Metric thread on anchor and erection bolts: tolerance classes, thread run-out, defects and inspection

What the designation M24-6g/6H means

A thread designation is read from left to right: the size, the tolerance class, and where needed the length-of-engagement group and the hand. M24 is a metric thread of 24 mm diameter. The pitch is not given, so it is the coarse one (3 mm); a fine pitch is written separately, for example M20×2.

The tolerance class consists of a digit and a letter. The digit is the tolerance grade: the smaller it is, the narrower the tolerance. The letter is the position of the tolerance zone relative to the basic profile, lower case for the thread of a bolt and upper case for the thread of a nut. So 6g is a bolt and 6H is a nut.

When the class is written as two pairs, the pitch diameter comes first and the crest diameter second: 7g6g means 7g on the pitch diameter and 6g on the crests. If both pairs are the same, the second is not repeated, and 6g is a short form of 6g6g. The fit of a pair is written as a fraction with the nut in the numerator and the bolt in the denominator: M24-6H/6g.

With no tolerance class in the designation, the standard assumes the medium class: 6g for the bolt and 6H for the nut (from 1.6 mm diameter). Still, it is better to always write the class, because a bare «M24» leaves purchasing free to take any variant.

Tolerance position: how much clearance is already built in

The tolerance position is set by the fundamental deviation: how far the tolerance zone is shifted from the basic profile. For a nut, H has zero deviation, while G shifts the zone up by a positive amount. For a bolt, h is also zero, while g, f, e and d shift the zone down by a growing amount.

This shift is what gives clearance in the thread even when the size is at the edge of the tolerance. It grows with the pitch, so M36 has more clearance than M16. The table gives the fundamental deviations for the pitches found on anchor and erection bolts of poles.

Bolt deviations (es) from the basic profile. For a nut, position H is zero and G is the same number with a plus sign. The standard intends positions e and f for threads with a thicker coating.

What a tolerance grade gives

From grade to grade the pitch diameter tolerance grows by a factor of 1.25. For M24 with a 3 mm pitch, the pitch diameter tolerance of a bolt is 160 µm in grade 5, 200 µm in grade 6, 250 µm in grade 7 and 315 µm in grade 8. For a nut of the same grade 6 it is wider by a factor of 1.32: 265 µm.

Which tolerance classes are chosen for bolts and nuts

The standard divides the classes into three tolerance qualities and three length-of-engagement groups. For general-purpose bolts and nuts with normal length of engagement the medium quality is enough: 6g for the bolt and 6H for the nut. These are the classes the standard selects for commercial fasteners.

The fine quality (4h for the bolt, 5H for the nut) is needed where the variation in the character of the fit must be small. It is not required in steel structures. The coarse quality (8g for the bolt, 7H for the nut) is recommended by the standard where manufacturing difficulties may arise, for example when cutting a thread on hot-rolled bars. That is close to anchor bolts, which are made from round rolled bar.

The standard ranks the classes by order of choice; 6g and 6H are in the first order and are selected for commercial fasteners, classes in brackets are the third order.

Length of engagement: S, N and L

The limits of the groups depend on the diameter and the pitch. For M24 with a 3 mm pitch the normal length of engagement is over 12 and up to 36 mm, for M36 (4 mm pitch) over 18 and up to 53 mm; less is group S, more is group L. A nut about 0.8 of the diameter high falls into the normal group. If a bolt is screwed into a long threaded sleeve, the length may move into group L, and the class is taken from the corresponding column.

The fit is chosen so that enough working height of the profile remains: the nut and the bolt should give fits H/g, H/h or G/h. A 6g bolt with a 6H nut is an ordinary clearance fit.

What the standards require from anchors: thread and position in the foundation

DBN V.2.6-198:2014 does not name a thread tolerance class for foundation bolts: the design tensile resistance of an anchor is taken from Table D.6, and the thread is set by the project. DSTU EN 1090-2:2019 (cl. 5.6.7) requires of a foundation bolt only mechanical properties to EN ISO 898-1 or steel to EN 10025-2…10025-4; it gives no thread accuracy class.

So a tight thread accuracy on an anchor is not a requirement of the standards but a choice of the project. For sets to EN ISO 898-1 it is 6g for the bolt and 6H for the nut; for an anchor made of hot-rolled bar, with the thread cut in the shop, the thread standard allows the coarser class 8g. The class is always stated on the drawing, but a tighter one than the joint needs is not demanded.

The position of anchors in the foundation is regulated more precisely. Under DSTU EN 1090-2:2019 the deviation of the centre point of a group of foundation bolts from the specified position must not exceed ±6 mm relative to the setting-out grid (cl. 11.2.3.2). The deviation from the vertical of the projecting part of a vertical anchor, under the tabulated functional tolerances, is not more than 1 mm in 20 mm of length (cl. 11.3.2).

