DIN 3570 Form A vs Form B: Differences and Selection Guide

Form A and Form B of a DIN 3570 U-bolt differ in one thing: how many legs are threaded. Form A has two equal legs, both threaded, so both take nuts. Form B has one long threaded leg and one shortened leg that carries no thread; the standard dimensions the shortened leg by its own height, and it is shorter than the long leg, so its end does not reach the mounting face of the support in the standard’s drawing. What the short leg does and how the bolt is fastened therefore depend on the support it is fitted to, which is why Form B belongs to installations that accommodate that geometry. The U-bolt size, the width across the inside of the bend, the leg centre distance, the thread and the bar diameter are the same for both, so the choice between them is an installation decision rather than a sizing decision.

What actually differs between Form A and Form B

DIN 3570 covers both configurations: the standard, dated October 1968, sets out a round-bar U-bolt for pipes of nominal width 20 to 500. Form A is the symmetric U-bolt you see in most catalogue photographs and price lists: two legs of equal length, each rolled with the thread the standard assigns to that size, each taking a nut and washer. Form B keeps the same bend and the same long leg, and shortens the second leg, leaving it plain; the standard gives that shortened leg its own height dimension rather than carrying the long-leg height across to it.

That single change carries consequences for the thread on the second leg and for the height it reaches, and nothing else moves.

The nut counts are the mounting recommendation suppliers publish, not a requirement written into the standard. They follow from the geometry: a leg that carries no thread cannot take a nut. Where the short leg reaches the support, a Form B bolt still takes its nut on the threaded leg, and suppliers commonly recommend one nut per bolt up to about size 64 and two above it. Confirm the nut requirement with the supplier for the actual support, because it depends on how the bolt is seated. What the standard fixes is the thread on the legs that are threaded, the minimum thread length b that goes with it, and the height h2 of the shortened leg.

What does not change between the two forms

This is where most of the confusion sits, because the part number changes while most of the drawing does not. Everything below applies to both forms at a given size; the one structural difference between them, the shortened leg, is covered at the end of the list, and the size range each form is specified for is a separate question dealt with further down.

• The pipe outside diameter range (d1). Both forms are made for the same pipes. Size 46 covers 38 to 42.4 mm outside diameter whichever form you order.
• The inside width of the bend. The size number — 46, 94, 120, 228 — is the same inner width in millimetres on both forms. This is the dimension that has to clear your pipe and its lining or sleeve.
• The leg centre distance (e). Both forms straddle the same hole pitch in the support: 56 mm at size 46, 136 mm at size 120, 248 mm at size 228.
• The thread size (d3) on the threaded leg. The thread steps with the size, not with the form — M10 up to size 52, M12 through 94, M16 through 202, M20 through 332, M24 above that.
• The bar diameter (d2). The U-bolt is bent from the same round bar either way: about 8.9 mm at M10, 10.7 at M12, 14.6 at M16, 18.2 at M20 and 22 at M24.
• The long-leg height (h1). The overall height in the standard’s table applies to the long leg, so it covers Form B as well as Form A — the published values, for example 171 mm at size 120 and 589 mm at size 530, are the same for both forms. The shortened leg is the exception, and it has its own height.
• The height of the shortened leg (h2). Form A has no such dimension because both its legs are equal to h1. For Form B the height of the short leg is a standard dimension, not a free choice: the standard’s table tabulates h2 for the smaller sizes — the range over which Form B is specified — and it sits well under h1, at 66 mm against an h1 of 138 mm at size 94. Use that value as the check on the short leg rather than assuming one.
• Minimum thread length (b). Set by size, and the same for the threaded leg or legs of both forms.
• Material and finish are not a free field. DIN 3570 is an old standard and its material clause belongs to its own era: it names USt 37-1, a general structural steel to DIN 17100, for the U-bolt and sets out requirements for the round steel semi-finished product and a strength grade for the nuts. Those historical designations are not the grades a mill or a supplier quotes today, so the material a supplier actually offers is a separate question — stainless steel, a current structural or strength grade, and a choice of coatings are supplier options confirmed at enquiry rather than standard requirements. What is not correct is to read the standard as silent on material: it does state a material, and an order that needs something else has to say so.

