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Thread: Replacement Stainless Steel Bolts

  1. #1
    Associate Engineer
    Join Date
    Jan 2018

    Replacement Stainless Steel Bolts

    WhileI was, before retirement, an engineer by profession, I’m wasn’t one that had todeal with mechanical component design so I need to ask here.

    I’m amotorcyclist and, like many bikers, I want to replace the old, tarnished bolts onmy bike with stainless steel bolts. My problem is the torqueing of them toprovide clamping forces without overstretching them.

    Allstainless bolt suppliers I’ve asked just say “Take your old bolts out, fit thes.s. bolts, lube them and torque to manufacturer’s spec.”. This is where I have theproblem. Many of the old bolts are marked as 8.8 steel with an accepted yieldstrength of 640 MPa, A2 stainless are 450 MPa.

    If Iuse a figure of 65% yield strength to stay on the elastic part of the material’scurve and, say, an M8 bolt with stressed csa at thread root of 36.61 mm^2 (fromtables):-

    For8.8 steel; 65% yield strength x csa = 15 kN clamping load.
    ForA2 stainless; 65% yield strength x csa = 10.7 kN clamping load.
    Doesthis make sense so far?

    If I thenput those into the simplified formula Torque = ‘nut factor’ x bolt diameter xclamping force = kDF and using a ‘nut factor’ of 0.16 for lubricated bolts, Iget:-

    For8.8 steel; T = 19Nm.
    ForA2 stainless; T = 14 Nm.

    Thereforeif I torque the s.s. bolt to the 8.8 figure, it’ll be loaded to about 90%yield strength and maybe at risk of breakage or, alternatively, the maximumtorque I can apply doesn’t provide the clamping force necessary.

    Isthere REALLY a problem or am I making a ‘mountain out of a molehill’?

    Cananybody offer some more knowledgeable insight?

  2. #2
    Administrator Kelly Bramble's Avatar
    Join Date
    Feb 2011
    Bold Springs, GA
    You math seems right as does your concern. In critical applications, like brakes or other essential places on the bike I would not change out the materials to something less capable. There is also a corrosion issue due to dissimilar metal contact at the point of installation (you bike is most likely aluminum). You might re-think your desire to have Stainless Steel bolts.

    Maybe chrome plated bolt heads with the same strength rating??
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

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