Wednesday, 17 July 2013

pins and rings

The gudgeon pin ( or wrist pin in the USA) on the right is the type favoured by JP pistons of Australian who supply my pistons with pins and rings. Presumably on a cost basis the hole through the middle is parallel . The original pin has double tapered hole being narrowest in the middle section and therefore lighter.
Since I have taken the bore back to the standard 71mm new rings are fitted. It is still a misconception among new boys to this game that the rings are made springy to form the gas seal. The fact is that at or approaching Top Dead Centre there has to be just enough ring gap to allow high pressure gas to get behind it and force it out onto the cylinder wall. Old books on engines tend to show illustrations of lapping the horizontal walls of the piston rings to ensure the gap is big enough.

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