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M3.5 screws

Arran Cameron

What is the origin of using M3.5 screws used to secure electrical fittings to backboxes?

They are a non-standard metric size that are almost synonymous with electrical fittings. 4BA screws were used until the 1960s, but what was the reason why they were changed to M3.5?

I estimate that over 99% of M3.5 screws in use have slotted heads although tamperproof Torx and 'snake eye' heads are available. M3.5 screws with aerospace Tri-wing heads; Reuleaux triangle heads - like on toys; or Pentalobe heads - like Apple uses, do not seem to be available.

  • 0

    3BA were used and M3.5 * 0.6  was the nearest Iso metric fit. 2BA in conduit boxed became M4.

    m6 & 0BA are the same pitch.

    M2.5 and M3.5 certainly are in the better kits, as are the coarse and fine thread options. But agree integer sizes have slowly become more common in general use since the 70s, displacing our previous  mess of UNC, UNF,  BSW, BSF, pipe threads, BA, cycle threads etc ,

    Even so the  more exotic length and headform combinations are quite rare in the non-common threads.

    Mike

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