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Stainless steel bolts for the engine

Discussion in 'The Garage' started by Bellerophon, Nov 7, 2019.

  1. Bellerophon

    Bellerophon XS400 New Member

    Hi All,

    So I have stripped and cleaned my XS400 engine and am about to start the rebuild. I purchased a set of SS bolts for the engine off've eBay ( *Link*). This doesn't contain all the bolts for the engine assembly so I was going to measure the left overs and order individually.

    But then I got thinking about stainless steel bolts in general, how the engine has been designed for standard bolts and how this could potentially throw up some problems.
    My thoughts and queries:
    • What grade are the Yamaha flange head hex bolts? They have a 6 or 9 stamped into the head - can we assume this either means a UTS of 600 or 900 MPa?
    • Your 'everyday' hex bolts are of grade 8.8 - 800 MPa UTS, 660 MPa Sy (*Link*). A2 grade bolts are significantly weaker - 700 MPa UTS, 450 MPa Sy (*Link*). As long as the torque settings are below the yield strength then this shouldn't be a problem - however with the cyclic loading nature of the engine could fatigue failure occur earlier?
    • Torque settings - tension. Given that the stiffness of steel generally stays around the 200 GPa mark can one use the same torque settings for the bolts and achieve the same required amount of tension?
    • Torque settings - strength. After checking the service manual against standardised torque settings (*Link*) some of the engine casing torque settings exceed not only those meant for A2 SS but 8.8 also (for instance the 8mm crankcase bolts have a torque setting of 2.2 mkg, which is 21.5 Nm). Can one assume, therefore that the standard bolts are higher grade than 8.8? This rings alarm bells if I'm going to be replacing them with a much weaker A2 grade.
    • Galvanic corrosion - I've read that this is much worse between stainless and aluminium than zinc coated mild steel and aluminium. Is this cause for concern? I've also heard that using zinc slip or copper grease can mitigate this but I'm not sure how.
    I may be overthinking this, but if I'm going off spec I want to be sure I'm not doing anything bad.

    To put it another way - has anyone here used stainless steel bolts on their engine and have they run into any trouble?

    All contributions appreciated.
     
  2. xschris

    xschris a lifestyle not a trend Top Contributor

    I would stick to stock bolts and torque specs for them. If you want stainless stuff make sure it has the same tensile strength and shape as the stock ones. Yamaha used specific bolts for a reason.
     
  3. Bellerophon

    Bellerophon XS400 New Member

    That's a fair point. So do you or anyone else know the grade of bolt Yamaha use? Or would going for the highest grade be the safest bet?
     
  4. xschris

    xschris a lifestyle not a trend Top Contributor

    I am not sure for certain all the grades they used. I only use stock bolts for all my bikes now. Some bolts have numbers already on them. A yamaha dealer my be able to help with this.
     

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