Superior Fluid Solutions · Procurement
Ordering Metric Tube Fittings Against an Imperial Drawing
The unit mismatch is a paperwork problem first, and it is solved on the requisition and on the label.
This article is not about working out what is already bolted to the rack. It is about the requisition you have to write when the drawing in front of you is in fractional inches and the tubing you are buying for is in millimetres, or the other way round. According to Collins-Oliver, a distributor publication rather than a standards body, “Fractional (1/16 in. to 2 in.) and metric (2 mm to 50 mm) fittings are not interchangeable.” That leaves a buyer with no rounding allowance, so the decision has to be made explicitly and written down. What follows is where mixed order lists come from, what to measure before you commit, what the purchase order has to say, and how to label the result so the next requisition is not written from the wrong page.
Where a mixed order list comes from
The mixture usually lives in the purchasing records before anyone notices it on steel. An item master is imported from an acquired site and arrives with millimetre descriptions sitting next to fractional ones. A category is set up in a hurry, so the same physical part exists twice under two spellings. A standard requisition template is copied year after year from a project that was metric. A contractor free issues his own material and it is booked in under whatever text the storeman typed. An approved vendor list carries a supplier who quotes in one system and a supplier who quotes in the other, and the buyer alternates between them without either description changing.
None of that is negligence, and none of it is visible from a stock report. The useful correction is to stop treating the unit system as an attribute of the site and start treating it as a mandatory field on every line of every requisition. A site described as “imperial with a few odd bits” will keep receiving the wrong parts for the odd bits, because the description itself is where the error is stored.
Measure for the order you are about to place
The number that decides what you buy is the outside diameter of the tube the fitting will go on. Get it with a vernier or digital caliper across bare tube, taken twice at ninety degrees apart and clear of any swage mark, dent or scratch. Do not measure the nut across the flats, do not scale it from a drawing, and do not work back from an inside diameter. Record the reading in millimetres to two decimal places, because two decimals is where the decision actually sits and one decimal hides it.
Then put the wall thickness on the requisition as its own field rather than folding it into the size. Wall schedules are quoted in both systems too, and a correct outside diameter with an unstated wall is still an incomplete order. Do the same with the thread standard at any pipe end. Outside diameter, wall and thread are three independent specifications, and a metric tube end can legitimately terminate in a tapered pipe thread, so a buyer who assumes one from another is guessing twice.
The two pairs that cause most of the trouble
Two substitutions account for most of the wrong deliveries, and they are the two workhorse sizes. A 1/4 inch tube measures 6.35 mm, because one inch is 25.4 mm exactly, and it sits beside a 6 mm tube. A 1/2 inch tube measures 12.7 mm and sits beside a 12 mm tube. Those are the sizes that dominate instrument work on both sides, so they are the sizes a buyer is most often asked to find in a hurry. Collins-Oliver, the distributor reference already cited, states the consequence without hedging: “1/4 inch is 6.35 mm and 6 mm is 6.00 mm. They are close but they do not seal in the same fitting.”
From a purchasing point of view the danger is not the geometry, it is that both descriptions look like the same request. A search for “quarter inch” in an item master will surface metric parts whose text happens to mention 6 mm, and an expediter chasing a shortage will find something that reads close enough. The defence is procedural: if the measured reading and the catalogue description do not use the same unit, the line does not get released.
Three procurement habits that create the problem, all of them well meant.
The helpful substitution
A shortage on one size, something similar available in the other system, and a swap made to keep the job moving. The requisition now says one thing and the carton holds another.
The bracketed line
A single line that offers a conversion in brackets. Whoever picks the order chooses, nobody records the choice, and the goods receipt confirms neither.
The unmarked handover
Parts fitted, drawing never touched, no label on the panel. The next requisition is written from the old page and repeats the error at full scale.
What the purchase order has to say
A requisition that cannot be misread is mostly a matter of splitting fields apart and refusing to abbreviate. The table below is the version we would rather receive, and the middle column is how the same line usually arrives.
| Field | How it usually arrives | How it should read |
|---|---|---|
| Unit system | Absent, or implied by the site | Stated on the line as fractional inch or metric |
| Tube OD | A bare number, or a bracketed conversion | One value with “in.” or “mm” written out |
| Wall thickness | Folded into the size or left off | Its own field, in the same system as the OD |
| Thread standard | Assumed from the tube unit | Named separately for every pipe end |
| Source of the size | Not recorded | Drawing reference, or measured on site with the date |
| Documentation | Requested after delivery | Asked for on the order, confirmed per order |
A documentation convention, not a product specification. Material grade, pressure rating and certification are confirmed per order.
