What the plain part is called, and what it does
Three names for the same thing, all correct depending on who you learned it from: the shank (the plain cylindrical body), the plain portion, and — the one that matters on a drawing — the grip, meaning the length of unthreaded bolt that sits inside the clamped parts. A bolt with a plain shank is a partially threaded bolt; one threaded to the head is fully threaded, and in the old German numbering those are DIN 931 and DIN 933 respectively, now ISO 4014 and ISO 4017.
The plain portion does three jobs a thread cannot. It puts solid metal in the shear plane, where a joint is loaded sideways. It gives the bolt a bearing surface that can be fitted closely to a reamed hole, so the joint locates rather than rattles. And it moves the smallest cross-section — the thread — outside the clamped stack, so that when something eventually yields it yields where the designer meant it to, in a ductile length of thread rather than at a hole edge.
Are fully threaded bolts weaker? The honest answer is “it depends what is pulling”
In pure tension, no. A bolt in tension fails at its thread stress area As, and both bolts have the same thread, so both have the same As — 58.0 mm² for M10×1.5, 84.3 mm² for M12×1.75, 157 mm² for M16×2. Threading a bolt to the head does not lower its tensile rating. This is why fully threaded bolts are perfectly respectable in joints that only ever see clamp load.
In shear, yes, and by a knowable amount. If the shear plane lands in the thread, the joint is working on As; if it lands in the plain shank, it is working on the full circle. For M10 that is 78.5 mm² of shank against 58.0 mm² of thread — 35 per cent more metal, and no thread root acting as a stress raiser at the exact plane where the load is trying to saw the bolt in half. Under a load that reverses, the difference shows up as fatigue life rather than as a single-shot failure, which is worse, because it arrives without warning.
There is a second, quieter reason designers reach for the plain shank: it can be made to fit. A thread is a clearance feature; a ground shank in a reamed hole is a locating feature. A joint that has to hold position under vibration — a linkage, a pivot, a bolted shear connection — wants the shank sized to the hole, which is a machining specification and not something a thread can offer at any price.
How much thread a standard bolt actually has — the ISO 4014 rule
Thread length on a standard partially threaded hex bolt is not a free choice; the standard fixes it from the diameter and the overall length. ISO 4014 gives thread length b as:
- b = 2d + 6 for nominal lengths up to and including 125 mm
- b = 2d + 12 for lengths over 125 mm up to and including 200 mm
- b = 2d + 25 for lengths over 200 mm
The standard also caps the imperfect thread at the run-out: x ≤ 2.5 P, where P is the pitch. Those two rules together tell you exactly how much usable plain length a catalogue bolt can give you, using the standard's own two dimensions: lg max = l − b (the longest the plain portion can be, imperfect thread included) and ls min = lg max − 5 P (the guaranteed length of full-diameter plain shank).
| Thread | Pitch P | b, l ≤ 125 mm | b, 125 < l ≤ 200 mm | b, l > 200 mm | Run-out x max (2.5 P) |
|---|---|---|---|---|---|
| M6 | 1.0 mm | 18 mm | 24 mm | 37 mm | 2.5 mm |
| M8 | 1.25 mm | 22 mm | 28 mm | 41 mm | 3.13 mm |
| M10 | 1.5 mm | 26 mm | 32 mm | 45 mm | 3.75 mm |
| M12 | 1.75 mm | 30 mm | 36 mm | 49 mm | 4.38 mm |
| M16 | 2.0 mm | 38 mm | 44 mm | 57 mm | 5.0 mm |
| M20 | 2.5 mm | 46 mm | 52 mm | 65 mm | 6.25 mm |
| M24 | 3.0 mm | 54 mm | 60 mm | 73 mm | 7.5 mm |
Where the catalogue stops: grip comes in 10 mm steps, your stack does not
Work the rule through one size and the commercial problem appears on its own. For M10, b is fixed at 26 mm, so every stock length hands you a grip that is that length minus 26 — and stock lengths above 50 mm advance in 10 mm steps. You are choosing from a menu, not specifying a dimension.
