Hobson technical document
Bi-Fix Bi-Metallic Stainless Self Drilling Screws: 304 and 316
A three-page Hobson technical document on Bi-Fix bi-metallic self drilling screws. It explains how the screws are made, the coatings, where the 304 and 316 versions are used, where they must not be used, and how the part numbers work.
What this covers
- How bi-metallic screws work
- Protective coating
- Applications
- Where not to use
- Chemical composition
- Part number system
Applies to Roofing & Cladding Screws and Tek Screws (Self-Drilling Metal Screws).
Key things to remember
The essentials from this document in plain words. Check the full document before you rely on a figure.
- A bi-metallic screw has a 300 series stainless steel (304 or 316) head and body fused to a heat treated high carbon steel point, so it self drills like a steel screw but resists corrosion like stainless.
- Coated with a metallic zinc layer and a baked top coat for extra corrosion protection and lubrication, which also reduces the risk of electrolytic corrosion: 1,000hr protection on the 304 range, 1,500hr on the 316 range.
- 304 bi-metallic: stainless or aluminium sheeting and cladding, and general purpose use with good corrosion resistance.
- 316 bi-metallic: also fibreglass sheeting, and where higher corrosion resistance is needed, such as severe marine environments, fertiliser facilities and chemical and petrochemical plants.
- Never use a bi-metallic screw on steel pre-painted roof sheeting or cladding: it causes an area of corrosion about 40mm across around the screw head.
- Roofing versions come with an EPDM bonded 19mm aluminium washer. Part numbers start T4X for 304 and T6X for 316.
Who it helps: Roofers and fabricators fixing stainless, aluminium or fibreglass sheeting, especially near the sea or chemicals.
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This document is published by Hobson Engineering, who supply this range to us. We show it here in full so you can read it next to the product; the summary above is ours. Read it on this page or download the PDF. For the most current version, ask us.
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