BZP spring steel for corrosion resistance. Can be squashed or stretched, depending on spring type. Compression spring sizes 9/32 – 1-1/2″. Extension spring sizes 13/64 – 17/64″. Handy storage case with compartments. BZP spring steel for corrosion resistance. Can be squashed or stretched, depending on spring type. Compression spring sizes 9/32 – 1-1/2″. Extension spring sizes 13/64 – 17/64″. Handy storage case with compartments.