Decarbonizing the Pistons

Rotate the crankshaft by pulling on the fan belt, or by jacking up one front wheel, engaging top gear and turning the wheel until two of the pistons are at the tops of the cylinders. Stuff clean rags into the bores of the remaining two cylinders and in the push-rod openings, the water-ways and the oil-feed drilling in the cylinder block.

Remove the carbon from the piston crowns with a suitable blunt scraper, taking care not to score the surfaces. Then burnish the crowns with a wire brush. Do not use an abrasive, such as metal polish, owing to the risk of particles being trapped in the piston ring grooves or between the rings and the cylinder walls.

Most authorities recommend that a narrow ring of carbon should be left around the edge of the piston crown and that the ring around the edge of the cylinder bore should not be disturbed as this can form a useful oil seal if the piston rings and bores are no longer in perfect con¬dition. This is where an old piston ring comes into the picture: placed on top of the piston, it protects the carbon seal from the scraper and the wire brush.