Washer Dimensions Calculator

Bore, rim, thickness and — the number that actually decides the choice — the bearing area of the annulus, for the small, normal and large series and for DIN 127 split lock washers. Computes the contact pressure that area puts on the clamped member, compares it with the member’s bearing strength, and gives the outside diameter you would need if it is over. Also the sourced position on split lock washers, which is that they do not lock and their DIN standards were withdrawn.

Last updated 24 Sep 2026

Washer dimensions

Size, series and preload → area and pressure

Bolt size the washer is forThe washer’s bore follows the bolt, not the hole in the part. DIN 127 split washers are tabulated here only from M16 up, because the smaller rows were not obtained from a source this batch would publish.

Washer standardISO 7090 is ISO 7089 with a chamfer and the same bore, outside diameter and thickness, so it is not a separate row; ISO 7091 is the same dimensions at product grade C and 100 HV rather than 200 HV. What changes the ANSWER on this page is the series — small, normal or large.

Preload in the bolt (kN)23 kN is roughly 70% of the proof load of an M10 class 8.8. The preload itself is a bolted-joint question; this page only spreads it over the washer’s annulus.

Bearing strength of the clamped member (MPa)What the washer is pressing INTO. About 250 MPa for structural steel, 100–200 for wrought aluminium alloy, 60 for a glass-filled nylon, 30–45 for unfilled thermoplastics, 5–10 across the grain of softwood, 2–4 for softwood end grain. The default is a filled plastic, which is where a washer earns its place.

Not a circuit: the three plain washer series as annuli, drawn to scale against each other on the left, and the contact pressure as a bar on the right. The three circles are the outside diameters of the large, normal and small series and the inner circle is the bore — the whole set is scaled so the large series just fills the space, which means the picture stays comparable as you change the size while the RATIOS between the three are true. The radial ticks mark the normal series' annulus, which is the area doing the work; the large series is about 2.7 times it and the small series about two thirds of it. The bar runs from zero to twice the member's bearing strength with the vertical line at 100%, so a bar past the line is a washer pressing harder than the material can take — which shows up as embedment and lost preload rather than as anything you can see. The bar is gated in steps of 5% of the strength, which is about a pixel, so it moves smoothly without shipping four hundred shapes.

168.4%Example

An ISO 7089 M10 washer, 23 kN of preload, on a member with a 60 MPa bearing strength — a glass-filled plastic

An annulus, a load, and a material that has to take it

bearing area = π(d₂² − d₁²) / 4 · contact pressure = F_preload ÷ area · utilisation = pressure ÷ member bearing strength · area needed = F_preload ÷ strength · d₂ needed = √(4·area/π + d₁²)

d₁, d₂: the washer’s bore and rim. The bore follows the BOLT, not the hole in the part — which is why a washer over an oversize or slotted hole needs the large series, not a bigger bore
area: the annulus. This is the whole of the washer’s contribution, and the three series differ by a factor of four across it
pressure: preload over area, assuming the pressure is uniform over the annulus. It is not: a thin washer bends and the pressure concentrates near the bore. The thickness ÷ radial width figure on this page is how you tell whether that assumption is safe
utilisation: the headline. A number over 100% means the washer will emboss itself into the member, which shows up as preload lost in the first few load cycles rather than as an immediate failure
d₂ needed: run the calculation backwards: the outside diameter that would keep the pressure inside the material’s bearing strength at the same bore. If it is bigger than the large series, you need a plate and not a washer

Worked example

An ISO 7089 M10 washer, 23 kN of preload, on a member with a 60 MPa bearing strength — a glass-filled plastic
The washer is 10.5 mm bore, 20 mm outside, 2 mm thick. Bearing area = π(20² − 10.5²)/4 = 227.6 mm²
Contact pressure = 23,000 N ÷ 227.6 mm² = 101.1 MPa
Against a 60 MPa bearing strength that is 168.4% — so the washer will press itself into the member, and the preload will fall as it does
Run it backwards: to stay at 60 MPa you need 23,000 / 60 = 383 mm², which at a 10.5 mm bore means an outside diameter of √(4 × 383/π + 10.5²) = 24.5 mm
The ISO 7093-1 large series at M10 is 30 mm outside, giving 620 mm² — 2.7 times the normal washer — and 37.1 MPa, which is 62% of the bearing strength. That is what the large series is for, and it is the answer here
For comparison: the same preload straight under an ISO 4032 M10 nut, with no washer at all, bears on 72 mm² and makes 317 MPa. The washer is not a nicety

