A tunnel crankcase, tunnel crankshaft or disc-webbed crankshaft is a design feature of a piston engine where the main bearings that support the crankshaft within the crankcase form the largest diameter of any part of the crankshaft and are larger than the crank webs. This requires a crankcase with a large tunnel instead of cross web supports for narrower main bearings, hence the name.
A tunnel crankcase, tunnel crankshaft or disc-webbed crankshaft is a design feature of a piston engine where the main bearings that support the crankshaft within the crankcase form the largest diameter of any part of the crankshaft and are larger than the crank webs. This requires a crankcase with a large tunnel instead of cross web supports for narrower main bearings, hence the name.
Source: Wikipedia "Tunnel crankcase" · CC BY-SA 4.0
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