Should my pistons be round

I have always taken pride for as long as I can remember on car & motorcycle engines. I have spent many a happy hour building & tuning motors & rebuilding gearboxes as a hobby from a young age of 15. From little BSA Bantams with George todd heads & padded flywheels etc etc. To Big block Chevy's LS6 engines for stock cars. + many Cosworth engines. My question now is are my pistons round or not.

My answer would have always been are you stupid, of course they are round, or should be. Remember the Honda NR750 that took the press by storm with it's oval pistons. When I 1st saw it at Donnington, I said it cannot work. It did not work & the project fell apart.

So back to today. Steve should pistons be round or oval. Round, are you stupid. If you have time guys & gals watch all 3. Absolutely amazed with what i saw. I fitted their pistons in a Calibra turbo i built for a friend but never knew the facts. Knowledge is everything. Almost. Just thought some of you on this forum would appreciate precise engineering at it's best. Regards. Just an add on. I always thought the biggest size you could take an A series block to was 1380cc. Wrong. 

Forged pistons

Steve.

Edited By Steviegtr on 28/02/2020 23:58:41

I haven’t stripped a full size engine for about forty years but was always told by the wise sages of the era that pistons were shaped slightly oval to allow for varying expansion to suit the fact that the con rod would push them more to the side of the bore and cause uneven expansion.

have to watch the video later as the Wife says we are going shopping now (arrrgh).

Edited for spelling.

Edited By V8Eng on 29/02/2020 09:55:54

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

On reflection I think John’s explanation is the correct one aging memory fail!

Edited By V8Eng on 29/02/2020 10:26:34

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.

No Steve, I C engine pistons are not round, nor should they be.They are very complex structures.

The form of the finished product is affected by the section of the particular areas of the Piston, which affect stiffness, and on the temperature gradient within the piston, as well as the temperature of the cylinder liner. The crown are has to be smaller in "diameter", because it is hottest and so expands most.

Piston Ring grooves and Gudgeon Pin bosses affect the stiffness in differing planes, so the skirt will need to be oval and barrel shaped.

The cold shape will not be that when the engine is running.

These are generalisations, since conditions will vary according to speed and load, but the piston is developed to take account of the worst case scenario, which is usually Full load, Rated Speed conditions.

But notice has to be taken of possible heat flows at Peak Torque, and of the inertia loads during overspeed conditions.

Even when you seem to have the ideal shape, you can run into longer term problems like cracking, and have to revisit the development process.

Howard

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.

All the above pretty much as the guy was saying. At least I learned something from it.

Steve.

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Sorry Bill!

Once the cylinder head is tightened, and / or the engine is run, the machined round bores be are unlikely to be so any more. The only possible way to avoid this is to use wet liners; but they bring other problems in their wake!

The dry liner block will stress relieve itself with the heat, (possibly with gas pressures and combustion forces ) and even if through bolted, may still distort from the clamping forces.

Piston rings do a fantastic job in sealing the gases, and controlling the oil!

Howard

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.

Not sure if I can even spell this properly but has anyone ever used ackrolite pistons. They were mainly used in highly tuned turbo charged engines & rattled like hell, they were so hard & undersized. For the expansion. Usually they were less than 8:1 comp.

Steve.

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.