Holes in base plates are made with a clearance that covers these deviations; for this, large-diameter washers may be placed between the nut and the plate (cl. 11.2.3.3). If anchors work with preload, the upper part of the bolt at least 100 mm long is made without bond to the concrete, and bolts that move are placed in sleeves with a diameter of three bolt diameters, but not less than 75 mm (cl. 9.5.2).

Erection starts only when the position and levels of the supports meet the acceptance criteria and the survey results are documented (cl. 9.5.2). The tolerances on anchor position and the way they are checked are written into the project and the as-built scheme; for the foundation input data see the article Input data for foundation design, for the bolts themselves the article Anchor bolts.

Classes 6g and 6H in numbers for common diameters

The deviations in the table are measured from the basic profile, in micrometres. The upper deviation of a bolt is the same for the crest diameter and the pitch diameter, the lower one differs. For a nut the lower limit is zero, that is, the smallest size of the nut coincides with the basic size.

d is the crest diameter of the bolt, d2 its pitch diameter, D2 the pitch diameter of the nut, D1 the crest diameter of the nut. Absolute sizes are taken from the table of basic dimensions of the metric thread, and the deviations are added.

How this reads on M24

The pitch diameter of an M24-6g bolt lies between 48 and 248 µm below the basic size, and that of a 6H nut between the basic size and +265 µm. The smallest clearance on the pitch diameter in the assembly is 48 µm, the largest 513 µm. This is the clearance the coating «eats», so it is taken into account in advance.

Thread on galvanized bolts and nuts

The tolerances in the thread system apply to the sizes before the coating. For thin coatings such as electroplating this is enough: the layer fits into the clearance of position g, f or e, but after coating the profile must not go beyond the basic one, that is, beyond position h for the bolt or H for the nut. Hot-dip galvanizing gives a layer over 40 µm. It lies on both flanks of the profile and increases the pitch diameter several times more than its own thickness, so the 6g clearance is not enough for it.

For hot-dip galvanizing of threaded products there are separate standards: ISO 965-4 for external threads and ISO 965-5 for internal ones. ISO 965-1 provides positions az, AZ and AX for such threads. A pair is assembled by one of two methods, and they must not be mixed.

Long rods and anchor bolts

Long rods and structures are galvanized by dipping, without spinning, so the layer on the thread is thicker there and the allowance for it has to be larger. A nut with an allowance is not tapped again, and the thread of a galvanized bolt is not measured by stripping the coating: the process itself dissolves some of the steel. Which nuts suit which bolt is agreed with the galvanizing plant before fabrication, not on site.

We supply anchor cages without a coating. If the project requires galvanizing anchor bolts or fasteners for connections, the drawing states the galvanizing method, the thread class of the bolt and the type of nut; more on galvanizing fasteners is in the articles Fastener strength classes, Design of structures for hot-dip galvanizing and Zinc coating thickness.

Rolling or cutting

A thread on a rod is made in two ways. Cutting removes metal with a tool, a die or a chaser, while rolling presses the profile in with rollers in cold metal, with no chips. Both methods give a thread in the same tolerance class, and the tolerance system does not depend on the method.

The difference is in the profile and the surface. In a rolled thread the fibres of the metal follow the profile and the surface is smooth and work-hardened; its characteristic defects are laps and seams on the crest or in the root. In a cut thread the fibres are severed, and it gives tears, burrs and steps from the tool.

For bolts of property class 8.8 and above, the root profile of the external thread must be a smooth curve, with a radius nowhere smaller than 0.125 of the pitch: for M24 that is 0.375 mm, for M36 0.5 mm. For lower classes working under alternating load or impact this is also desirable. A root with a sharp corner from a worn or wrongly ground tool will not meet the requirement.

The diameter of the blank for rolling and for cutting is different, so the method is agreed before the rod is cut. On an anchor bolt drawing the method need not be fixed, but the tolerance class and the thread length always are.

The company POLYGONAL cuts the thread on anchor bolts itself. The tolerance field and the length of the threaded part are set by the drawing, so they are stated in the KMD or in the request for anchor bolts.

Thread length and run-out

The full thread profile does not end abruptly. After the full thread comes a run-out: a section where the profile height falls because the tool or the roller leaves the metal, and then the smooth part of the rod. A run-out is not a defect, but a nut does not sit on it, so the thread length under the nut is counted up to the full profile.

The size of the run-out depends on the pitch and on the lead angle of the tool: the smaller the angle, the longer the run-out. In a rolled thread a short length remains between the thread and the head or the smooth part. The table gives indicative maximum values for common diameters; in a part, what the drawing sets applies.

Maximum values. Run-out x is for a cut thread, short length a for a rolled one.

How much thread to leave on an anchor bolt

Above the concrete the thread must cover the thickness of the flange, the washer, the nut and a projection of the rod beyond the nut of at least one pitch. If the flange stands on levelling nuts, one more nut and the gap under the flange are added. The run-out is not part of this count.

A margin for the error in the height of the cage is added separately: the flange may sit higher or lower than the design level. For the thread in the shear plane and the length of a bolt in the plate pack see the article on bolted connections of steel structures; for cages and bolts under poles see Anchor bolts for poles and masts.

