Why one diameter has more than one pitch

The choice that exists at all

An M8 screw is 8 mm nominal diameter whichever pitch it has. What changes is how far the thread advances per turn: 1.25 mm for the coarse series, 1.0 or 0.75 mm for the fine ones. The pitch diameter, the minor diameter and the thread depth all move with it — which is exactly why the numbers below come out the way they do.

Each diameter has one coarse pitch listed, and one or more fine ones. ISO 261 defines the coarse series as the largest pitch in current use at each diameter; it does not say coarse is a default, and the reason coarse usually wins is worked out below rather than read off the standard.

The screw side: fine is slightly stronger

Tensile capacity comes from the stress area, computed from the pitch and minor diameters rather than from the nominal 8 mm. A finer pitch cuts a shallower groove, so both diameters are larger and so is the area.

Real, and worth knowing, but single digits. A joint that needs more than that needs a larger diameter or a higher class, not a different pitch.

The nut side: pitch cancels, and this is the part people get wrong

The intuitive argument is that a finer pitch packs more threads into the same depth of hole, so there is more metal to shear and the female thread is stronger. That argument is wrong, and it is wrong in an instructive way.

The standard geometric shear area for an internal thread is A = π·L_e·d·[0.5 + (0.57735/P)(d − d₂)]. For the ISO basic profile d − d₂ = 0.649519 P, so the pitch inside the bracket cancels against the pitch outside it:

One clarification, because the usual sources appear to say the opposite. They say a fine thread is stronger “in tension and in shear”, and they are not disputing the result above — they mean a different failure. Their shear is the bolt being cut across, which is why they attribute it to the minor diameter. Stripping is the internal thread being pushed off a cylinder. One catalogue makes the split explicit by listing resistance to stripping as a coarse advantage on the same page as “stronger in tension and shear” for fine.

Which of them fine actually wins depends on where the shear plane falls, and the codes do not agree on the area. Eurocode 3 uses the tensile stress area when the plane passes through the threads, worth about 7% to fine, and the gross section when it passes through the shank, worth nothing. AISC works from the nominal unthreaded body area and handles threads through the allowable stress instead, so pitch buys nothing there either. VDI 2230 picks the diameter to suit the plane. So: fine has an edge on bolt shear in some cases and none in others, and on stripping the basic-profile geometric area is the same for every pitch.

This is the basic-profile result. A full calculation uses limit sizes rather than basic ones, which reintroduces a pitch-dependent term through the tolerance classes, and real stripping strength also depends on the relative strength of the two materials, nut dilation and the uneven load sharing that puts most of the force on the first engaged thread. None of that restores a simple “finer is stronger” rule. See thread engagement for how the length is chosen, length and grip for what the rest of the screw is doing, and stripped threads for which side actually fails.

So what is choosing the pitch

If strength is a few percent on one side and nothing on the other, then pitch is settled by manufacturing and service conditions, and those genuinely point both ways.

The coating point, stated properly

Room for a coating is not a fixed clearance shared by all pitches. It is set by the tolerance position — the letter in 6g — which places the tolerance band below basic size deliberately, and by the grade, which sets its width. The plated thread must still stay inside the maximum material limit.

Pitch enters indirectly: tolerances in the ISO system scale with pitch, so a coarse thread's bands are wider in absolute terms and there is more absolute room to work with. That is a real advantage and a different mechanism from the one usually given. See what the tolerance class means and what happens when the allowance runs out.

The vibration claim, with the numbers attached

Fine threads are widely said to resist loosening because the helix angle is shallower. The angles are 3.17° for M8×1.25 and 2.48° for M8×1. Whether that counts as self-locking depends on friction, which the claim usually omits: the equivalent friction angle for a 60° thread is arctan(μ/cos 30°), giving 2.6° at μ = 0.04 and 3.3° at μ = 0.05.

Where this sits in the decision order

Pitch belongs in step two because it follows from the female side, which was settled in step one. A casting or a plastic boss that cannot hold a thread on its own usually argues for coarse, on tapping and damage grounds — usually, not always, since a hard parent or a shallow wall argues the other way and those do occur together.

What should not decide it on its own is the load, or the turn count. Fine buys single-digit percentages on one side of the joint and, in the basic profile, nothing on the other, so it is rarely enough on its own to rescue a marginal design. Diameter, property class and engagement length are the levers that move. See what the property class numbers mean and the full decision order.

One level up, the same shape of answer applies to how many starts the thread has — and why it is almost always one.

This page covers step 2, the thread. The whole order is substrate, thread, head, drive, finish, documentation, and why doing it out of order is rework rather than a tweak is in specifying a screw.

Common questions

Is a fine thread stronger than a coarse one?

On the screw, yes, by about 6 to 9 percent of stress area for common sizes. On the nut side the pitch term cancels out of the basic stripping geometry, so there is no gain there in that model; using limit sizes reintroduces a small tolerance-dependent difference that can go either way. It is not a lever worth pulling for strength.

Does a finer pitch put more metal in the hole to shear?

It puts more turns in, but each turn is proportionally thinner, and the two effects cancel exactly in the basic profile. The geometric shear area works out at 0.875 times pi times engagement length times diameter regardless of pitch.

Do coarse taps break less because they have a thicker core?

Not for that reason. The basic minor diameter of M8x1 is 6.77 mm against 6.47 mm for M8x1.25, so the fine thread has the larger unfluted envelope, though that is not the tap core itself once flutes are cut. Core section does affect torsional stiffness, but so do chip volume, torque per tooth, hole depth and lubrication, and we found no comparison holding those constant.

Does a fine thread resist vibration loosening?

Only at the margin. The helix angles are 3.17 and 2.48 degrees, and the equivalent friction angle is about 2.6 degrees at a friction coefficient of 0.04, so at low friction the coarse thread has little self-locking margin. Loosening in service is driven by transverse slip shedding preload, which a shallower helix does not address.

When should I specify fine?

When the wall or nut depth is too shallow for enough coarse threads, when travel per turn is being used to adjust something, when the NASA guidance on extra-fine threads for hard materials or thin walls applies, or on clean machined assemblies. Otherwise coarse is the safer default.