The keg neck – the component nobody talks about
Which spear goes into a keg is not decided by the spear. It is decided by the neck. Nine systems are in circulation, two fastening principles – and one standard that has won for good reasons.
A note on terminology, because it regularly causes ordering errors
The welded-in component that takes the spear is called Fassmuffe in German – Muffe for short – and the thread in it Muffengewinde. In English-language catalogues you will find the same component as “neck”. The literal translation “Hals” is wrong in German usage and leads to mix-ups when ordering. Where a manufacturer’s catalogue speaks of “BS neck” or “drop-in neck”, the Muffe is meant.
Why the neck matters more than the spear
A spear can be swapped. The neck is welded into the upper dome and stays there as long as the keg lives. It therefore determines, for decades, which spears can be considered at all, which couplers the operation has to keep, which spare parts belong in the store and how expensive reconditioning becomes.
And it is the reason why a conversion is usually not needed at all. If the standard thread sits in the neck, flat (A-type), EU Sankey (S-type), combi (M-type), triangular (G-type) and US Sankey spears (D-type) can be swapped for one another without changing anything on the keg. Anyone wanting to move a fleet to a different coupler system as a rule changes only the spear.
Nine neck systems, seven spear families
This overview follows the system of a leading spear manufacturer, which lists the available neck variants for every spear system.
| Neck system | Principle | Fits | Distribution |
|---|---|---|---|
| 2" × 14 TPI DIN | Thread, right-hand | A · G · M · S · D · U | market standard in continental Europe |
| 2" × 14 TPI, left-hand | Thread, left-hand | A | special case |
| 2" × 14 TPI short | Thread | A · G · M · S · L | short design |
| Rd 52 · Rd 52.8 · Rd 53 × 1/6" | Coarse thread | A · G · M · S | above all kegs built in the eighties · common on polyurethane-clad kegs |
| 2 1/8" × 7 TPI short | Thread | A · G · S | very rare |
| BS neck | Bayonet + snap ring | A · G · M · S · D | United Kingdom |
| SOS neck | Snap ring | A · G · S · D | Europe, secondary range |
| US drop-in | Snap ring + two lugs | S · D | USA, the standard there |
| Groove neck (Guinness) · SAF-2 | Groove | U | Ireland, brand-specific |
| Weld-in neck M46 × 1.5 | Thread, welded in | L (soft drinks) | special application |
The coarse thread – where you can expect it
The round threads Rd 52, Rd 52.8 and Rd 53 are called coarse threads in German usage; in English-language trade you will find them as “big thread”. They are not exotic, but they occur in clusters: above all on kegs from the eighties, and there particularly often on the polyurethane-clad designs.
That makes a practical rule of thumb for goods inwards: an old build year plus a polyurethane jacket means measure before ordering spare parts. Order standard spears instead and you get parts that will not go in – and you notice only out in the yard.
Assigning the coarse thread to build year and design is an operating figure from our reconditioning, not a manufacturer’s figure. Kegs of these vintages are everyday work for us – which is why we know the pattern. If your experience differs, write to us.
What of this we carry
2″ × 14 TPI only. The other neck systems are listed here so that you can place your keg – not because we have them in the range. We have deliberately chosen depth in the standard rather than breadth across eight secondary threads: all spear types, all versions, all spare parts, immediately ex stock.
The decisive line is the first one. The standard thread is the only neck system that takes six out of seven spear families. Every other system covers only part of them. Have it in the neck and every option stays open – and you never have to choose between markets.
Two principles, one identifying feature
All nine systems come down to two ways of holding the spear in the neck.
Principle 1
Screwed in
The spear has an external thread, the neck an internal thread. It is tightened to a defined torque and re-tightened after the first fill. Sealing is by a square seal resting on top.
Identifying feature: square seal, often a star- or triangle-shaped laser mark on the body. The safety hook itself is only visible on the removed spear – the laser mark is an indication, not proof.
Principle 2
Set in
The spear is inserted and held by a retaining ring – also called a snap ring or C-clip – which engages in a circumferential groove. On the bayonet neck, two lugs and a cam track are added. Sealing is by an O-ring.
Identifying feature: an O-ring instead of a square seal, a tamper-evident ring on the neck, and on the SOS system a groove visible from above.
The quickest test in the operation
A square seal means threaded. An O-ring means threadless. That is recognisable without tools as soon as a spear has been removed once – and more reliable than measuring around on the neck.
Why 2" × 14 TPI is the better choice
Six reasons. The first weighs more than the rest together.
1. It is the only principle the standard knows
DIN 3542 governs the connection dimensions of keg necks and of the keg connection parts that screw into these necks – including the safety requirements for those parts. The threadless principle does not fall within this scope. So for the threaded case there is a normative backbone with safety requirements; for the threadless case there are manufacturers’ designs.
That is not formalism. On a pressure vessel, what the closure was designed and tested against is the central question of all.
2. The retaining ring is a single-use part, the thread is not
On the snap ring of the US system the documentation is unambiguous: once it is removed from the neck it is damaged and unsafe for further use. At every installation a new O-ring and a new retaining ring go in.
A thread, by contrast, can be used practically without limit. It does not wear from use. Over thirty years of keg life and an overhaul interval of seven to eight years that is four to five removals – and afterwards the thread is as good as on the first day. With the snap ring, by contrast, a new part every time and a groove that is loaded every time.
If a neck thread does break, it was not from use
There are two causes, and both are avoidable:
The impact wrench. It has no business anywhere near a keg spear – ever. Impact pulses load the thread flanks abruptly, in a way that cannot be controlled. What that produces cannot be seen on the thread from outside, and the keg goes back into circulation with it.
