Use the 40 mm Black Magic Cardo AirBlock for jib and mainsheet systems on hi-performance dinghies. For the ultimate system, use Technora, Dyneena, Kevlar, or Spectra line with the cover stripped where it run through the blocks; however, the 40c will take line up to 3/8" (10 mm). These blocks are also ideal for highly-loaded control lines on small keelboats. The 40C double and triple can lock in 2 directions or swivel freely. 3/8 in LINE, SWL 485#, BS 1620# WT. 1.7OZ SHEAVE 19/16 in