To use the donut gasket effectively, the manifold flange and the pipe flange must both be chamfered to mate with the shape of the donut. If they're flat, you'll need to use the gasket, or spend some time with a die grinder, modifying the flanges to match the donut.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!