§1Locate, don’t clamp
A bolted joint holds two parts together but does not fix where they sit — every bolt lives in a clearance hole, and within that clearance the parts can lie anywhere. Pins exist to remove that freedom.
The clearance is not negligence; it is necessity — bolts could not be assembled without it, as the fits pages explain. But it means a bolted cover, bracket or gearbox housing can shift by the clearance each time it is disturbed, and wherever position matters — bearing alignment, gear centres, a die set’s two halves — that shift is intolerable. The division of labour is therefore strict: bolts provide the clamping force; pins provide the location, sitting in holes reamed to a precise fit so the parts can go back together in exactly one place. Everything on this page is a variation on that idea — the dowel that does it perfectly (§2), the taper pin that does it removably (§4), the spring pin that does it cheaply (§5) — plus one relative that is not a pin at all but belongs to the same protect-the-precision family: the stud (§6).
Contents§2Dowels and the two-pin rule
A dowel is a hardened, ground parallel pin pressed into one part and closely fitted in the other — and exactly two of them fix a joint completely.
The geometry is worth seeing plainly. A part resting on a flat mating face has three freedoms left in the plane: it can slide in x, slide in y, and rotate. One dowel removes the two slides but leaves the part free to swing about the pin. A second dowel, set as far from the first as the parts allow, removes the rotation — and the joint is fully located: three freedoms gone. A third dowel adds nothing the first two have not already fixed; what it adds is over-constraint — three reamed holes in each part that must all agree to within their fit, which manufacturing tolerance cannot guarantee, so the third pin either will not enter or forces the joint into a strained position. Hence the universal pattern on machine assemblies: two dowels, placed diagonally and asymmetrically (so the part cannot be refitted rotated half a turn), with the bolts doing the clamping in their sloppy clearance holes nearby. Practice adds two refinements: dowel holes are reamed through both parts together at assembly, which is what makes the location exact and repeatable; and blind dowel holes are vented or the dowel relieved, since a ground pin in a blind reamed hole is otherwise an air spring going in and a vacuum lock coming out.
Contents§3What a pin carries
When a located joint is loaded sideways, the pins carry the shear — and the capacity arithmetic is one line, with one distinction that doubles it.
Take a hardened Ø8 dowel at a shear strength of 300 N/mm². Bridging two plates — one shear plane — it carries F = 300 × π × 8²/4 = 15.1 kN. In a clevis or fork arrangement, where the pin passes through three members and is cut at two planes, the same pin carries 30.2 kN — double, for free, which is why hinge and linkage pins are almost always double-sheared and why the rivets page found the same factor between lap and butt joints. Two cautions temper the number. First, dowels in a bolted joint should be sized to carry the service shear alone, without help from bolt friction — friction is the first thing lost when preload relaxes. Second, a hardened dowel is strong but brittle in bending: a pin loaded across a gap between parts, rather than at a tight interface, bends and snaps at loads far below its shear figure. Pins are interface devices; keep the shear plane tight.
§4Taper pins
The taper pin is the removable locator — a pin with a gentle standard taper that seats dead tight with a tap and releases with a tap from the other side.
The standard taper is 1:50: the diameter changes by 1 mm for every 50 mm of length, a slope so slight it is invisible to the eye and thoroughly self-holding — the same shallow-taper logic as the Morse tapers of the toolholding pages, where the wedge angle lies far inside the friction angle. Driven home in a taper-reamed hole, the pin wedges into full-length contact and locates as positively as a dowel; driven out from the small end, it releases cleanly and can be refitted many times, seating at the same depth each time. That combination — precise, tight, removable, re-seatable — made it the classic fastening for handwheels, levers and collars on shafts, cross-pinned through hub and shaft together: the assembly is drilled and taper-reamed in place, so the pin records the parts’ exact relationship, and (unlike the set screws of the cap-and-set-screws page) it transmits torque positively through its own shear section rather than by friction. Its costs are the reamed taper hole — a special reamer per size, and a hole that must go right through — and the pin’s intolerance of being driven from the wrong end, which jams it tighter. Small ends carry the size marking for exactly that reason.
§5Pins that make their own fit
Reamed holes are expensive. The elastic pins — spring pins and grooved pins — trade a little precision for the ability to seat tightly in a plain drilled hole.
The slotted spring pin is a tube of spring steel rolled with a lengthwise gap, made deliberately larger than its nominal hole: driven in, it compresses, and its spring-back loads the hole wall continuously — a press fit generated by the pin itself, in an ordinary drilled hole of normal tolerance. The coiled spring pin wraps the spring through more than two turns, spreading the elasticity through the coil; it is gentler on the hole, more uniform in its radial pressure, and better in fatigue, which suits repeated-shock duty. The grooved pin is the solid-metal answer: three longitudinal grooves are swaged into a solid pin, raising ridges beside each groove, and driving the pin burnishes the ridges into the hole for a tight mechanical grip. All three locate to drilled-hole accuracy — good, not dowel-grade — and all are retained by their own elasticity, needing no head, no thread and no retaining device. Their natural home is the vast middle ground of pinning: hinges, linkages, rollers, keying light hubs, retaining clevis pins’ humbler cousins — wherever a dowel’s precision is not needed and a dowel’s reamed hole is not worth paying for. One rule of use: a spring pin’s gap is oriented along the load, not across it, so the shear plane crosses the full tube and not the slot.
Contents§6Studs
A stud is a headless bolt threaded at both ends — screwed permanently into a casting at one end, taking the nut and all the wear at the other — and it exists to spend the casting’s thread only once.
The inserts page computed the problem: an internal thread in aluminium or cast iron is the joint’s weakest, least replaceable element, and every assembly cycle of a bolt spends it again. The stud’s answer is to make that thread’s engagement a once-only event. The stud’s casting end — often an interference (oversize) fit thread — is run home at manufacture and never disturbed; every subsequent service cycle works the nut on the stud’s outer end, wearing a cheap steel-on-steel thread pair that can be renewed by replacing stud or nut. The cylinder-head stud is the canonical case, and it carries the family’s other virtues too: a stud gives an exact, repeatable clamp length and lets the gasketed joint be assembled by sliding the head down over standing studs — guided, aligned, threads protected — rather than fishing bolts through blind. Torque practice follows the geometry: the nut end is tightened, per the torque-and-tension page, while the casting end merely holds; and studs are removed, when they must be, with a proper stud extractor or two nuts locked together — never pliers on the thread the next nut has to run down. It is the least glamorous fastener on this page and the one that most directly embodies the section’s recurring principle: put the wear, and the failure, where they can be afforded.
Contents§7Quick reference
The working core of the page on one card rack.
Division
bolts clamp · pins locate
clearance is why
Dowels
two pins fix x, y, θ
a third over-constrains
Capacity
Ø8: 15.1 kN single shear
30.2 kN double
Taper & elastic
1:50 — 1 mm per 50 mm
spring pins fit drilled holes
Studs
casting thread spent once
the nut takes the wear