The practical reading of that list: Form B is not a different size and not a different strength class. It is the same bolt with one leg cut short and left plain, and with the height of that leg given by h2 instead of h1. Moving from one form to the other does not change whether the bolt fits the pipe or the support — it changes how the bolt is fastened and how many nuts the assembly needs.

Choosing between them

Work through these in order. The first two decide the form; the rest confirm that the form you have chosen can actually be supplied in the size and material you need.

1. Do both legs need to be fastened? Form A threads both legs, so both can be bolted to a support that presents a face on each side — a channel, a bracket with through-holes, a beam flange. Form B threads only the long leg: the short leg is held to the height h2 and its end does not reach the mounting face in the standard’s drawing, so it does not grip the support from behind during tightening. Treat a Form B bolt as fastened by the long leg, and choose it only where the support and the surrounding structure suit that arrangement.
2. Match h2 to the support before you order. The short leg is a standard dimension, not a free choice, so the check is against the drawing: take h2 for the size you need and confirm what it has to clear or sit against on your support. Some published tables treat h2 as a minimum dimension tied to a panel thickness, so if the assembly needs something different it has to be stated when ordering rather than assumed.
3. Check the size against the Form B limit, not just against your supplier’s list. The two forms do not cover the same nominal sizes, and the boundary is part of the standard rather than a commercial choice. Form A is specified for the whole DIN 3570 range, from size 30 to 530. Form B is specified up to nominal width 80, which is size 94. Above that, there is no Form B in DIN 3570 to order — one German manufacturer, for example, prices Form A for all nominal widths and quotes Form B only up to nominal width 80, which is simply its catalogue following the standard’s limit.
4. Then check what your supplier practically holds. Within that standard range, availability is a supplier matter and it varies a good deal. One German supplier lists Form B only up to size 64, about DN 50, and makes it to order rather than holding it; another describes its entire Form B range as a special order made to request. A size that appears in the standard and not in a supplier’s catalogue is a size to ask about specifically — it is inside the standard’s Form B range, but it may come with a longer lead time than the stocked sizes.
5. Keep the pipe, thread and height checks unchanged. Both forms are selected by measured pipe outside diameter rather than by nominal pipe size, and both need the thread, the minimum thread length and the overall height confirmed against what is above the pipe. The full set of values for every size is in the DIN 3570 size chart, and the same table is reproduced with DIN’s permission on fasten.it. Use one of those rather than reproducing dimensions from a catalogue that may label its columns differently.
6. Read the standard’s material clause before you accept a substitute. DIN 3570 names USt 37-1, a structural steel to DIN 17100, and sets nut requirements, so it is not a document that leaves material open. Those designations are historical and are not what a supplier quotes today; stainless and current structural or strength grades are supplier options. Where the drawing or the application needs one of those, say so in the enquiry rather than assuming that a “DIN 3570” line settles the material as well as the shape.

Reading Form A and Form B in supplier catalogues

The two form designations have to be verified before anything else, because catalogues are not consistent about them. Having compared supplier and standards records, the position is this.

The two-form convention is solid. German manufacturers and distributors describe the same pair — a two-leg threaded bolt and a bolt with one shortened plain leg — as Form A and Form B, and the English versions of those catalogues use Form A and Form B, or shape A and shape B. The part descriptions repeat it: one supplier prices a Rundstahlbügel DIN 3570 Form A across all nominal widths, while another publishes a product sheet for a stainless steel U-clamp DIN 3570 shape B — the same two products named two ways. The geometrical description behind the name is consistent across the supplier documents examined, which is what matters when you are buying.