Buy the crossover as a component
Where a system genuinely has to change units, put that change in a part you can point at and pay for, rather than absorbing it into a size. An adapter or a reducer gives one identifiable location where the standard changes, which is worth more on a handover document than a joint that looks ordinary and is not. Terminations and straight runs sit under connectors, and the full range is under tube fittings. Ask for the material documentation on the same order and check what you need against the certifications page.
Labelling so the next requisition is right
Manufacturers already accept that metric parts need marking, which is a useful precedent for a site to copy. Swagelok® metric catalogue notes that on its metric fittings “a stepped shoulder is machined into the body and nut hex for easy identification,” and that shaped fittings are “stamped MM.” If the maker thinks a part needs a permanent mark, an installation certainly does. Band or tag each tube run with the outside diameter and the unit. Put a label inside the enclosure door naming the unit system for that panel, and where a panel carries both, name the location where the change happens and which side is which. Mark the as-built while the access is still there, because corrections deferred to the paperwork stage do not get made.
Carry it through to the spares. Bags of ferrule sets and loose fittings held for that panel should show the unit on the label rather than a number on its own. Superior Fluid Solutions lists Superlok® fittings in corrosion-resistant 316 stainless steel, with the finest interior surface finish of any tube and pipe fitting on the market, and none of that quality reaches the joint if the part is right in every respect except the unit. Where you are matching a metric drawing to a part number you can actually order, our instrumentation cross-reference notes and the cross-reference page cover that step, and the distributors page covers where to route the order.
Ordering questions we get asked
Can I release a 1/4 inch line item against a 6 mm requirement?
No. A distributor reference states that fractional and metric fittings are not interchangeable, and that 6.35 mm and 6.00 mm do not seal in the same fitting. If the measured reading and the catalogue text disagree on the unit, hold the line rather than releasing it.
Our item master has the same part twice, in two units. What now?
Treat them as two items, because they are. Do not merge the records to tidy the list. Add the unit to both descriptions so a text search cannot return one when the other was meant, and retire whichever one your site does not buy.
Does ordering a metric tube fitting mean ordering a metric thread?
Not automatically. Tube outside diameter and thread standard are separate specifications, so put the thread on its own line. A metric tube end paired with a tapered pipe thread is an ordinary combination, not a mistake.
How should a requisition read when I measured rather than read a drawing?
Say so on the line. Record the measurement, the unit, the date and who took it, and keep the drawing reference beside it. A quote that comes back against a measured size, clearly flagged as measured, is auditable. One that silently overrides a drawing is not.
Which metric sizes can I actually order?
Catalogued metric ranges differ by manufacturer and by fitting series, so check the series you are buying rather than assuming from another maker’s range. Availability, material grade and rating for a given size are confirmed per order.
Order in the unit you measured
Fractional and metric tube fittings, listed by size, shape and material.
Superior Fluid Solutions
One unit system per line, every line.
Sources
- Collins-Oliver, “Tube Fitting Sizing Chart: Fractional and Metric Tube OD Reference,” distributor publication, accessed September 2026. Supports that fractional and metric fittings are not interchangeable, and that 6.35 mm and 6.00 mm do not seal in the same fitting. collins-oliver.com
- Swagelok®, “Metric Tube Fittings,” MS-12-06, manufacturer catalogue. Supports the instruction that these metric fittings are for metric tubing sizes only, and the use of a stepped shoulder on the body and nut hex plus MM stamping for identification. swagelok.com
- Jon Kestner, “Why You Shouldn’t Use Tube Fittings from Different Manufacturers in the Same System,” Machine Design, 10 August 2023. Supports the point that no industry wide standard governs tube fitting geometry, so tolerances are set by each manufacturer and a mixed assembly is unvalidated. machinedesign.com
Technical note: the sources above are a distributor reference page, a manufacturer catalogue and an engineering trade article. They establish that fractional and metric tube fittings are not interchangeable, that metric fittings are specified for metric tubing, and that manufacturers mark metric parts so they can be told apart. The millimetre values quoted here follow from the exact conversion of one inch to 25.4 mm and are dimensional facts only: they are not test results, not sealing data, and not a suggestion that any size may stand in for another. No stock level, price, lead time or part number is stated anywhere in this article, and nothing here proves what is installed on your panel today. Material grade, pressure rating and certification for any item are confirmed per order.
Trademark disclaimer: Swagelok® is a registered trademark of Swagelok Company. DK-Lok® is a registered trademark of DK-Lok Corporation. Superlok® is a registered trademark of its owner. All other marks belong to their respective holders. Superior Fluid Solutions is not affiliated with, endorsed by or sponsored by any of these companies. Cross-references are made by form, fit, and function only.
