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I'm a shade tree mechanic with an old car that's got a fair amount of rust on various fasteners (particularly in the under-regions). When I'm doing maintenance, I often like -- or find the need to -- replace those rusted bits with new pieces. The problem that I run into is that I don't know for sure what I need to buy until I take the thing apart and can look at the fasteners in detail. It would make life a lot more pleasant if I could have the things I needed on hand before disassembly.

I've noticed some correlation between dimension and torque as specified in my FSM -- in fact, there's a table at the beginning that gives the torques for bolts by diameter/pitch and grade -- but there's nothing like "bolt 35 in this diagram is 12mmx1.25" (because of course shops just have bolt bins), and not every bolt has a torque spec. Nor does the Haynes manual have specific info.

Can I rely on the torque/size relationship? What should I do for fasteners with no torque spec? Is there another measurement, resource, trick, or rule of thumb that I'm not aware of that will allow me to be reasonably confident of purchasing the right material? Or should I just resign myself to afternoon bike rides to the Fastenal store?

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Each manufacturer will probably have it's own system for them, unfortunately. For my 1970's Triumph, I had to get hold of a parts book for the car, then a separate Fasteners list to cross-reference from the OEM part number to the actual bolt size.

Even if you can guess the diameter & pitch from the torque rating, you probably won't be able to guess the length...

A lot of firms will use common sizes to reduce the number of different parts to stock, so once you've identified some of them it can be worth stocking up, which will at least reduce the number of trips...

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  • A parts book is an item the manufacturer distributes, and separate from the service manual? I have exploded diagrams, of course, but the fasteners aren't listed. There's a website, opposedforces.com/parts that has part numbers, including some bolts, linked to what seem to be the same diagrams; maybe these come from the parts book, though?
    – jscs
    Commented Jul 6, 2013 at 4:55
  • Yep - The one I have is a series of exploded diagrams, with part numbers attributed to each bit. They would have been intended for dealers/traders to work out what part to order to fix something, before it all went computerised...
    – Nick C ♦
    Commented Jul 8, 2013 at 9:34
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There is usually a generic specification depending on diameter and material. Genuine service manuals will have half page or so on common bolt and screw sizes but you may have to hunt for it as it could be in specifications, servicing or somewhere else Grade 4 bolts will snap way below the minimum torque for grade 8 of same diameter Metric use slightly different classifications, but, as you mentioned 12x1.25, thats a Japanese standard fine thread (12x1.75 is ISO metric thread) If it's a stud, it will have a rounded punch mark on one end, thats a grade 9~10 equivalent. Hex head bolts can have manufacturer markings or actual numbers formed on tops You can find length from part number, you'll have to search online for how it's related to your application At one time (early/mid 1980's) Yamaha had bolts listed on exploded drawings with diameter and length, pitch wasn't given as they were all Japanese standard

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