Three series, three bearing areas — the number a washer is chosen for

A washer’s job on a soft member is to spread load, so the column that matters is the area of the annulus between its bore and its rim — and the three series are not small variations on each other. At M10 the small series gives 148 mm², the normal 228 and the large 620: the large series is 2.7 times the normal one and 4.2 times the small. The last column shows why it is described as “outside diameter about 3 d”, which it is, to within a few per cent, across the whole range. Note where the small series is the right answer: inside a counterbore, or under a socket head where a normal washer would not fit — not for load spreading. These dimensions come from a published standard’s table, not from a formula. The standard itself is cited below and the printed values are attributed to the catalogue they were taken from; a different publisher may round differently in the last digit.
Size	Small d2 (mm)	Normal d2 (mm)	Large d2 (mm)	Small area (mm²)	Normal area (mm²)	Large area (mm²)	Small ÷ normal	Large ÷ normal	Large d2 ÷ bolt size
M5	9	10	15	42	56	155	0.736	2.738	3.00
M6	11	12	18	63	81	222	0.777	2.747	3.00
M8	15	16	24	121	146	397	0.833	2.726	3.00
M10	18	20	30	168	228	620	0.738	2.726	3.00
M12	20	24	37	181	320	942	0.568	2.948	3.08
M14	24	28	44	276	439	1,344	0.628	3.061	3.14
M16	28	30	50	389	480	1,737	0.810	3.619	3.13
M18	30	34	56	423	624	2,179	0.678	3.491	3.11
M20	34	37	60	562	729	2,481	0.770	3.404	3.00
M24	39	44	72	704	1,030	3,581	0.683	3.477	3.00
M30	50	56	92	1,209	1,708	5,792	0.708	3.391	3.07
M36	58	66	110	1,567	2,346	8,309	0.668	3.542	3.06

What actually stops a fastener loosening

This table is the useful part of the page. The split washer is at the bottom not because it is weak but because it is answering a question it cannot answer: it stores spring energy, and a preloaded joint compresses it flat long before the fastener reaches working tension. Everything above it either works without stored spring force (prevailing torque, adhesive, positive locking, a ramp that the helix cannot climb) or removes the cause rather than the symptom (preload). Note that the ISO spring-washer standard was itself published and then withdrawn, so a drawing that calls for one by ISO number will be supplied to DIN 127 — which is also withdrawn. This page sizes a part; it does not certify one. Where the answer carries a consequence — a load path, a lifting duty, a pressure boundary, a fastener holding something that can fall — confirm it against the design code that governs the application, and against the manufacturer’s own rating, before relying on it.
Method	How it works	What the sources say
Correct preload	A joint that never goes slack never loses its friction. Most self-loosening starts with transverse slip, and slip needs the clamp force to have fallen far enough for the joint to move	This is the first answer in every serious treatment and the cheapest. It is also the one a split washer cannot help with, because it is flat long before the preload is reached. The preload calculation belongs to the bolted-joint pages, not here; what this page contributes is the embedment question — a washer over the member’s bearing strength loses preload in the first few cycles.
Prevailing-torque nuts — nylon insert or all-metal	The nut resists rotation by friction on the thread flanks, independently of the clamp force, so it still works when the joint is slack	NASA RP-1228 recommends a locknut over a nut-and-jam-nut assembly in terms: “A locknut is a much more practical choice than a regular nut and a jam nut.” Its own caution on the deformed-thread type is that “the nut can be reused approximately 10 times before it has to be discarded for loss of locking capability.” ISO 7040 and ISO 10511 are the nylon-insert forms; ISO 7719 is all-metal. Dimensions on the hex nut dimensions calculator.
Wedge-lock (ramped) washer pairs	Two washers with matched cams between them and teeth outward. Any rotation of the fastener has to climb the ramp, and the ramp angle exceeds the thread helix angle, so loosening would have to stretch the bolt	A different mechanism from a spring washer entirely: it does not rely on stored spring force, so flattening is not a failure mode. Take the specific figures from the maker; this page does not carry them because there is no public standard to cite.
Thread-locking adhesive	Fills the clearance between the flanks and cures, so the assembly cannot rotate without shearing the film	Works, with the usual caveats about cleanliness, cure time, temperature and the fact that a joint you may need to take apart wants the right grade. Also fills the gap that lets a loose joint move at all.
Positive locking — split pin through a castle nut, lock wire, tab washer	Mechanical obstruction. Independent of friction and of preload	The price is that you can only tighten to the next slot or hole, so the preload is whatever that position gives. Right where position matters more than clamp force. NASA RP-1228 recommends lockwiring for critical aerospace applications. Castle nut slot dimensions are on the hex nut calculator.
Helical spring (split) lock washer — DIN 127	Nothing, on a properly preloaded joint. It is a spring that has already been fully compressed	NASA RP-1228: “The lockwasher serves as a spring while the bolt is being tightened. However, the washer is normally flat by the time the bolt is fully torqued. At this time it is equivalent to a solid flat washer, and its locking ability is nonexistent. In summary, a lockwasher of this type is useless for locking.” ASME B18.21.1’s own scope says the same thing positively: these washers are for compensating for developed looseness, distributing load and providing a hardened bearing surface — not for preventing loosening. And DIN 127, 128, 137, 6797, 6798 and 7980 were, per Würth, “withdrawn without replacement, as there is no functional guarantee in combination with high-strength screws and bolts.”

