OT piston direction.

Red460 · Dec 13, 2008

I am putting together a 1941 LeRoi engine for a museum. The cylinders are cast in pairs and the connecting rods are offset. The pistons have one piston pin boss slightly larger than the other.
My question is why is the rear (I think) slightly larger than the front and would you put all the pistons in facing the same direction ?
Thanks.

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Janicholson · Dec 13, 2008

I think, but do not know, that the center to center distance on the paired cylinders is the reason. each "jug" is treated separately. If you measure the location of the journals on the crank (from one another) and the bore spacing of the two cylinder units. the space between cylinder #2 and #3 (in different castings will require the offsets you have found. If you lay it out it should be clear which way they assemble. I had a 2 cyl Le Roi and it had offset rods on its assembly. JimN

Red460 · Dec 13, 2008

Thanks Jim. The offset in the connecting rod puts the little end right in the middle of the bore, so the slight offset in the piston is not required for spacing. Naturally I want to get it correct but how important is it. Why are pistons made with a front and a back ?

mattofvinings · Dec 13, 2008

I think there is some geometry different for the "major thrust" side.

For example, on the power stroke, there's a "ton" more load on the piston and so the direction it is travelling requires more thrust (supported by cylinder sidewall) support, and possibly a geometry change to the rod as well to encourage crank rotation in the normal direction.

New pistons that go into the latest 4-cylinder automotive engines have a completely different geometry from one side to the other in order to provide support where it is needed and take away drag where support is unneccessary.

OT piston direction.

Red460

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Janicholson

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Red460

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mattofvinings

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