Machine Design
Allen bolts are

Self locking bolts
Designed for shock load
Used in aircraft application
Provided with hexagonal depression in head

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Machine Design
A bolt of M 24 × 2 means that

The nominal diameter of bolt is 24 mm and pitch is 2 mm
The pitch of the thread is 24 mm and depth is 2 mm
The effective diameter of bolt is 24 mm and there are 2 threads per cm
Cross-sectional area of the threads is 24 mm²

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