DIN 127 B split lock washers — the dimensions, and what they come to once flattened

Only the sizes a reliable table was found for are printed; the rows below M16 were not obtained from a source this batch would publish, so they are absent rather than estimated. Read the last three columns together with the section below. Once the washer is flat — which NASA’s Fastener Design Manual says it normally is “by the time the bolt is fully torqued” — it is a flat washer, and it is a poor one: 67% to 80% of a normal ISO 7089 washer’s bearing area, which is about what the SMALL series gives, and with the annulus interrupted by the split. The h ÷ s column is the amount of spring available, and it is between 2.2 and 2.7 thicknesses — a few tenths of a millimetre of travel against a joint that closes by far less than that under preload. This page sizes a part; it does not certify one. Where the answer carries a consequence — a load path, a lifting duty, a pressure boundary, a fastener holding something that can fall — confirm it against the design code that governs the application, and against the manufacturer’s own rating, before relying on it.
Size	d1 (mm)	d2 max (mm)	Thickness s (mm)	Radial width b (mm)	Free height h max (mm)	h ÷ s — how much spring there is	Flattened area (mm²)	÷ ISO 7089 area	÷ DIN 433 area
M16	16.2	27.4	3.5	5.0	9.2	2.63	384	0.799	0.987
M18	18.2	29.4	3.5	5.0	9.2	2.63	419	0.671	0.989
M20	20.2	33.6	4.0	6.0	10.4	2.60	566	0.777	1.008
M24	24.5	40.0	5.0	7.0	13.0	2.60	785	0.763	1.116
M30	30.5	48.2	6.0	8.0	16.1	2.68	1,094	0.640	0.905

ISO 7093-1 against DIN 9021 — the same washer, two bores

Distributors sell these as one item — “ISO 7093 / DIN 9021 fender washer” — and at most sizes they are. At M18, M20 and M24 they are not: the DIN bore is a millimetre larger. At M5 the thicknesses differ instead, 1.0 against 1.2. The outside diameters agree everywhere. It is a small difference and it matters in exactly one situation: a washer chosen to bear on a narrow land, where an extra millimetre of bore eats into the annulus at the end where the pressure is highest. The two columns of area are computed so you can see what it costs. The same designation can mean different dimensions in different standards families — ANSI against ISO, inch against metric, one national standard against another. The family used here is named beside every figure; check which one your part was made to.
Size	ISO 7093-1 bore (mm)	DIN 9021 bore (mm)	Bore difference	ISO 7093-1 thickness (mm)	DIN 9021 thickness (mm)	Thickness difference	ISO 7093-1 area (mm²)	DIN 9021 area (mm²)
M5	5.3	5.3	same	1.0	1.2	+0.2	155	155
M6	6.4	6.4	same	1.6	1.6	same	222	222
M8	8.4	8.4	same	2.0	2.0	same	397	397
M10	10.5	10.5	same	2.5	2.5	same	620	620
M12	13.0	13.0	same	3.0	3.0	same	942	942
M14	15.0	15.0	same	3.0	3.0	same	1,344	1,344
M16	17.0	17.0	same	3.0	3.0	same	1,737	1,737
M18	19.0	20.0	+1.0	4.0	4.0	same	2,179	2,149
M20	21.0	22.0	+1.0	4.0	4.0	same	2,481	2,447
M24	25.0	26.0	+1.0	5.0	5.0	same	3,581	3,541
M30	33.0	33.0	same	6.0	6.0	same	5,792	5,792
M36	39.0	39.0	same	8.0	8.0	same	8,309	8,309

