Superior Fluid Solutions · Cross-Reference
DK-Lok® vs Swagelok®: Inch and Metric Tube on the Same Panel
Mixed-unit panels are normal in the field, and the 1/4 inch against 6 mm decision is where a tube list quietly goes wrong.
The awkward panels are rarely the ones built wrong. They are the ones built correctly, twice, ten years apart, by two different contractors working to two different unit conventions. According to Swagelok®’s own published metric tube fitting catalog, MS-12-06, which is a manufacturer publication, “These metric fittings are to be used on metric tubing sizes only,” and the same document notes that shaped metric fittings are “stamped MM.” That note exists because inch and metric tube sizes sit close enough to be confused and far enough apart to matter. This post is about telling them apart on an existing panel, and about what to confirm before a mixed-size list goes out for quote.
Why one panel ends up with two measuring systems
Instrumentation panels accumulate. A skid arrives from a supplier who works in millimetres. It is tied into a plant whose tubing stores are stocked in fractional inch. An analyzer is retrofitted, and the analyzer vendor ships its own tube runs. A valve is replaced under a shutdown with whatever the storeroom had. None of that is negligent, and both unit systems are fully catalogued by the manufacturers involved. DK-Lok®’s own tube fitting catalog lists fractional tube outside diameters from 1/16 through 2 inches, and it also lists metric tube sizes in millimetres, with the size element of the ordering number carrying an M, so a 6 mm union appears as DU-6M. Swagelok®’s metric catalog states that its metric tube fittings are “available in 2, 3, 4, 6, 8, 10, 12, 14, 15, 16, 18, 20, 22, 25, 28, 30, 32, and 38 millimeter tube sizes.” The units are a fact about the panel, not a fault in it.
The 0.35 mm nobody sees
The inch is defined exactly. NIST states that the inch “is exactly equivalent to 25.4 mm,” which makes quarter inch tube 6.35 mm and six millimetre tube 6.00 mm. The difference is 0.35 mm, which is roughly the thickness of a business card and comfortably inside the range a person can fail to notice by eye or by thumb. Step up the sizes and the gaps change but never become obvious: half inch tube is 12.7 mm against 12 mm, three quarter inch is 19.05 mm against 20 mm, and one inch is 25.4 mm against 25 mm. In three of those pairings the metric tube is the smaller of the two and in one it is the larger, which is exactly why guessing from memory does not work.
Both manufacturers publish their sizes precisely so that none of this has to be guessed, and Swagelok®’s instruction that metric fittings are for metric tubing only is a plain statement of that boundary.
| Inch tube OD | Exact value in mm | Nearest catalogued metric OD | Difference |
|---|---|---|---|
| 1/8 in. | 3.175 mm | 3 mm | 0.175 mm |
| 1/4 in. | 6.35 mm | 6 mm | 0.35 mm |
| 3/8 in. | 9.525 mm | 10 mm | 0.475 mm |
| 1/2 in. | 12.7 mm | 12 mm | 0.7 mm |
| 3/4 in. | 19.05 mm | 20 mm | 0.95 mm |
| 1 in. | 25.4 mm | 25 mm | 0.4 mm |
The millimetre values in column two are arithmetic, derived from the exact inch definition. The metric sizes in column three are taken from the manufacturer catalogs cited below. Placing two sizes side by side here is a statement about dimensions only, and is not a suggestion that one may be used in place of the other.
Identifying what is already installed
Work from the tube, not from the paperwork. A caliper reading across the tube outside diameter, taken at two points ninety degrees apart, will separate 6.35 mm from 6.00 mm without argument. Take that reading on more than one run, because the whole point of a mixed panel is that the runs do not agree with each other. Then read the fittings: Swagelok® publishes that its shaped metric fittings are stamped MM, and DK-Lok® publishes an M inside the size element of its metric ordering numbers, so a stamping or a legible ordering number is worth photographing. Check the tube itself for printed markings, which often carry the size and the specification. Finally, pull the original bill of materials if it exists, and treat it as a hypothesis rather than a conclusion.
