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Gudgeon Pin Offset

Engine Building Fundamentals

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Discussion and questions related to the course Engine Building Fundamentals

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What is the best way to identify the offset? (1mm) Measurements?

1) My pistons only have valve reliefs on one side, Surmise Inlet?

2) Skirt on the relief side is slightly different to the opposite side. Cutout on the relief side

3) Arrow inside the piston is facing the relief cutout.

Hello Generally the valve cutouts are on the inlet valve side but 1mm offset can be measured by using a small bar thu the gudgeon pinhole to a flat surface

Regards Ross

Hi there, thanks. makes sense.

I did measure from the skirt to the ouside of the gugdeon pin housing. (either side)

The arrow in the inside of dome also points to the Valve cutouts, nevertheless, the manufacturer does not state/supply the info. (wouldnt think you need to measure and make sure?)

Thanks again,


There should be a notch or pointer on the top of the piston, that points to the front of the engine, use that and don't worry too much about the pin offset, it isn't really relevant unless you're running VERY close clearances.

It isn't unusual or specialist, race pistons to have an asymetric skirt design, as the major thrust side is loaded much higher than the minor thrust side, and weight savings can be made on the latter by reducing it. I would expect the pin offset to be towards the side of the piston with the reduced skirt?

As Ross said, it's usually the inlet that needs the notch, if there's only one, as it's larger and usually has more lift if different for the valves..

Can you provide pictures of the pistons, if you still have concerns? Maybe engine and piston makes and versions, if you can't provide that?

Will do, once I,ve taken them. Thanks

To clarify, in case it's a transverse installation - by "front of" I mean the crank pulley end of the engine, not as fitted in the vehicle. ;-)

Understood, thanks.

Pics as promised

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Arrow pointing to the Valve reliefs and this shaped skirt

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Opposite skirt

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