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Why do lag screws have the unthreaded body between the head and the threads?

And why is that 'Body' the same diameter as the outer 'Thread' diameter?

Why doesn't the transition immediately abut the head, letting the threads run down a greater portion of the bolt?

Diagram of a lag bolt

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    Just so you know the fastener in your photo is a hex head bolt. Lag bolts have a pointed courser thread and are for screwing into wood. – ojait Jan 7 '16 at 21:27
  • Yeah.... I couldn't find a goodpicture and I was being lazy. Just edited the one i had into a point. – Billy C. Jan 7 '16 at 21:57
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    "Now there be's a lags bolt says I". Nice edit BTW – ojait Jan 7 '16 at 22:00
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In many cases an unthreaded shank provides a better, more secure fit when mating to the components that it connects. Where lateral movement is a concern, this type of bolt should be used.

In other cases, it's important that there be no pull on the upper portion of the material being joined. Threads would create drag that may counter the bolt's ability to bring two parts together with force.

There are two reasons that the unthreaded portion has the same diameter as the threaded portion. The first relates to what I mentioned earlier--a close-tolerance fit to the other parts of the system. If the threads were raised with respect to the shank (and some bolts are like this for various reasons), the bolt would be sloppy in the bore after insertion.

The other is more mundane--it's easier from a manufacturing standpoint. Threading machines typically remove material by cutting. This obviously leaves threads that have inner diameters smaller than the bolt's shank.

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    I believe most if not all commercially produced threaded fasters are produced with die rolling rather than cutting, as they are superior in strength and smoothness. And you can find bolts that have shanks narrower than the threaded portion. Those are produced to allow uniform stiffness over the bolt, where a thicker shank would be too stiff. (Engine head bolts are the common example I can think of there.) – Tim B Jan 7 '16 at 19:15
  • My understanding is that lag screw's shaft matches the threads is because they have to be pre-drilled. Since you want the hole larger than need be, the shaft is the same as the threads is the same as the hole. Structural screws on the other hand--which can be used when allowed in place of lag screws--have a consistent shaft size throughout for easier driving since they don't need to be pre-drilled. – DA01 Jan 7 '16 at 22:15

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