ORIGINAL: da Rock

The head for a diesel has the ability to adjust the compression of the engine. And of course, there is no provision for a glow or spark plug.

If you've been to Davis Diesel, then you've seen their pictures. The thing sticking out of the head is basically a bolt. Turn it and that changes the compression of the engine.

You have to adjust the compression of a diesel to match the "weather" encountered. You adjust it as well as the needle valve.