Superior Fluid Solutions · Cross-Reference
Cross-Referencing a Bulkhead Union: Panel Thickness Decides
A bulkhead union is still just a union underneath. What actually decides whether a replacement fits is the panel it has to pass through.
A bulkhead union is, on both sides of the wall, an ordinary union: two tube ends, a body, a set of ferrules each side. What makes it a bulkhead fitting is the middle, the threaded section that passes through a drilled hole in a panel, skid wall or cabinet, plus the nut that holds it there. According to the bulkhead union tables in Swagelok®’s MS-01-140 catalog of gaugeable tube fittings, a manufacturer publication, these fittings are tabulated with two columns the plain unions do not get: a panel hole size and a maximum panel thickness, printed for every listed tube size beside the usual lettered dimensions. Two extra numbers on the page, and they are the two that decide whether a cross-referenced part goes in or comes back.
A union that has to fit a hole that already exists
Swapping a plain straight union means matching two tube ends. Swapping a bulkhead union means matching two tube ends and a hole drilled years ago through a panel nobody has measured since. That second condition is the whole difference between this shape and every other union in the catalog. How to cross-reference the tube-end half of any union, plain or bulkhead, is its own method with its own steps, and the union fittings range is the place to work that side. From here on this post is about the panel.
The two figures that are published per size
Because both figures are tabulated row by row, they have to be read row by row. Swagelok®’s catalog puts the maximum panel thickness at 0.40 in. against 1/4 in. tube outside diameter, 0.44 in. against 3/8 in., and 0.50 in. against 1/2 in. Run the arithmetic on that and the point makes itself: the tube size doubles between the first and last of those rows while the panel allowance rises by a quarter. Nothing here scales the way intuition suggests, so nothing here can be interpolated. The panel hole size sits in its own column for the same reason. It is neither the tube outside diameter nor the width across the fitting’s flats, and copying it from a fitting of a different size, or re-cutting an old hole from memory, is where a bulkhead replacement fails before the tube is even offered up.
Those figures belong to the parts that catalog lists and to no others. They are quoted here to show the shape of the problem, not as a specification for any part we sell. For a cross-reference, the numbers that matter are the ones on the current page for the exact part proposed, plus the thickness you measured at the actual penetration, which is a different thing from the thickness the panel was drawn at.
| Detail | Why it matters | Confirm before ordering |
|---|---|---|
| Panel hole size | The threaded section has to pass a hole that is already cut, or about to be | The published hole size on the row for that exact part, never one borrowed from a neighbouring size |
| Panel thickness | The nut and the body shoulder only clamp across a published maximum | Thickness measured at the penetration itself, checked against that part’s stated limit |
| Locknut | It holds position; it is not the pressure boundary | Whether it comes with the fitting, and the wrench size its flats need |
| Seal location | Separates what stops a leak from what stops rotation | The ferrule seal on each tube end, and separately whether the panel face wants its own weather seal |
| Tube ends | A bulkhead fitting is still a union on both sides of the wall | Outside diameter and wall each side, measured, since a bulkhead can also be a reducing pattern |
Every row here is a check to carry out, not a figure to copy. The thickness values quoted earlier in this post are one manufacturer’s published maxima for its own listed parts and do not transfer to any other part or brand. Superlok® dimensions and grade are confirmed per order.
The locknut’s job, and where the seal actually is
The locknut is the most visible extra part compared with a plain union, so it collects the blame for leaks it has nothing to do with. On a compression tube fitting the pressure boundary is at the tube ends, where the ferrules do the work. Swagelok®’s catalog describes that division plainly for its own design: the gripping duty and the sealing duty are split between two ferrules, each of which “is optimized for its function,” the back one advancing the front one while the front one seals. Nothing in that mechanism involves the panel. The locknut clamps the body against the panel so the assembly cannot rotate or work its way loose in service. It holds position. It does not hold pressure, and tightening it harder will not cure a weep at a ferrule.
Whether a given penetration also needs a weather seal, a gasket or O-ring against the panel face to keep wash-down water and dust out of a cabinet, is a question about the enclosure rather than about the tube joint. Settle it separately and do not assume the locknut has answered it either way.
Three ways a bulkhead replacement fails before anyone reaches the leak test.
Right tube, wrong hole
Drilling to the tube outside diameter rather than to the panel hole size printed on that part’s own row.
Thickness taken on trust
Ordering against the thickness a similar-looking skid used last year instead of putting a caliper on the panel in front of you.
Locknut treated as a seal
Hauling on the locknut to chase a leak that is actually happening at a ferrule on one of the tube ends.
Superlok® markets its own fittings on a single claim, the finest interior surface finish of any tube and pipe fitting on the market, describing Superlok® machining and saying nothing whatsoever about your panel. Bulkhead unions are listed in 316 stainless steel, corrosion-resistant metal that earns its keep on an outdoor skid. Panel hole size, allowable panel thickness, grade and working pressure are each settled for the exact part at the order stage. Beyond the unions there is the full tube fittings catalog, the ferrule sets that make the actual seal on each side, a certifications record for the paperwork, and the semiconductor fab piece for panel work under tighter cleanliness rules.
Questions about cross-referencing a bulkhead union
Does a bulkhead union seal at the panel or at the tube?
At the tube. Both sides seal through the same ferrule arrangement any union uses. The nut at the panel secures the body in place and is not part of the pressure boundary.
Can I reuse the hole that is already there?
Only where the replacement part’s own published hole size and thickness limit both suit the hole and the panel as they stand. If either is out, the choice is a fresh hole or a different mounting, not a forced fit.
Does a larger tube size allow a thicker panel?
Not in any fixed proportion. In the figures quoted above, doubling the tube outside diameter lifts the published maximum by about a quarter. Read the row for the part rather than scaling from a neighbouring size.
What is the locknut actually for?
Clamping the body against the panel so the assembly cannot spin or back out under service loads. That is retention, and it is a separate job from the seal at the ferrules.
What should reach us for a bulkhead cross-reference?
Measured panel thickness at the penetration, the diameter of the hole as it exists, and the tube outside diameter and wall on each side. We work from those four numbers.
Measure the panel first
Thickness at the penetration, the hole you already have, and the tube size each side.
Superior Fluid Solutions
The panel decides. Not the catalogue.
Sources
- Swagelok, “Gaugeable Tube Fittings and Adapter Fittings,” MS-01-140 (manufacturer publication), supports the bulkhead union table carrying a panel hole size column and a maximum panel thickness column alongside the lettered dimensions for every listed tube size, the published maximum panel thickness figures of 0.40 in. at 1/4 in. tube, 0.44 in. at 3/8 in. and 0.50 in. at 1/2 in., and and the quoted statement that each ferrule is optimized for its function within a design that splits gripping from sealing. swagelok.com
Technical note: the source above establishes how one manufacturer catalogues its own bulkhead unions, including panel hole size and maximum panel thickness as published columns, the maximum panel thickness figures quoted for three tube sizes, and how that manufacturer accounts for the sealing and gripping duties within its own two-ferrule design. Those figures apply only to the parts that catalog lists, at the edition read. They do not establish a panel hole size or a thickness limit for any other size, any other part or any other brand, they do not establish that components may be assembled together across brands, and they do not certify a working pressure for your service. Superlok® panel dimensions, grade, working pressure and documentation are confirmed per order.
Trademark disclaimer: Swagelok® is a registered trademark of Swagelok Company. Superlok® is a registered trademark of BMT Co., Ltd. Superior Fluid Solutions is not affiliated with, authorized by, endorsed by or sponsored by either company, and the marks are used here only to identify the products they name. Cross-references are made by form, fit, and function only.
