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I'm getting replacement for some M5 x 30mm hex bolts. The original ones are fully threaded, i.e. the whole shank length. The ones I can find have an unthreaded portion (i.e. grip) but their overall length matches the original.

I wonder if the unthreaded portion can also fit through the hole because I need the whole length inside the wedge that the bolt goes into. Or does the bolt stop where the threaded portion ends?

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Unthreaded portions of bolts are generally just a tick under their nominal diameter. So an 8mm bolt might be 7.8mm across the shaft.

Typically, the OD of threaded areas is smaller than that, so it might be an issue. You also have the possibility of bottoming out the threads if the screw goes deep into the threaded portion.

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How much clearance do you have between the threaded portion and the hole you want to insert the bolt in? In most cases,the shank is cut first and then the threads are cut so the threaded portion will usually be a bit smaller than the shank. If the threaded portion fits snugly into the hole, you'll have a problem. If it's a loose fit, you'll be OK. You might want to get a caliper or micrometer to check the difference in diameter of the threaded portion of your existing bolts and the shanks of the new ones.

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Most bolts that have an unthreaded portion or shank are designed that the shank is to support the shear loads and provide more loadbearing surface than just threads.

The shank is usually larger or the same as the outer diameter of the thread so it fits reasonably tightly in the holes of the pieces to be secured. Thise pieces are designed so the depth of theunthreaded portions match the length of the shank.

So if your piece is fully threaded then you need fully threaded bolts or bolts with a turned down shank.

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