Standard Honda piston size...

[Ruairidh Dunford]

Can anyone confirm actual metric size of a standard Honda piston please?

I know they are not round.

[Austin in the Shed]

57 mm

[Ruairidh Dunford]

Thank you for confirming that so quickly Dave - much appreciated.

[Jacob Fowkes]

I would have said 57 but didn’t want to be wrong as mine are certainly not genuine. What’s the pin size on the pucker pistons ?

[Ruairidh Dunford]

Austin gudgeon pins are 1/2", Honda pistons are 13mm.

[John Mason]

Piggy backing on this thread and Ruairidh mention of Honds pistons not being round. I thought all pistons were made slightly oval in shape but have never known the reason why. Can anyone with more knowledge than myself explain this and also how do they do it when the castings are turned in a lathe.

John Mason.

[Ruairidh Dunford]

I think that I am correct in saying that the skirt has ovality (possibly taper too?) and the crown is round - my understanding is that it allows for heat expansion and helps with load contact where it is needed on the cylinder wall.

I have a feeling the actual science behind how much ovality is best suited to certain applications is very much more complex than that...

[Jacob Fowkes]

Cheers I may try more accurate Honda copies on my next build then. Went Suzuki copy this time

[Hedd_Jones]

My mate made some Cast iron pistons for the 1917 Thornycroft lorry he bought off my father. And when I say made, he made the pattern and machined them himself.

They were round, and picked up in the bore and the engine stuck for a moment. They picked up near the gudgeon pins.

A good agressive hone of the bores, and a light skim of the pistons to increase the clearance and all was well

[Bob Culver]

The original solid pistons were round with the fore and aft sides relieved except for a narrow band so the pistons were free to rock fore and aft in bores to some extent.(some after market ommitted and often emerge scuffed)
Then car pistons were made oval with the thrust dia clearance too small for normal expansion. Heat barrier slits concentrated the heat into the fore and aft direction which had very generos clearance (hence oval piston). The greater conducted heat expands and so pulls the thrust dia smaller under load. Engines can get stiff after stopping. Some makes have cast in steel struts to distort the expansion pattern.
Chev 6 engines had ci pistons into the 1930s, possibly 1950s. Dont know if were contoured.