Machine Design
The designation M 33 × 2 of a bolt means

Bolt of 33 mm nominal diameter having 2 threads per cm
Metric threads of 33 mm outside diameter and 2 mm pitch
Metric threads with cross-section of 33 mm
Metric threads of 33 numbers in 2 cm

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Machine Design
In replacing the V-belts, a complete set of new belts is used instead of replacing a single damaged belt because

The new belt will carry more than its share and result in short life
Only one belt cannot be fitted with other used belts
New and old belts will cause vibrations
Belts are available in sets

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