A 38 mm OD cuplock spigot joint pin, supplied in 150, 200, 225, 250 and 300 mm lengths to fit 48.3 mm scaffold tubes, is the standard vertical-stacking connector for cuplock systems, with mild steel and high-strength steel both listed as common base materials [S1][S5].
Square-profile spigots give better rotational lock and higher load capacity for heavy slab casting and infrastructure, while round spigots remain the economic default for medium-load residential, commercial and rental scaffold fleets, and internal joint pins are a separate expanding/locking-pin family used inside the standard tube [S1][S2][S5].
Two Pin Families, Two Different Jobs on the Same Standard
Cuplock vertical standards stack through a spigot (external) joint pin, while a separate internal joint pin (also called expanding, gravity-locking or pigtail pin in manufacturer catalogues) engages inside the standard tube to lock the joint from the inside [S1][S5]. The spigot carries the concentric alignment, the load transfer and the vertical compression path; an internal pin is an additional anti-separation, anti-lift device, not a direct substitute for the spigot itself [S1][S2].
Manufacturer listings routinely catalogue both side by side, for example "Scaffolding Joint Pin, Ms Spigot Pin, MS Pigtail Pin, Gravity Locking Pin, Expanding Joint Pin" as five distinct SKUs in the same joint-pin product group, which confirms they solve different problems rather than competing on the same axis [S5]. The spigot determines vertical rigidity and concentricity; the internal pin mainly prevents the upper standard from lifting off if a ledger is struck or a bay racks sideways [S1][S7].
38 mm Spigot Geometry and the 48.3 mm Tube It Drops Into
The de-facto cuplock spigot geometry is 38 mm OD on a 48.3 mm OD scaffold tube, with the 10.3 mm radial gap allowing a clearance fit plus a small horizontal play in the joint, per Enright's structural stability study of braced scaffolding and formwork systems [S3]. That clearance is intentional, not a defect, because the cup-and-blade node above the spigot is what locks the four-way ledger connection; the spigot only has to keep the two standards coaxial and carry the axial compression [S3][S4].
Length options on the 38 mm OD spigot family are 150, 200, 225, 250 and 300 mm, with 250 mm and 300 mm being the most commonly stocked industrial sizes; mild steel is the default body material, with stainless steel and high-strength steel offered for corrosive or heavy-duty service [S1][S5]. Finishes range from paint-coated (cheapest) through zinc plated to hot-dip galvanised (HDG), and HDG is the finish that APAC's cuplock component guide specifies for the broader system, which sets the corrosion benchmark spigots are usually ordered to match [S1][S4][S5].
Square vs Round Spigot: The Real Decision Criteria
Square spigot pins give "better alignment stability, reduced rotational movement, higher load-bearing capacity and better vibration performance" than round pins, while round pins win on "fast installation, easy replacement availability, economical for rental scaffolding and suitability for medium-load applications" [S1]. On a metro infrastructure project that initially used round pins, switching to square spigots after slab-casting alignment problems gave the contractor "better vertical locking, reduced movement, improved scaffold stability and faster inspection approval", with the site crew noting the scaffold "became noticeably more stable during concrete pouring" [S1].
Side by side, the comparison matrix from the same source rates square pins High on alignment stability, Excellent on movement resistance and Heavy Duty on load, against round pins Medium, Moderate and Standard respectively, with square carrying a small cost premium and round scoring faster to install and cheaper for rental fleets [S1]. For cuplock specifically, the spigot is one of two vertical-stack components, the other being the cup-and-blade node itself, and the choice of profile interacts with how the standard above seats into the lower cup, which is detailed in the cup-and-blade node reference [S1][S4].
Internal Joint Pin: When You Actually Need One
An internal joint pin is a separate device that drops inside the standard tube and either expands, drops under gravity or hooks onto a pigtail to stop the upper standard sliding up or lifting off the lower one [S2][S5][S7]. Manufacturer product copy for an internal spigot-style pin lists "Pipe Size: 38 OD, Length: 250 mm and 300 mm, Type: Internal, Finish: Paint Coated, Material: Mild Steel, Base Material: Stainless Steel, Steel", which is the same 38 mm OD family as the external spigot but specified as an internal-type fit [S5].
In use, an internal pin is specified where the scaffold sees uplift loads (formwork pressure rebound, wind on a sheeted face, crane-handled bays) rather than just dead compression, and it is sold alongside expanding, gravity-locking and pigtail variants for those different service conditions [S5][S7]. Round spigots are usually paired with a separate internal locking pin on rental fleets, while square spigot systems tend to rely more on geometry at the spigot-cup interface and add the internal pin only when the site risk assessment calls for it [S1][S5].
Cuplock Node Context: Why the Spigot Only Carries Axial Compression
Cuplock joints rely on cups rather than wedge-and-rosette connections, which is efficient for vertical loads but less so for diagonal forces, and that is exactly why the spigot only has to be good at concentric compression, not at transferring shear [S6]. APAC's cuplock-component reference confirms standards are built with "fixed lower cups welded at 0.5-meter intervals, with movable top cups that lock the horizontal ledgers into place", and the spigot/lock-pin combination is what "offers height flexibility" between those node points [S4].
The 0.5 m node spacing is fixed by the welded lower-cup pattern, and the spigot is what lets you stack to whatever total height the structure needs, which is covered in more depth in the cuplock 500 mm node-spacing reference [S4]. For diagonal-load-heavy applications, contractors frequently move from cuplock to ringlock, whose wedge-and-rosette node delivers better shear resistance but loses the cuplock speed advantage, which is a system-level trade-off, not a spigot issue [S6].
Specification and Sourcing Checklist for a 38 mm Cuplock Spigot
A procurement-grade spec for a cuplock spigot joint pin is: product type Spigot/Joint Pin, OD 38 mm, tube compatibility 48.3 mm scaffold tube, length from the 150/200/225/250/300 mm option set, material mild or high-strength steel, finish painted, zinc plated or HDG, and application vertical scaffolding connection [S1]. For corrosive or coastal service, upgrade the base material to stainless steel or specify HDG to match the rest of the cuplock system, which is itself hot-dip galvanised as standard [S1][S4][S5].
Two trackable signals to watch before ordering: (1) confirm whether the site risk assessment requires a separate internal joint pin (expanding, gravity-locking or pigtail) on top of the spigot, because that adds a second SKU, not a replacement [S5][S7]; and (2) confirm tube OD and tolerance, since the 38 mm spigot is designed for 48.3 mm OD standard tube and will sit loose on a 42 mm or 60 mm system [S1][S3].
The underlying component specifications are covered under vertical lift module, expansion joint, and universal joint.