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Glossary
Inside Diameter (ID)
Inside diameter (ID) is the diameter of the internal opening of a circular or cylindrical part, measured across the inside surface through the centerline. It describes the “hole size” of components such as tubes, bushings, sleeves, bearings, and washers, and it’s commonly specified along with outside diameter (OD) and length/thickness to fully define a part.
In fastener-related applications, ID is critical because it controls fit and clearance—for example, a washer’s ID must clear the bolt shank, a sleeve’s ID must fit over a pin or fastener, and a bushing’s ID must meet the required running or press-fit relationship with a shaft. For internally threaded parts, the “ID” is often discussed in terms of minor diameter (the smallest diameter at the thread roots), which influences thread engagement, strength, and tap-drill sizing.