DIN 931 vs DIN 601: Product Grade and Dimension Differences

DIN 931 and DIN 601 are both partially threaded metric hexagon head bolts with a coarse pitch thread — the difference the designations draw is the product grade, not the thread. DIN 931 specifies product grades A and B; DIN 601 specifies product grade C. Product grade is a dimensional accuracy class defined in ISO 4759-1, so it controls how tightly the bolt is made, not how strong it is. Neither designation fixes a strength level: property classes such as 4.6, 8.8 and 10.9 come from ISO 898-1 and are selected separately. On the overlapping sizes the two tables give the same nominal head height and the same standard thread length, and both cover the M5 to M24 range of the industry’s standard sizes; what differs is the permissible dimensional tolerance attached to those nominal values. The place where they genuinely part company is therefore the tolerance, not the nominal dimension — chiefly on the shank diameter and the head height — and, for a buyer holding a drawing that says “DIN 601 M10”, the head width as well.

Two conclusions follow from that table, and they point in opposite directions. Where strength is the question, “DIN 601” tells you nothing that “DIN 931” does not — both are dimensional designations with the property class left open. Where dimensional accuracy is the question, the two designations are not interchangeable at all, and the difference is measurable on the shank.

What product grade C actually means

The term product grade is defined by ISO 4759-1, Tolerances for fasteners — Bolts, screws, studs and nuts — Product grades A, B and C. It is a tolerance class for the dimensions of the fastener. Grade A is the tightest and applies to the smaller diameters and shorter lengths; grade B applies to the larger diameters and longer lengths of the same product; grade C is the loosest of the three. A grade C product is therefore one whose dimensions are allowed a wider band than the same product made to grade A or B. It is not a weaker product, and it is not a differently shaped product.

The confusion starts because both systems use short designations and, in casual trade use, the word “grade”. A product grade is dimensional. A property class is mechanical. The two live in different standards and are chosen separately.

ISO 4016:2022, the current standard behind DIN 601, states this in its own scope. It specifies hexagon head bolts in steel, with metric coarse pitch threads M5 to M64, and with product grade C — and then adds that where other specifications are requested, property classes can be selected from ISO 898-1 and dimensional options from ISO 888 or ISO 4753. The standard identifies itself as the grade C document and hands the strength question to a different standard entirely.

The published technical requirements of the grade C document make the split visible clause by clause. Mechanical properties are governed by property class, with classes 4.6 and 4.8 tabulated up to M39 in the standard’s demand table and other classes available from ISO 898-1. Tolerances are governed by “product grade: C”, referencing ISO 4759-1. Thread tolerances are a separate line again, at 8g. Three clauses, three different subjects, and strength appears only in the one about property class.

So the answer to the question this comparison usually provokes is no: product grade C does not mean a DIN 601 bolt is weaker than a DIN 931 bolt. A grade C bolt and a grade A bolt at the same diameter, pitch and property class have the same specified minimum strength. What the grade C bolt has is a wider permissible range of dimension, and what it may lack is the specific property class the application needs — which is a question about the order, not about the grade.

The dimensional difference, in numbers

Accuracy differences are easy to assert and hard to act on, so it is worth putting the actual tabulated limits side by side. The shank diameter is the dimension where a grade C product most clearly differs from a grade A or B product of the same size, because the shank is the plain, unthreaded section that sits in the hole and in the bearing length.

Values are as tabulated in the withdrawn DIN 601 dimension table, the withdrawn DIN 931 dimension table and the DIN EN ISO 4014 dimension table, all reproduced by permission of DIN. Grade A applies to threads M1.6 to M24 with nominal lengths up to 10d or 150 mm, whichever is shorter; grade B applies above that, so the two right-hand minima in the grades A and B column are the two tolerance classes within the same standard. The upper limit of both bands is the nominal diameter, which puts the whole grade A to grade B spread at M10 between 9.64 mm and 10.00 mm.

The pattern repeats at every size: the grade C shank tolerance band is 1.16 mm wide at M8 and M10, 1.40 mm at M12 and M16, and 1.68 mm at M20 and M24, against a combined grade A and grade B envelope — measured from the grade B minimum to the nominal upper limit — of 0.36 mm at M10, 0.43 mm at M12 and M16, and 0.52 mm at M20 and M24. Thread length b for the standard length bands is identical in all three documents at every size they share.

