SR25 Retro Arms SiCr spring with progressive irregular M120 coil

Retro Arms SiCr SR25 spring with progressive irregular M120 coil

Mainspring in chromium-silicon doped steel, for Long type gearboxes.
The progressive coil makes it easier to tension the spring without depriving it of power.
Spring dimensions:
Internal: 11.3 mm
Outer: 14.5 mm
Length: 185 mm