Re: piston protrusion

Posted by Bob Bidonde on May 06, 1998 at 06:27:44

In Reply to: Re: piston protrusion
posted by Marco Tahtaras on May 05, 1998 at 23:56:35

Marco's comment about the relief in the cylinder head for the piston is a good one. I have a milled head on my Model B engine that no longer has any piston relief left in it. When carbon builds up on the piston top, it hits the head and causes a knock. Fortunately, carbon knock does not happen often because I have learned to run my engine a tad lean.
At first I removed the head and scrapped the carbon, but later on I learned that I could run the engine lean and burn off the carbon. I don't recommend running a head without some piston relief as Marco suggests.
It is also important thatthe cylinder head gasket fits an oversize bore. I have worked on engines that were ticking to find that the pistons protruded and hit the cylinder head gasket. Some rebuilders chamfer the pistons to assure clearance to the gasket.
The down side of a large bore gasket is that the amount of gasket material between cylinders is small and as a result, these large bore gaskets may leak compression between cylinders. So carry a spare cylinder head gasket with you and be sure to have the tools to replace it at roadside.
Experience among the Model A'ers running high compression heads in my area suggests that the composition gaskets with a steel combustion seal ring are best - they don't leak between cylinders. McCord steel cylinder head gaskets are also better than the copper, but the steel gaskets aren't made anymore and are hard to find.
Your engine is at the maximum practical bore diameter. If the cylinder walls get any thinner, an over heat situation can do some serious cylinder damage. Consider having this engine sleeved to standard size at its next rebuild.

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