TOOL SPECS: Helix mortise arbor with 5/16"-24 thread, 1/4" shank, and 1-3/4" overall length for use in routers with a 1/4" collet. HELIX MORTISE ARBOR: These arbors securely hold replaceable helix mortise heads, ensuring stable and reliable performance for your mortising needs. STEEL ARBOR: Steel provides the strength and durability that is required for long production runs when mortising. THREAD TYPE: Features a 5/16"-24 thread. SHANK SIZE: Designed for use in a 1/4" router collet.