An exhaust pipe gasket, sometimes referred to as a “donut” because of its shape, is a seal between components of the exhaust system that runs from front to back along the underside of your vehicle.
An internal combustion engine, like the one in most automobiles, relies on the rapidly expanding gasses produced by burning fuel to propel the pistons inside and produce power. Just as a fire in the fireplace gives off heat and smoke, the combustion process in your engine emits gasses that are extremely hot and hazardous to both humans and the environment. Not only that, but an engine is loud when it is running.
Therefore, an exhaust system is in place to collect those gasses as they leave your engine, remove many of the hazardous compounds, keep the engine quiet, and route the remaining fumes to the rear of your vehicle.
Attached to the cylinder head on an engine is the exhaust manifold that collects the exhaust gasses and passes them off to the catalytic converter. From there, the exhaust passes through a pipe ultimately to the muffler and out the tailpipe. These components are frequently bolted together through mating flanges sealed with an exhaust gasket. The gasket can be made from a fibrous material or from a combination of metal and graphite, and it can be flat or donut-shaped depending on the type of exhaust flange it seals.