Where catalogues diverge is in what they attach the letter to. Three failures recur, and all three can put the wrong part on site.

• A form table with the sizes moved into the wrong form. One widely copied supplier page prints two tables side by side — one under “Form A” covering the small sizes, one under “Form B” covering everything from size 120 upwards. That is not a form split at all; it is the size ladder cut in half and mislabelled, since the sizes listed under “Form B” there are ordinary two-leg threaded bolts. Nothing about the form can be read from that page.
• A weight column printed as part of the dimension table. The same page prints its “h2” column with values such as 64.0, 72.7, 83.4 and 95.8 against the larger sizes, and a footnote instructing the reader to order by the dimension in the table. Those are not height dimensions and there is no millimetre column they belong to. A height taken from that column will not match the part.
• Form letters used for something else. In the pipe-clamp family, form letters describe the halves and variants of a DIN 3567 pipe clamp, and clamp manufacturers use further letters for other bracket shapes. A search result titled “Form B” may therefore be a clamp half rather than a U-bolt. The standard number in the line is the check: DIN 3570 is the U-bolt, DIN 3567 is the two-part clamp.

So the working rule is to read the geometry rather than the letter. If the description says one leg is shortened and unthreaded, it is the single-sided form whatever the catalogue calls it. If it says two equal threaded legs, it is the form that can be bolted on both legs. Anything else — a form letter on a dimension table with no description of the legs — is not enough to order against.

Specifying the form so it cannot be misread

The failure mode this article exists to prevent is a delivered part with the wrong number of threaded legs. It is easy to avoid, because the form is a single word in the line and the rest of the specification is the same either way.

U-bolt DIN 3570 / Form A / size 120 / M16 / pipe 108–114.3 mm / USt 37-1 to DIN 17100 / hot dip galvanised / four nuts and washers per bolt

U-bolt DIN 3570 / Form B / size 94 / M12 / pipe 88.9 mm / short leg h2 = 66 mm / stainless 1.4301 / two nuts and washers per bolt

Five things belong in the line whichever form you are buying: the standard, the form, the size, the thread, and the material or finish. Add the pipe outside diameter when the bolt is being bought for a known line. Add the short-leg height for a Form B order as well: h2 is a standard dimension and belongs in the line as the standard gives it, so a short leg that differs from h2 is a deviation and should be written as one, with the supplier’s confirmed drawing as the reference for that dimension. State the nut count separately rather than leaving it implied, and check it against the supplier’s assembly recommendation for the support — the Form A example above carries four nuts because at that size suppliers commonly recommend two per leg, and the Form B example carries two, while a smaller Form B bolt may take one. A Form B order that arrives with the wrong number of nuts is a small thing to fix, but an order that arrives as Form A when the installation called for Form B is not.

The short answer for the next order

Use Form A — two threaded legs — for almost everything, because it is specified across the full DIN 3570 range and both legs can be bolted. Move to Form B when only the long leg is to be bolted and the support suits the shortened leg at height h2: the long leg takes the nut or nuts, and the short leg is held to its standard height rather than reaching the mounting face. Form B is specified up to nominal width 80, which is size 94, so above that size there is no Form B to order.

In both cases the pipe side of the specification is unchanged. The size still follows the measured pipe outside diameter, the leg centres still have to match the holes in the support, and the thread, thread length, overall height and material still have to be checked against the drawing and what is above the pipe. The form determines how the bolt is fastened. It does not determine whether the bolt is the right size — and in a catalogue that mislabels its tables, that distinction is the one worth holding on to.

ChinaBolts is a fastener manufacturer and supplier, and DIN 3570 U-bolts are part of the range we quote, alongside the wider bolt range. Send the size, the form, the thread, the pipe outside diameter and the material or finish — plus the short-leg height if the form is B, and noting if it has to differ from the standard h2 — and we will confirm what we can supply against the actual specification before the order is placed.