round, but from the gudgeon pin downwards, the piston is turned oval, with the large dimension at the front to back. Slipper pistons take this to extremes by doing away with the lower sides altogether. High silicone low expansion pistons have always been finish turned using diamond tools. Older designs used to have wire wound around below the pin, and split skirts on the non load bearing side was common.

Or the machine shop uses "torque plates" when boring and finish honing, with the cylinders torqued to the plates. A neccessity on long alloy barrels with relatively thin iron sleeves and thru bolts/studs from the crank cases, thru the barrels and thru into the top surface of the head.