A tunnel crankcase, tunnel crankshaft or disc-webbed crankshaft engine is a design feature of a diesel engine or petrol engine where the crankshaft is designed so that the main bearings (the bearings that support the crankshaft within the crankcase) are enlarged in diameter, such that they are now larger than the crank webs (the radial arms that link the big end bearings to the main bearings). They thus form the largest diameter of any part of the crankshaft. Rather than a conventional crankcase that has webs across it to support the narrow bearings of a conventional crankcase, the crankcase now has a large tunnel through it, hence the name.