Hardness, and the one washer requirement people skip

Würth state the requirement plainly and note that it is “often ignored in practice”: “it is absolutely necessary to use at least hardness class 200 HV for high-strength screws and bolts of property class 8.8 and higher.” That rules out ISO 7091 and DIN 126, which are 100 HV, under an 8.8 bolt — a soft washer under a hard head is a washer the head embeds into, and every micron of that is preload lost. ISO 7090 differs from ISO 7089 only in having a chamfer on the bore, which is why it is not a separate row on the calculator: the bore, outside diameter and thickness are the same, so the bearing area is the same. These dimensions come from a published standard’s table, not from a formula. The standard itself is cited below and the printed values are attributed to the catalogue they were taken from; a different publisher may round differently in the last digit.
Standard	Series and grade	Hardness	DIN equivalent
ISO 7089	normal, product grade A	200 HV	DIN 125 Form A
ISO 7090	normal, grade A, CHAMFERED	200 HV	DIN 125 Form B
ISO 7091	normal, product grade C	100 HV	DIN 126
ISO 7092	small outside diameter, grade A	200 HV	DIN 433
ISO 7093-1	large, about 3 d, grade A	200 HV	DIN 9021
ISO 7094	extra large	200 HV	no DIN equivalent

The area is the answer, and the split washer is not

A washer’s only real contribution is its bearing area, and the three series differ by a factor of four. At M10 the small series (ISO 7092 / DIN 433) gives 148 mm², the normal series (ISO 7089 / DIN 125 A) 228 mm², and the large series (ISO 7093-1 / DIN 9021) 620 mm² — 2.7 times the normal one. Put 23 kN through each and the contact pressure is 155, 101 and 37 MPa. Against a glass-filled plastic at 60 MPa the first two crush the member and the third does not. That is the whole decision, and it is why the large series exists: soft members, oversize holes, slotted holes. Worth noting what a washer is NOT for: it does not make the bolt stronger, and on a hard member with a clean hole it is often doing nothing at all except protecting the surface from the spanner.

And it is worth seeing how small the bearing face is without one. A hex nut does not bear on its flats; it bears on a circle of diameter d_w, and over a clearance hole an ISO 4032 M10 nut leaves an annulus of 72 mm². The same 23 kN through that is 317 MPa, which will emboss structural steel. The washer triples the area for a few pence, and the large series multiplies it by nine.

Now the honest part: split lock washers do not lock. This is not a controversial opinion. NASA Reference Publication 1228, the Fastener Design Manual, says it in four sentences: “The lockwasher serves as a spring while the bolt is being tightened. However, the washer is normally flat by the time the bolt is fully torqued. At this time it is equivalent to a solid flat washer, and its locking ability is nonexistent. In summary, a lockwasher of this type is useless for locking.” ASME B18.21.1 makes the same point from the other direction by stating what these washers ARE for — compensating for developed looseness, distributing load, providing a hardened bearing surface — and not listing loosening resistance among it. And the standards themselves are gone: Würth’s DIN-to-ISO comparison records DIN 127, 128, 137, 6797, 6798 and 7980 as “withdrawn without replacement, as there is no functional guarantee in combination with high-strength screws and bolts”, and the ISO spring-washer standard was published and then withdrawn too, which is why a drawing calling for one by ISO number will be supplied to DIN 127 anyway.

The mechanism is worth understanding rather than just believing. A DIN 127 washer’s free height is between 2.2 and 2.7 times its own thickness, so the spring travel available is a fraction of a millimetre. A preloaded M16 joint closes by far less than that before the bolt reaches working tension, so the washer bottoms out early and everything after that is the bolt stretching, not the washer pushing. And once it is flat it is a flat washer with poor geometry: this page computes its flattened annulus against the plain series at the sizes where both are published, and it comes to 67% to 80% of a normal washer’s area — roughly what the SMALL series gives — with the annulus interrupted by the split and a sharp end that digs into the surface. You have paid for less bearing area and a stress raiser.