Surface and thread defects: what is permitted

Bolt defects are divided by origin. Some come from the material (laps in the rolled bar), some from forming (forging cracks, folds), some from heat treatment (quench cracks), and some from thread cutting or rolling (tears, burrs, nicks). The limits are set as fractions of the diameter d, so they are easy to calculate for a specific bolt.

The values are taken as a guide for bolts cut from rod: the standard for surface defects covers bolts, screws and studs from 5 mm diameter, and the requirements for anchor bolts from bar are set by the project.

What makes a defect dangerous

Worst are quench cracks and folds under the head or at the transition to the smooth part: fracture starts from them under overload and alternating load. A nick on a thread crest is almost harmless for strength, but the nut will not run on past it. So acceptance goes by symptom: if the nut or the gauge does not run, the cause is looked for.

Inspection: visual check and gauges

A visual check is made without magnifying instruments; in disputed cases a 2.5–3× loupe is used. The thread is examined over its whole length, along with the transition to the smooth part and the end face. Cracks, folds and tears show best in raking light.

A thread is checked with a pair of gauges: go and no-go. For a bolt these are rings or snap gauges, for a nut plugs. The go gauge must screw on over the whole length of the thread with no more effort than 0.06d N·m: this confirms that the size has not gone beyond the maximum material limit and the profile is not damaged.

The no-go gauge must not screw on over the whole length: it checks the pitch diameter. The design of gauges and the measuring procedure are set by the standard for gauges for metric threads.

Hidden defects are found by a laboratory: deep etching of a section or the magnetic particle method. The property class is confirmed by a tensile test and a hardness measurement. If a batch raises doubts (no maker’s mark, a thread that looks very different, a gauge that does not run), it goes to a laboratory test rather than being corrected on site with a tap or a die.

What to state in the shop drawings and the request

The schedule and the drawing of an anchor bolt state the diameter and the pitch (if not coarse), the tolerance class for the bolt and for the nut (for example, M24-8g and M24-6H for an anchor made of hot-rolled bar; M24-6g and M24-6H for a set to EN ISO 898-1), the thread length and from which end it is counted, the property class or steel grade, and the coating. An entry such as «metric thread» without a tolerance class fixes nothing.

If the fasteners are galvanized, the galvanizing method and the pair variant are added: a nut with an allowance tapped after galvanizing, or a bolt with a reduced thread. An example note: «Thread M24-8g, thread length L1 from the free end, run-out not included in the length; nuts M24-6H as per schedule».

We prepare KM and KMD drawings and fabricate steel structures to the customer’s drawings: KM and KMD design, steel fabrication. We supply fasteners for connections together with the structures, and erection is carried out by our partner construction company. Sizes of bolts and nuts are in the fastener range tables, an example schedule is in the article on a KMD sheet and schedule, cages are in the article on foundation anchor cages.

Where mistakes are most common

• no tolerance class stated: purchasing takes what is in stock, and nuts of different batches give different clearance;
• the bolt was galvanized and an ordinary 6H nut was taken: it will not run on, and tapping again on site reduces the load capacity of the thread. For galvanized bolts the nut is taken according to the agreed pair variant;
• a bolt to the standard for galvanized external threads was taken with a nut to the standard for nuts for galvanized bolts: this pair is not assembled, the thread strips;
• «thread 150 mm» with no note on where it is counted from and whether the run-out is included: the nut sat on an incomplete profile, or there was not enough travel for the levelling nut;
• the thread was not protected on site: an impact during unloading or concrete splashes, and the nut no longer runs freely;
• the property class was confused with the tolerance class: 8.8 speaks of the metal of the bolt, while 6g speaks of the size of its thread. One does not replace the other.

Standards and validity

The tolerance system for general-purpose metric threads is set out in ISO 965-1; the fifth edition came out in April 2026 and replaced the 2013 edition. For threads for hot-dip galvanizing there are ISO 965-4 (external thread) and ISO 965-5 (internal thread). The basic dimensions of metric threads are given in ISO 724:2023, gauges and gauging in ISO 1502.

The mechanical properties of bolts, screws and studs are covered by DSTU EN ISO 898-1:2022, and those of nuts by ISO 898-2:2022. Hot-dip galvanizing of threaded products is covered by ISO 10684:2004 (confirmed in 2021), and surface defects of bolts by ISO 6157-1. Thread run-out and undercuts are covered by ISO 3508 and ISO 4755; the run-out table above is for orientation.

The position and tolerances of foundation bolts, the requirements for sleeves and the acceptance of supports are in DSTU EN 1090-2:2019 (current, cl. 5.6.7, 9.5.2, 11.2.3.2, 11.2.3.3, 11.3.2).

Which current DSTU reproduces ISO 965-1, ISO 965-4, ISO 965-5, ISO 724, ISO 1502, ISO 6157-1, ISO 3508 and ISO 4755 is checked before they are cited in project documents. The figures in this article are for orientation; a project uses the current text of the standards.