| Stock bolt | Thread length b | lg max (plain incl. run-out) | ls min (full-diameter shank) | In the catalogue? |
|---|---|---|---|---|
| M10 × 40 | 26 mm | 14 mm | 6.5 mm | Yes |
| M10 × 50 | 26 mm | 24 mm | 16.5 mm | Yes |
| M10 × 60 | 26 mm | 34 mm | 26.5 mm | Yes |
| M10 × 70 | 26 mm | 44 mm | 36.5 mm | Yes |
| M10 × 80 | 26 mm | 54 mm | 46.5 mm | Yes |
| A 38 mm grip — two plates and a spacer | your choice | 38 mm | 38 mm if you ask for it | No — it falls between M10×60 and M10×70 |
| A 30 mm grip with only 12 mm of thread (short nut, tight envelope) | 12 mm | 30 mm | 30 mm | No — b is not adjustable in the standard |
| Plain shank held to h9, or to a reamed-hole fit | — | as drawn | as drawn | No — that is a shoulder bolt or a fitted bolt |
| Two threads, one plain middle (stud through a stack) | both ends | as drawn | as drawn | No — see double-ended studs |
The everyday workaround is a washer stack, and it is fine right up to the point where it is not: washers under the head move the run-out out of the joint but also raise the head above the counterbore, add a settling interface to a joint that is trying not to settle, and eventually put the thread back inside the clamped parts anyway. If the grip you need is more than about half a stock step away, cutting the bolt to the grip is cheaper than the argument.
“The stack is 38 mm. A 60 shows thread inside the joint and a 70 sticks out past the nut by half a diameter. I need one in between.”
— the shape of the enquiry this page exists for. Grip, thread length, class and head form: that is the whole intake.
How to specify grip length so the bolt arrives right
Give us three numbers rather than one, and in this order.
1. Grip. Add up the parts the bolt passes through — plate, plate, spacer, washer — and give that total. Then say whether you want the plain shank to be exactly that, or a millimetre or so shorter so the nut is certain to pull the joint tight rather than bottom on the shank. Our default, when nobody says otherwise, is grip 0.5 to 1 mm under the measured stack, because a nut that lands on the shank clamps nothing at all and gives no warning that it is doing so.
2. Thread length. Enough for full nut engagement plus a couple of pitches of adjustment, which for a standard-height nut means roughly 1×d of thread plus 2×P. Say if a lock nut, a jam nut or a split pin has to fit on the end as well, because that comes out of the same length.
3. Overall length falls out of the first two, and is the number we machine to. Where the shank has to fit rather than merely pass through, name the tolerance — h9 for a close clearance, or the reamed hole size you are fitting into, and we grind to it. That is the point at which a partially threaded bolt has become a shoulder bolt, which is a different page and often the more honest description of what you need.
Run-out, undercut, and the millimetres nobody counts
Where the thread stops it does not stop cleanly. The tool leaves an imperfect thread — the run-out, x, which ISO 4014 limits to 2.5 P, so 3.75 mm on an M10 and 5 mm on an M16. Those millimetres are neither reliable thread nor full-diameter shank, and they are the reason ls min sits 5 P below lg max in the standard's own tables. If your drawing needs a clean transition — a nut that must seat right up against a shoulder, or a shank that must enter a reamed hole without a taper leading it — the answer is a thread undercut or relief groove turned to the standard rather than an assumption that the thread simply ends.
The other quiet consumer of length is the head radius under the flange. A bolt seats on the flat, not the radius, so a hole without a chamfer lets the radius foul and the head sits proud. If your parts are laser-cut and unchamfered, say so and we cut the under-head radius to suit rather than sending a bolt that will not sit down.
Materials and classes
The grip decision is independent of the material, but the material decides how much use the grip is. If a fastener is going into a shear joint, tell us — it changes the recommendation.
| Material | When it is the right call | Note |
|---|---|---|
| Class 8.8 / 10.9 / 12.9 alloy steel | The default for anything structural or shear-loaded | Where the shank does the work, this is where a specified grip pays for itself |
| A2-70 stainless (304 / 1.4301) | Corrosion service at moderate load | A2-70 is roughly a class 7 in strength terms — do not swap it into a 10.9 joint on appearance |
| A4-80 stainless (316L / 1.4404) | Chloride and washdown exposure | Specify for the chemistry; galling on assembly is the real risk, so the plain shank helps here too |
| 1.4305 free-machining stainless | Small non-structural fasteners in quantity | Machines cheaply, corrodes sooner than 304 — never marine, never structural |
| 6061-T6 aluminium | Weight-critical, low-load fixings | Around 310 MPa tensile — a shear joint in aluminium wants a bigger shank, not a stronger thread |
| 7075-T6 aluminium | Only where grams genuinely matter | Near 570 MPa but stress-corrosion prone; the wrong bolt for a damp bay whatever the grip |
| Fully threaded, class 8.8, standard length | Pure tension joints on a shelf near you | Buy it from stock — no one-off can match a stock DIN 933 price, and we will say so in the quote |
How to order
Send thread and pitch, grip length, thread length (or what has to fit on the end), class and head form. If measuring the stack is easier than describing it, photograph the parts with a rule in frame and give us the caliper readings — no CAD is required, and a hand sketch is a normal way to order here. Anything critical comes back as a dimensioned drawing for approval before we cut. Quote within 12 hours, MOQ 1, worldwide shipping by DHL.