What does work is in the table below, and the first answer is preload. Self-loosening under vibration generally begins with transverse slip, and slip needs the clamp force to have fallen far enough for the joint to move — so a joint that keeps its preload does not need a locking device. That is where this page connects back to the arithmetic at the top: a washer working above the member’s bearing strength loses preload to embedment in the first few load cycles, and that loss is the beginning of the problem the lock washer was supposed to solve. After preload: prevailing-torque nuts, which resist rotation by thread friction independently of clamp force; wedge-lock washer pairs, whose ramp angle exceeds the thread helix so loosening would have to stretch the bolt; thread-locking adhesive; and positive locking — a split pin through a castle nut, lock wire, a tab washer. NASA recommends lockwiring for critical work and a locknut over a nut-and-jam-nut pair. Nut dimensions for all of those are on the hex nut dimensions calculator, the head end is on the hex head bolt dimensions calculator, and if the washer has to sit inside a recess the counterbore and countersink calculator sizes it. A bolted mounting that sees a shock load from a belt or chain drive is the classic case for checking the pressure under the washer rather than the bolt itself.

Frequently asked questions

Do split lock washers work?

No, not as locking devices on a properly preloaded joint. NASA Reference Publication 1228 is unambiguous: the washer “is normally flat by the time the bolt is fully torqued. At this time it is equivalent to a solid flat washer, and its locking ability is nonexistent. In summary, a lockwasher of this type is useless for locking.” ASME B18.21.1 lists their purposes as compensating for developed looseness, distributing load and providing a hardened bearing surface — locking is not among them. And the DIN standards for split and serrated lock washers were withdrawn without replacement, per Würth, “as there is no functional guarantee in combination with high-strength screws and bolts”. The reason is geometric: the free height is only two or three times the washer’s own thickness, so there is almost no spring travel, and a preloaded joint uses it up long before working tension.

If a split washer does not lock, is it worse than a plain washer?

Yes, at the same nominal size. Flattened, a DIN 127 washer’s annulus is 67% to 80% of a normal ISO 7089 washer’s at the sizes where both are published — about what the small series gives — and the annulus is interrupted by the split, whose sharp end bears into the surface. So you have bought less bearing area and a stress raiser. If the surface needs protecting, use a plain hardened washer; if the joint needs locking, use something from the table on this page.

What should I use instead?

In order: get the preload right, because a joint that does not go slack does not loosen. Then, if the duty needs it, a prevailing-torque nut — nylon insert (ISO 7040, ISO 10511) or all-metal (ISO 7719) — which resists rotation by thread friction whatever the clamp force is doing. Wedge-lock washer pairs work by a different mechanism again: a ramp between the two halves steeper than the thread helix, so the fastener cannot back off without stretching the bolt. Thread-locking adhesive works and fills the clearance. And positive locking — a split pin through a castle nut, lock wire, a tab washer — works regardless of friction, at the cost of only being able to tighten to the next slot. NASA RP-1228: “A locknut is a much more practical choice than a regular nut and a jam nut.”

When do I need the large series rather than a normal washer?

When the member is soft, the hole is oversize or slotted, or the computed contact pressure is over the member’s bearing strength. The large series is about three times the bolt diameter across and gives roughly 2.7 times the bearing area of the normal series at the same bore — 620 mm² against 228 at M10. Run this page with the member’s bearing strength as the input and it will tell you which side of the line you are on, and if neither series is enough, what outside diameter would be. Past that, the answer is a load-spreading plate.

Does the washer bore follow the bolt or the hole?

The bolt. A washer’s bore is nominally the bolt diameter plus a small clearance — 10.5 mm for an M10 — which is why using a washer with a bigger bore to suit a bigger hole is the wrong move: it removes area from the bore, which is exactly where the pressure is highest on a washer that bends. The right move for an oversize or slotted hole is a bigger OUTSIDE diameter, and the outside diameter has to be enough to land on solid material either side of the slot.

Does the washer hardness matter?

Under a high-strength bolt, yes, and Würth say it is “often ignored in practice”: “it is absolutely necessary to use at least hardness class 200 HV for high-strength screws and bolts of property class 8.8 and higher.” ISO 7089, 7090, 7092 and 7093-1 are all 200 HV. ISO 7091 and DIN 126 are 100 HV, and a soft washer under a hard head is a washer the head embeds into — every micron of which is preload gone. The bearing area is the same; the hardness is not.

Is ISO 7090 a different washer from ISO 7089?

Only in having a chamfer on the bore. The bore, the outside diameter, the thickness and the hardness class are the same, so the bearing area — which is all this page computes — is the same, which is why the two are not separate options. The chamfer matters where the washer has to sit against a fillet radius or under a head with its own fillet, and it is the Form B of DIN 125 where ISO 7089 is Form A. ISO 7091 is the one that is genuinely different: product grade C and 100 HV.

