Guide · Measuring and buying
How to measure a dust port: inside or outside diameter
Measure both diameters of every port and hose end, note tapers and which end goes inside, and treat inch-to-millimeter conversions as arithmetic only.
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Short answer
Measure both the inside and the outside diameter of the port and of the hose end you want to join. The port's outside diameter has to match the hose's inside diameter, or the other way round.[1] Write down which surface each number is from, and whether the end tapers, before you shop for any hose or adapter.
Which surface joins which
Every joint has one end that goes inside and one that goes over. A male end slides inside; a female end slides over. The numbers that matter are the two surfaces that touch: the male end's outside diameter (OD) and the female end's inside diameter (ID).
POWERTEC's dimensioned parts show why one number is never enough. Its 70136 cone reducer has a "4-inch" end listed at 3-61/64 in OD and 3-41/64 in ID.[2] The "2-1/2-inch" end is listed at 2-15/32 in OD and 2-9/32 in ID.[2] Neither end is exactly the number on the label, and the inside and outside of each end differ by the wall thickness.
POWERTEC's 70291 shows the other direction: its 4-inch end tapers from 3.51 to 3.73 in and friction-fits inside a 4-inch nominal OD machine port.[3] That part goes inside the port, so the port's ID is the number to compare.
How to measure
A caliper is the easiest tool. Close the outside jaws across the widest point of the end for the OD, and open the inside jaws across the opening for the ID. Take two readings at right angles; plastic ports are often slightly oval. A ruler laid across the center of the opening works too if you read it at the widest point.
Measure the end itself, not a cuff or clamp that happens to be on it. If a rubber cuff is fitted, note that separately.
Tapers: record a range
Many ports and fittings taper so they bind as you push them together. A tapered end has no single diameter. Measure at the tip and at the base and write both. Makers publish tapers the same way: POWERTEC gives the 70291's 4-inch end as 3.51 to 3.73 in rather than one figure.[3]
Inches and millimeters
One inch is exactly 25.4 mm.[4] So 35 mm is about 1.378 in, and 1-3/8 in is about 34.9 mm. That is arithmetic. It does not mean a metric part fits an inch part of a near size; the fit still depends on which surfaces meet and how each maker makes its end.
When the maker's label is ambiguous
Some makers' own pages disagree with themselves. Makita's SP6000J page calls the rear dust port 1-7/16 in in one place and a 1-1/2 in OD port in another.[5] Its XPS01PTJ page gives 1-7/16 in without saying whether that is the inside or the outside.[6] In cases like these, measure the port, then ask the maker before you buy.
What no source gives
No source in our register gives a general tolerance for "a good fit" between two dust fittings. Treat any rule of thumb you read as someone's opinion, and compare your numbers with the maker's stated dimensions for the part you want.
Your worksheet
Copy this table and fill in one row per end.
| End | ID | OD | Tapers (yes/no, range) | Male or female | Lock or thread | Where the number came from |
|---|---|---|---|---|---|---|
| Tool port | ||||||
| Hose, tool end | ||||||
| Hose, vac end | ||||||
| Vac or collector inlet |
An adapter's maker should state both of its ends. POWERTEC's 70136 is one that does, and it is a hose-to-hose part: POWERTEC says it is not intended to fit a machine port directly.[2]
Picks that fit1 product
Choose the single 70136 4 in to 2-1/2 in cone reducer only to join a 4 in ID hose to a 2-1/2 in ID hose: POWERTEC says it is for hoses only, not machine ports.
More on choosing
Compare your hose IDs with its stated end sizes first. On Amazon, check that 'Ships from / Sold by' shows Amazon.com or POWERTEC.
Next
Questions
Do I measure the inside or the outside of a dust port?
Both. Rockler advises measuring the inside and outside diameter of each port and hose connector, because the port's outside diameter has to match the hose's inside diameter.[1]
What tool do I use to measure a dust port?
A caliper is easiest: close the outside jaws across the widest point for the outside diameter and open the inside jaws for the inside diameter. A ruler laid across the center of the opening works if you read it carefully at the widest point.
My port tapers. Which number do I use?
Record both the smallest and the largest diameter. Makers publish tapered ends as a range, for example POWERTEC lists the 4-inch end of its 70291 as tapering from 3.51 to 3.73 inches.[3]
Is 35 mm the same as 1-3/8 inches?
Not quite. One inch is exactly 25.4 mm, so 35 mm is about 1.378 inches and 1-3/8 inches is about 34.9 mm. That arithmetic does not make a metric part and an inch part interchangeable.[4]
The maker's label for my port is unclear. What now?
Measure the port yourself and ask the maker. Makita's SP6000J page, for example, describes the same port as 1-7/16 inch in one bullet and as a 1-1/2 inch OD port in another.[5]