Neighbouring parts: shoulder bolts when the plain length has to be ground to a fit, double-ended studs when both ends are threaded, threaded rods and studs when there is no head, large and oversize bolts above M20, drilled-head and cross-drilled bolts when the end has to take a split pin, and custom bolts as the general entry point.
Frequently asked questions
The shank, or the plain portion. The part of that shank which sits inside the clamped parts is the grip, and grip length is the dimension your joint actually cares about. A bolt with a plain shank is partially threaded (ISO 4014, formerly DIN 931); one threaded to the head is fully threaded (ISO 4017, formerly DIN 933). The standard also names two sub-dimensions you will see on tables: lg max, the longest the plain portion can be including the imperfect thread, and ls min, the guaranteed length of full-diameter shank.
So that solid metal, rather than a thread, sits in the shear plane. The plain shank does three things a thread cannot: it puts the full circular section where the joint is loaded sideways, it can be sized to fit a reamed hole so the joint locates instead of rattling, and it keeps the smallest cross-section outside the clamped stack so that yielding happens in ductile thread rather than at a hole edge. For a joint that only ever sees clamp load in tension, none of that matters and a fully threaded bolt is the right part.
In pure tension, no - both fail at the same thread stress area, so an M10 is 58.0 mm2 either way and the tensile rating is identical. In shear, yes: if the shear plane lands in the plain shank you are working on 78.5 mm2 for M10 instead of 58.0 mm2, which is 35 per cent more metal, and there is no thread root acting as a stress raiser exactly where the load is trying to cut the bolt. Under reversing load the difference shows up as fatigue life rather than as an immediate failure.
ISO 4014 fixes it from the diameter and the length: b = 2d + 6 for nominal lengths up to 125 mm, b = 2d + 12 for lengths over 125 up to 200 mm, and b = 2d + 25 above 200 mm. So an M10 up to 125 mm long always carries 26 mm of thread, an M12 carries 30 mm and an M16 carries 38 mm, no matter how long the bolt is. The imperfect thread at the run-out is limited separately to x = 2.5 P, which is 3.75 mm on an M10.
Add up everything the bolt passes through - plate, plate, spacer, washer - and that total is your grip. Then decide whether you want the plain shank exactly that long or slightly shorter. Our default, if nobody says otherwise, is a grip 0.5 to 1 mm under the measured stack, because a nut that bottoms on the shank clamps nothing and gives no sign that it is doing so. Thread length then needs full nut engagement plus a couple of pitches for adjustment, which is roughly 1 x d plus 2 x P for a standard-height nut.
That is the reason this page exists. Thread length b is fixed by the standard and stock lengths above 50 mm advance in 10 mm steps, so an M10 can only give you a 14, 24, 34, 44 or 54 mm plain portion - a 38 mm grip falls between two catalogue parts. We turn the bolt to the grip you state, with whatever thread length you need rather than the standard's, and can hold the shank to h9 or to a reamed-hole fit if it has to locate as well as clamp. MOQ is 1 piece.
EKINSUN LTD is a custom parts manufacturer that turns partially threaded bolts and studs to a stated grip: any plain-shank length, any thread length, threads M3 to M36 and imperial equivalents, in classes 8.8, 10.9 and 12.9, A2-70 and A4-80 stainless, titanium and aluminium. Shanks ground to h9 or to a reamed-hole fit where the bolt has to locate the joint. From a drawing, a sketch or the old bolt, MOQ 1 piece, quote within 12 hours.
Send the stack, not the bolt
Grip length, thread length, class and head form. If measuring is easier than describing, photograph the parts with a rule in frame. One piece minimum, quote within 12 hours.