Related calculators

References

HUIHUI. ISO 7089, 7090, 7092 & 7093 Washers Explained — ISO Standards & DIN Equivalents and DIN, ISO & ASME Washer Standards: Cross-Reference & Equivalents. Source for the family map used here: ISO 7089 = DIN 125 Form A, 200 HV, grade A; ISO 7090 = DIN 125 Form B, chamfered; ISO 7091 = DIN 126, grade C, 100 HV; ISO 7092 = DIN 433, small series; ISO 7093-1 = DIN 9021, large series, outside diameter about 3 d. Also notes that the ISO spring-washer standard was published and then withdrawn, so a drawing calling for one by ISO number will be supplied to DIN 127.
Mech Codex. Metric Flat Washer Dimensions (ISO 7089 Normal / ISO 7093 Large). Source for the ISO 7093-1 column used here, printed beside ISO 7089 so the two can be read together. Its bore at M18, M20 and M24 is 19, 21 and 25 mm where Aspen Fasteners’ DIN 9021 sheet prints 20, 22 and 26 — a real fork between the ISO and DIN publications of what a distributor sells as one washer.
Aspen Fasteners. Metric DIN 9021 Flat Fender Washers, Metric DIN 433 Flat Washers, Metric DIN 127 Type B Helical Spring Split Lock Washers and Metric DIN 471 External Retaining Rings for Shafts. Distributor specification sheets, used for the printed dimensions those standards are cited for.
BigBoltNut. DIN 127 Spring Lock Washer Dimensions. Source for the DIN 127 form B rows at M16, M18, M20, M24 and M30 — inside diameter, outside diameter, thickness s, radial width b and free height h. Its table starts at M16, and the smaller sizes were not obtained from a source this batch would publish, so they are absent rather than estimated.
NASA Reference Publication 1228, Fastener Design Manual, Richard T. Barrett, 1990. On helical spring (split) lock washers, quoted: “The lockwasher serves as a spring while the bolt is being tightened. However, the washer is normally flat by the time the bolt is fully torqued. At this time it is equivalent to a solid flat washer, and its locking ability is nonexistent. In summary, a lockwasher of this type is useless for locking.” On the two-nut assembly: “this type of assembly is too unpredictable to be reliable … It would be rare to get the correct amount of torque on each nut. A locknut is a much more practical choice than a regular nut and a jam nut.” The NTRS PDF could not be fetched directly here; the text was read through Engineering Library’s Fastener Design Criteria transcription of it, which names RP-1228 as its source.
Robert Medure, Helical Spring Lock Washers: Separating Myth from Truth, 20 September 2011. Collects the NASA finding above with ASME B18.21.1’s own scope statement — that helical spring washers are not intended to prevent loosening, their stated functions being to compensate for developed looseness, to distribute load and to provide a hardened bearing surface — and with Bolt Science’s transverse-vibration testing.
Würth Industry. Fasteners: Differences between DIN – EN – ISO standards. The single most useful document this batch read. It tabulates DIN 934 against ISO 4032 at the four changed sizes, states that “nuts according to ISO 4032, ISO 4033, ISO 4034 … may not be replaced by nuts according to DIN standards which only have got a reduced loadability according to DIN 267 Part 4 … such as DIN 934”, that “it is absolutely necessary to use at least hardness class 200 HV for high-strength screws and bolts of property class 8.8 and higher”, and that DIN 127, 128, 137, 6797, 6798 and 7980 were “withdrawn without replacement, as there is no functional guarantee in combination with high-strength screws and bolts”.
Matrix Engineering. The Use of Two Nuts to Prevent Self Loosening. The stacking order, quoted: “the thin (jam) nut nearest the joint, tightened first to 25–50% of the final torque value. The thick nut is then tightened on top to full torque while the thin nut is held against rotation.” Wikipedia’s Jam nut article, citing the same paper and NASA RP-1228, adds that once assembled “the outer nut bears the full tension of the joint. The inner nut functions merely to add a small additional force to the outer nut and does not need to be as strong.”
ISO 273, Fasteners — Clearance holes for bolts and screws. Cited by number; the fine / medium / coarse columns used here are taken from EKINSUN’s What Size Clearance Hole for a Metric Bolt? ISO 273 Chart, which labels them H12, H13 and H14.