The wrong torque. Tightened too hard, the thread is overstretched; too loose, the seal does not hold and the spear works itself free. That is why there are figures for both: around 70 to 80 Nm for tightening, 40 to 50 Nm for re-tightening after the first fill.
Keep to those two rules and you will hardly ever destroy a neck thread in service. That is the real point: with a thread, handling decides the service life; with a snap ring, the design decides. The one can be trained, the other cannot.
3. The preload is defined and testable
With a threaded spear there is a number: it is tightened at around 70 to 80 Nm and re-tightened at 40 to 50 Nm after the first fill. That is measurable, documentable and verifiable in a test record. With the snap ring the holding force follows from the groove – with no adjustable, testable parameter.
And the torque is not only a test value, it is the protection of the thread itself. Keep to it and you have no wear problem.
Honesty requires this: the manufacturers’ figures for the tightening torque diverge. We found values of 70 to 80 Nm, 80 ± 5 Nm and 75 Nm. Anyone installing spears themselves should use the sheet for their own make and not whatever value comes to mind.
4. No special tools, no proprietary tools
Threadless systems require their own assembly and removal tools, some of them tied to the manufacturer and expressly described as such. For the threaded system there are assembly tools on the open market – and at a pinch a threaded spear can be handled without a power tool.
That sounds like a detail and, in a service case, is the difference between “done today” and “next week, when the tool arrives”.
5. It is cheaper to buy
In one manufacturer’s price list the threadless flat (A-type), triangular (G-type) and combi spear (M-type) costs €27.30 against €22.00 and €25.50 respectively for the threaded version. On the EU Sankey spear (S-type) both versions cost the same. So the premium runs up to a quarter – on every spear you will ever buy.
6. Reconditioning is cheaper and less troublesome
Probably the most solid point, because it does not come from a manufacturer but from a fleet operator. A British keg rental company that has converted more than 30,000 containers from the BS neck to the standard thread gives as its reasons: lower reconditioning costs, more efficiency and a lower failure rate, better occupational safety.
Anyone converting 30,000 containers has done the sums first.
What speaks for the threadless systems
They are not more poorly designed. They are built for a different purpose.
- Tamper protection is built in. A threadless spear comes with a tamper-evident ring. Unauthorised opening is meant to leave visible and permanent damage – that is exactly the design intent. For threaded spears there is no comparable standard range.
- No torque, no overtightening, no cross-threading. Three common assembly errors of the threaded system are designed out.
- Forced pressure relief on removal. At least one bayonet design provides for a two-stage removal in which pressure is released in the first step before the spear comes free. In safety terms that is elegantly solved.
- Fast assembly on the line. No tightening operation, no re-tightening after the first fill.
For a brewery with its own well-set-up line and a closed loop, that is a coherent concept. For a fleet that is reconditioned, converted and moved across borders, the disadvantages outweigh it.
Both principles carry one risk
If a spear is undone while the keg is under pressure, it can be thrown out of the neck. For the US system it is documented that this applies from about 30 psi – around 2 bar –; the patent literature describes the same risk for threaded spears. Pressure is released before anything is undone, at a distance, with a long tool, and nobody stands in the line of ejection. This rule knows no exception and no spear type.
What that means for your fleet
You have a standard thread in the neck
Then you need no conversion, at most different spears. Flat (A-type), EU Sankey (S-type), combi (M-type), triangular (G-type) and US Sankey (D-type) can be swapped for one another without changing anything on the keg. That is the real value of this standard.
You have a coarse thread
Then the neck is fine – it is a thread, not a snap ring system. You only need the matching spears, and those exist for flat (A-type), EU Sankey (S-type), combi (M-type) and triangular (G-type). The effort is a question of procurement, not of the workshop.
Spears for coarse threads are something we do not carry, though – we concentrate on the market standard. Whether a conversion to the standard thread is worthwhile is purely a matter of arithmetic: it would mean machining out the existing neck and setting in a new one. With small stocks it rarely pays. Ask us with quantity and build year and we will work it through.
You have a threadless neck
Then the fleet keeps running – just with narrower options, more expensive spears and more effort in reconditioning. A conversion to the standard thread is technically possible: the old neck is machined out and a threaded neck set in. Service providers in Germany and Great Britain offer this, in quantity.
And the direction is always the same. Conversions run from threadless to threaded. Nobody offers the reverse – and that is perhaps the most telling argument in this whole account.
You are buying new
Then the matter is simple: the standard thread. Anything else restricts you over the whole service life of the keg without giving back anything you need.
Where these figures come from
The neck variants for each spear system follow the public catalogue of a leading spear manufacturer. The prices of €22.00 and €27.30 come from a manufacturer’s price list of 2021 and are to be read as an order of magnitude, not as a current market price. The tightening torques are manufacturers’ figures from assembly and maintenance sheets of various makes; they diverge from one another, which is marked as such in the text. The statement on the retaining ring as a single-use part and the figure of 30 psi come from safety documents of the American brewers association and from the trade press. The experience with more than 30,000 converted containers is a public statement by a British keg rental company. The description of the bayonet mechanism with two-stage removal and pressure relief, as well as the disadvantages of both principles, come from published patent specifications – where the weaknesses of existing solutions are usually named openly.
Assigning the coarse thread to build years in the eighties and to the polyurethane-clad designs is an operating figure from Keg Exchange, taken from ongoing reconditioning, not a manufacturer’s or standards figure.