That is what the grade difference is worth in practice. A grade C shank may sit below the nominal diameter by up to 0.58 mm at M10, against 0.36 mm for the grade B minimum, and the gap in the permitted band widens with size — the same two figures are 0.84 mm and 0.52 mm at M24. The consequence is a looser fit in a close-tolerance hole, which is why a grade C bolt suits a clearance hole in general steelwork and is a poor choice where the shank locates the joint. The shank is not the only dimension affected. Head height is specified as a range rather than a single value, and the grade C range is wider: at M10, ISO 4016 permits 5.95 mm to 6.85 mm against 6.22 mm to 6.58 mm in ISO 4014 grades A and B. That is the distinction worth carrying away — the nominal dimension may be the same while the tolerance around it is not, so a grade C part should not be assumed to match a grade A or B part on a controlled dimension without checking the permitted limits.

Head width: the size that trips up a substitution

The width across flats is the other place the two designations differ, and it is the one that stops a job because a socket either fits or it does not. It has to be handled in two steps, because two separate things changed.

First, the older DIN 601 range used the older, larger heads at the smaller sizes. The withdrawn DIN 601 table gives an M10 bolt 17 mm across flats and an M12 bolt 19 mm. Those values are confirmed in the trade: Würth product references for DIN 601 hardware are spanner-referenced as WS19 at M12 and WS24 at M16, which is the DIN series.

Second, the DIN-to-ISO move changed the head at exactly four sizes, in three cases downwards. The published cross-reference tables and German technical literature record the same four sizes for the hex head family: M10 from 17 mm to 16 mm, M12 from 19 mm to 18 mm, M14 from 22 mm to 21 mm, and M22 in the other direction, from 32 mm to 34 mm. Every other size in the hex range takes the same spanner before and after the change.

The current ISO 4016 values were read from the published DIN EN ISO 4016 dimension table and cross-checked against the scope of ISO 4016:2022. Every size outside M10, M12, M14 and M22 takes the same spanner in the DIN and ISO tables — which is why “DIN 601 or ISO 4016?” is a harmless question at M16 and a real one at M10.

The practical reading is narrower than it looks. A DIN 601 bolt bought today may carry either head, depending on the table the maker worked to, and an M10 order described only as “DIN 601” does not say which. If the assembly has already been tooled for 17 mm at M10 or 19 mm at M12, specify the head width rather than the historical standard number. If the head sits in a recess, a counterbore or a clearance pocket, the same 1 mm applies to the clearance as well as to the socket.

DIN 601 and DIN 931 are withdrawn — what governs a new order

Neither designation is a current German standard, and neither passed straight to its ISO successor. The chain has an intermediate European step, and drawings that cite it are still in circulation.

• DIN 601:1987 — hexagon head bolts, product grade C. Withdrawn. The dimension table carries the cancellation notice and records the replacement as DIN EN ISO 4016.
• DIN EN 24016:1992-02 — the European adoption of ISO 4016, published as Hexagon head bolts — Product grade C. This is the intermediate document, with its own number, and a drawing citing it is citing neither DIN 601 nor ISO 4016 directly.
• DIN EN ISO 4016:2001, then DIN EN ISO 4016:2011 — the EN ISO editions, the second superseding the first.
• DIN EN ISO 4016:2022-10, the German edition of ISO 4016:2022 — current. ISO 4016:2022 was published in June 2022 and replaced ISO 4016:2011.

The DIN 931 chain runs in parallel. DIN 931-1:1982-07 carried the German title for hexagon head screws with shank, threads M1.6 to M39, product grades A and B, described as ISO 4014 modified; DIN 931:1987 is the edition reproduced in the dimension tables and superseded the 1970 edition. Its recorded replacement is DIN EN ISO 4014, and the current German edition is DIN EN ISO 4014:2022-10, the adoption of ISO 4014:2022 — hexagon head bolts, product grades A and B, coarse pitch M1.6 to M64.

Two practical points follow. First, a new part should be specified against the current standard, with the DIN number kept as the commercial description the market recognises — which is exactly what a catalogue line such as “DIN 601 / ISO 4016” is doing. Second, the ISO numbers need an edition as well as a number. ISO 4014 and ISO 4016 have both been published in several editions, and a call-out that gives only the number leaves the edition to be agreed.

Where DIN 933 fits, and why it is a different question

DIN 933 is the third designation that gets pulled into this comparison, and it differs from DIN 601 in two respects rather than one. DIN 933 is a hexagon head screw threaded up to the head — a fully threaded product, with no plain shank. DIN 601 and DIN 931 are both partially threaded, with an unthreaded shank below the head. That is a difference in thread configuration, and it is the reason the two answer different questions in a joint: a partially threaded bolt puts plain shank in the shear plane and in the bearing length, while a fully threaded screw gives adjustable engagement and suits tapped holes and varying grip lengths.

