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Imperial versus metric dust ports

Dust fittings come in both imperial and metric sizing, and the near-misses between them are the source of most loose or over-tight connections in a mixed workshop.

The near-miss sizes

Four inches is about 101.6 millimetres, so a 4 inch fitting and a 100 millimetre fitting differ by around 1.6 millimetres. That gap is small enough that parts often push together, but large enough that the seal can be loose or need forcing depending on which is the port and which is the hose.

The same happens lower down. Two and a half inches is about 63.5 millimetres, so a 2.5 inch part and a 63 millimetre part are close cousins rather than identical. A metric 32 or 35 millimetre tool port is genuinely different from an imperial 1.25 inch, which is about 31.75 millimetres, though 1.25 inch and 32 millimetres are almost the same.

Which way the parts overlap

Whether a near-miss fits depends on the direction of overlap. If the metric part is the male end and the imperial part is the female socket, a slightly smaller metric spigot slides into a slightly larger imperial port and can be sealed with tape. If the sizes are reversed, the metric socket may be too tight for the imperial spigot.

So the practical answer is not simply that 4 inch and 100 millimetres are the same. It is that they are close, and whether they seal depends on which is inside the other and how much play there is.

Making a mixed system seal

A wrap of self-amalgamating or cloth tape takes up a millimetre or two of play and turns a loose near-miss into an airtight joint. For a permanent or frequently used connection, a proper adapter machined to bridge the exact sizes is better than relying on tape.

Where a system is entirely one standard, keep it that way. Mixing imperial and metric ducting invites a series of small leaks at every junction, each of which costs suction.

Reading a spec sheet

Manufacturers may quote a port in whichever unit suits their market, so a tool sold in Europe often lists 35 millimetres while a similar one sold in North America lists 1.25 inch. Convert both to the same unit before deciding they match, and remember that a stated nominal size is not always the measured diameter.

Related

Sources: Oneida Air Systems ducting reference · Festool hose and adapter documentation