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scott12180

History of hardened bolts

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Today we commonly have Grade 2, Grade 5 and Grade 8 bolts at our disposal depending on the severity of the application. 

When did this distinction come about?

 

On cars I've worked on from the teens, 20's and 30's, I've never seen any markings on a bolt to indicate its relative hardness.  Even in the engine for connecting rods, main bearings or driveline.  It seems back then a bolt was a bolt was a bolt.  It was just the diameter that determined its strength.

 

True?  If not were there different hardness specs?

If not, Was it also WWII which inaugurated the use of hardness markings on bolts?

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The SAE published specs for appropriate steels for bolts, and most other things, quite early on. The earliest SAE handbook is 1926 but most of the specs listed in it are older and must have been circulated to members when they were arrived at. I suspect the handbook was only published when the quantity of recommended procedures got to the point that it was difficult to find out what they were if they weren't all in one place.

 

When you get to brass cars, most bolts are very soft, roughly equivalent to Grade 2. The use of hardened bolts for special applications was certainly understood at the time and they were frequently used in building machine tools. Practically all the screws in my early B&S milling machines have been hardened, no matter what the application was. One can only assume that the stresses associated with automobiles didn't demand them – or at least they didn't think they did. The usual solution, if there was a question, was to use a bigger fastener.  RR used small fasteners but a lot of them. The original con rod bolts on my 1910 Mitchell are 7/16"... an analysis of the stresses involved suggests that 5/16" grade 8 would be more than sufficient but I'll make them 3/8 and grade 8 just to be safe.

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