The diameter of the head of the copper Heat pipe insert is 14 mm, the length of the head is 70 mm

More information:

Vacuum tube with copper insert

15 vacuum pipes for the Heat-Pipe system, together with the Heat-Pipe!

Vacuum tube solar collectors generally utilize the sun's energy efficiently, and the vacuum helps minimize heat loss to the environment. The copper insert on the inside of the vacuum tube helps the solar energy to be absorbed and transmitted faster in the system. Solar collectors of this type can be used to produce hot water and supplement heating systems.
This product - due to its fragility - can only be delivered in a unit package.
The diameter of the head of the copper Heat pipe insert is 14 mm, the length of the head is 70 mm, the length of the jacket is 55 mm.

15 solar collector vacuum pipes for 15 vacuum pipe Heat Pipe systems complete, including Heat-Pipe copper pipe, 52 mm head!

15 vacuum tube solar collector parts

Vacuum tube with copper insert