On top of that, the standards split the same way they do for the bolt family. DIN 933 maps to ISO 4017:2022, Fasteners — Hexagon head screws — Product grades A and B, while DIN 601 maps to ISO 4016, product grade C. So “DIN 933 vs DIN 601” combines a thread configuration change with a product grade change, whereas “DIN 931 vs DIN 601” is a product grade comparison between two bolts of the same form. Substituting a fully threaded DIN 933 hex head bolt for a DIN 601 bolt therefore changes the joint, not just the part number, before the grade is even considered.

Two further distinctions are worth keeping straight. The first is that the corresponding standard for the partially threaded grade A and B bolt is DIN 931, the partially threaded hex bolt — not DIN 933 and not DIN 601; how the DIN 931 designation is read is covered separately. The second is that a size range is not a form. DIN 601 reaches a larger diameter in its own table than DIN 931-1 does, but that says nothing about which bolt a given joint needs.

Checking a DIN 931 / DIN 601 substitution

A nominally similar hex head bolt should not be treated as interchangeable without checking the dimensions and technical requirements that the application actually depends on. Most of the risk sits in six checks.

1. Record which document and edition the drawing cites — DIN 601, DIN EN 24016, DIN EN ISO 4016 or ISO 4016, and the year. A drawing citing a DIN number is describing a withdrawn document; the current standard is the reference point for anything new.
2. Check the width across flats at M10, M12, M14 and M22. This is the check that prevents a line stoppage, and the one most easily skipped, because the difference is invisible until a socket is offered up. At M10 the withdrawn DIN 601 table gives 17 mm and the current ISO 4016 table gives 16 mm.
3. Decide deliberately whether grade C tolerance is acceptable. If the shank locates the joint, fits a close-tolerance hole or is used in shear, the grade C band at M10 is wider than in grades A and B and may be too wide for the fit, and the grade A or B product is the correct one. If the bolt passes through a clearance hole, the wider band is normally irrelevant.
4. State the property class or stainless grade separately — 4.6, 4.8, 8.8, 10.9 or 12.9 under ISO 898-1, or a full stainless designation such as A2-70 or A4-80. Neither designation carries this information, and a change of designation on its own tells you nothing about the grade of the parts behind it.
5. Confirm the thread length if the shank position is functional. The standard b values agree between the two documents, so the check is whether the standard thread length is what the drawing needs rather than a special.
6. Confirm the finish and whether it affects thread fit — plain, zinc plated, hot dip galvanized or another coating. Hot dip galvanizing in particular has to be allowed for in the thread tolerance, which is why galvanized nuts are tapped oversize.

Write the call-out in one line so that none of it is left to inference. A workable example is ISO 4016 - M16 × 80 - 4.6 - hot dip galvanized, and the DIN 601 number can be kept alongside it as a cross-reference if the market expects the DIN designation. Where the drawing depends on a dimension, give the dimension rather than the historical standard number.

The difference in one paragraph

DIN 931 and DIN 601 describe the same product — a partially threaded metric hexagon head bolt with a coarse pitch thread — and they are split by product grade. DIN 931 specifies product grades A and B, the tighter dimensional tolerances; DIN 601 specifies product grade C, the loosest. Product grade is a dimensional accuracy class from ISO 4759-1 and carries no strength meaning, so a DIN 601 bolt is not a weaker bolt, only a less tightly toleranced one; the strength comes from the property class selected from ISO 898-1. The nominal head height and the standard thread length are shared at the overlapping sizes, but the tolerances around them are not — the difference is one of permitted dimensional tolerance, not of nominal dimension, and it reaches the head height as well as the shank. The grade C shank tolerance band is 1.16 mm wide at M8 and M10, 1.40 mm at M12 and M16, and 1.68 mm at M20 and M24, against a combined grade A and grade B envelope of 0.36 mm at M10, 0.43 mm at M12 and M16, and 0.52 mm at M20 and M24; a grade C shank may sit up to 0.58 mm below nominal at M10, against 0.36 mm at the grade B minimum. The head widths add a second, separate issue: the withdrawn DIN 601 table used the older DIN heads (17 mm at M10, 19 mm at M12), while the current ISO 4016 table uses 16 mm and 18 mm at those sizes, with M14 at 21 mm and M22 at 34 mm, so an M10 head can differ by 1 mm within the same designation. Both DIN numbers are withdrawn, replaced through DIN EN 24016 and DIN EN 24014 into ISO 4016:2022 and ISO 4014:2022. Whether one can stand in for the other is settled by the drawing, not by the name on the box: check the across-flats at M10, M12, M14 and M22, decide whether the grade C tolerances on the shank diameter and head height are acceptable for the joint, and state the property class, the finish and the current standard with its edition.