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A hole in one… piston

October 7, 2008
Holed piston

See the hole in the piston? (Orin O'Neill photo)

You may recall back in August, Doc and our friend Chuck embarked on a road trip to Montana. You may also recall, near Wenatchee the engine in Doc’s P200 stopped running, and wouldn’t restart. The picture makes it very clear why.

The combustion chamber of any internal combustion engine must be sealed tightly for the engine to run most efficiently. The piston rings are part of that equation, and so are intake/exhaust valves in 4-stroke engines.

But a big ol’ hole like this one lets all the hot gases resulting from combustion escape, instead of pushing the piston down (and therefore turning the crankshaft).

Doc said the temperature in Eastern Washington was in the high 90s, and that the P200’s carburetion was leaner than it should have been. An IC engine running lean runs hot, and it doesn’t have to be extremely lean to melt a hole in the piston, especially in hot weather.

The lesson here: when you change an exhaust pipe and/or carburetor (or operate at a significantly different elevation), you will need to assess the effect on the fuel mixture and make adjustments as necessary. You’re shooting for the ideal Stoichiometric ratio of fuel to air, which in the case of gasoline is 14.7 parts air to 1 part fuel. In the blogroll under “Resources” there’s a link to a chart showing how to read a spark plug; that’s how you determine whether to go richer or leaner.

Putting a Sito Plus pipe on a PX introduces more air into the combustion process due to freer air flow through the engine. That’s why you must also install a bigger main jet in the carburetor—you need to get back to the ideal air/fuel ratio.

You’ll end up with a paperweight if you don’t. Favicon

One Comment
  1. October 7, 2008 7:37 pm

    What a costly paperweight, indeed! Nice to see this rare shot; too bad for Doc.–Lorenzo

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