The WBT crimp sleeves are made of oxygen free copper and plated with 24k gold. They have a transparent insulation collar that provides additional strain relief. We offer 3 sizes: WBT-0443: 4mm (12 AWG, will fit 4TC or 4PR) WBT-0444: 6mm (10 AWG, will fit 8TC or 8PR) WBT-0445: 10mm (8 AWG will fit 12TC) WBT crimp sleeves work well with WBT speaker connectors, WBT-0610WBT-0661, and WBT-0681. WBT recommends using the WBT-0403 crimping pliers for WBT crimp sleeves.