Usually fastener standards specify two types of marks to be on the head of a bolt. The manufacturer's mark is a symbol identifying the manufacturer (or importer). This is the organisation that accepts the responsibility that the fastener meets specified requirements. The grade mark is a standardised mark that identifies the material properties that the fastener meets. For example 307A on a bolt head indicates that the fastener properties conform to the ASTM A307 Grade A standard.

Normally it will not matter whether the bolt head or the nut is torqued. This assumes that the bolt head and nut face are of the same diameter and the the contact surfaces are the same (giving the same coefficient of friction). If they are not then it does matter.

Say the nut was flanged and the bolt head was not. If the tightening torque was determined assuming that the nut was to be tightened then if the bolt head was subsequently tightened instead then the bolt could be overloaded. Typically 50% of the torque is used to overcome friction under the tightening surface. Hence a smaller friction radius will result in more torque going into the thread of the bolt and hence being over tightened.

Historically the difference between a bolt and a screw was that the screw was threaded to the head whereas the bolt had a plain shank. However I would say that now this could cause you a problem if you made this assumption when specifying a fastener. The definition used by the Industrial Fastener Institute (IFI) is that screws are used with tapped holes and bolts are used with nuts.

Torque to yield is the method of tightening a fastener so that a high preload is achieved by tightening up the yield point of the fastener material. To do this consistently requires special equipment that monitors the tightening process. Basically, as the tightening is being completed the equipment monitors the torque verses angle of rotation of the fastener. When it deviates from a specified gradient by a certain amount the tool stops the tightening process. The deviation from a specified gradient indicates that the fastener material as yielded.

A fastener joins together two existing elements or parts and a fixing attaches a moveable element to a fixed one. This is only a general rule and there are many exceptions and the terms are often used interchangeably. For example, a wood screw is a fastener but is often used as a fixing.

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