Record the result per run rather than per panel: line number, tube outside diameter as measured, wall thickness, tube material and specification, and the fitting shapes at each end. That record is what a cross-reference is actually built from, whether the shapes involved are unions, elbows or reducers.
Building a mixed-size order list
Keep the two unit systems in separate columns and never let a spreadsheet round between them. Give every line its measured outside diameter in the units it was measured in, plus the wall thickness, because a fitting list without wall thickness cannot be checked against a pressure requirement. Flag every place where an inch run meets a metric run, since those junctions need a deliberate decision rather than a default. State the service medium, temperature and design pressure per section, say whether documentation is required, and mark the lines you are confident about separately from the lines you inferred.
Superlok® tube fittings are listed in 316 stainless steel, a corrosion-resistant grade widely used in instrumentation service, and Superlok® states that its fittings carry the finest interior surface finish of any tube and pipe fitting on the market. Grade, pressure rating and certification are confirmed per order rather than assumed from a size list. Browse the range of tube fittings and gauge valves to see the shapes available, see our distributors page for where we supply, and read our Hy-Lok® cross-reference post for a different angle on the same problem.
Questions about mixed inch and metric panels
Is 6 mm close enough to 1/4 inch to treat them as one size?
No. They are 0.35 mm apart and each manufacturer catalogues them as separate sizes with separate part numbers. Swagelok®’s metric catalog states plainly that its metric fittings are for metric tubing sizes only. Measure and order to the size that is actually installed.
How do I tell an inch fitting from a metric fitting without taking the line apart?
Start with a caliper on the tube, which is non-intrusive and decisive. Then look for markings: Swagelok® publishes that its shaped metric fittings are stamped MM, and DK-Lok® carries an M in the size element of its metric ordering numbers. Photograph anything legible.
Can a reducing fitting bridge an inch run to a metric run?
Reducing shapes exist in both unit systems, and which shape suits a particular junction depends on the sizes, the service and the space available. Send us the measured sizes at that junction and the service conditions, and we will work through the options with you rather than guess.
Do I need to convert the whole panel to one unit system?
That is an engineering decision for the plant, not something a fitting supplier should decide for you. What matters for ordering is that every line on the list carries the size that is genuinely installed, in the units it was measured in, with the wall thickness alongside it.
Send us the measured list
Sizes as measured, wall thickness, service conditions. We will cross-reference from there.
Superior Fluid Solutions
Measure the tube. The paperwork can wait.
Sources
- Swagelok, “Metric Tube Fittings,” MS-12-06 (manufacturer publication), supports the quoted note “These metric fittings are to be used on metric tubing sizes only,” the statement that shaped metric fittings are “stamped MM,” and the quoted metric size list. swagelok.com
- DK-LOK USA, “DK-Lok Tube Fittings” product catalog (manufacturer publication), supports the fractional tube outside diameter range of 1/16 through 2 inches, the availability of metric tube sizes in millimetres, and the M carried in the size element of metric ordering numbers. dklokusa.com
- NIST Office of Weights and Measures, “SI Units: Length,” supports the exact conversion, the inch being “exactly equivalent to 25.4 mm,” from which every millimetre value in the table above is calculated. nist.gov
Technical note: the sources above establish the published size ranges and marking conventions of the named manufacturers and the exact definition of the inch. They do not establish that any part from one manufacturer performs the same as any part from another, they do not establish that components may be assembled together across brands, and they do not certify a pressure rating for your tubing or your service. Superlok® grade, pressure rating and documentation are confirmed per order.
Trademark disclaimer: Swagelok® is a registered trademark of Swagelok Company. DK-Lok® is a registered trademark of DK-Lok Corporation. Hy-Lok® is a registered trademark of Hy-Lok Corporation. Superlok® is a registered trademark of BMT Co., Ltd. Superior Fluid Solutions is not affiliated with, authorized by, endorsed by or sponsored by any of these companies, and the marks are used here only to identify the products they name. Cross-references are made by form, fit, and function only.
