A double start thread has lead of 5 mm. The pitch of the thread is:

Concept:

• Threads are formed on screws in a helix.
• Helix is the path of a point travelling around an imaginary cylinder such that its axial and circumferential velocities maintain a constant ratio. 
• When a single helix is making a screw, it is called a SINGLE START thread. In a single start thread, the LEAD and PITCH are the same. 
• 
• In the case of TWO START (DOUBLE START) threads, one thread is wound within the other exactly in the middle. This enables the lead of the helix increased without increasing the pitch. 
• Lead = Pitch x Number of starts 
• 
• A screw-thread may have any number of starts. The general term for such threads other than single start is MULTI- START.
• Application of multi-start threads can be found in fly presses, pen caps etc. A multi-start thread makes it possible to keep the depth of Thread less, and provides a rapid axial movement of the screws.

Calculation:

Given:

Lead = 5 mm

Lead = Pitch x Number of starts 

​⇒ 5 = 2 × P 

P = 2.5 mm