Trojan Special Fasteners Ltd | Specialist Nuts Fasteners | Nuts Bolts Washers | Precision Polygon Turning | Fastener Manufacturer

FAQ Section - Pre War Nuts, Stay Nuts & Historical Standards

Pre War Nuts & BS190/BS191 Standards

Q: What exactly are pre-war nuts and why are they called "pre-war"?
Pre-war nuts refer to fasteners manufactured to the original British Standard BS190:1924 specifications. The term "pre-war" is somewhat of a misnomer as the dimensional change actually occurred in 1924, not specifically related to either World War. Prior to 1924, hexagon nuts and bolt heads were manufactured with larger across-flat (A/F) dimensions. The reduction in hexagon sizes was implemented due to steel conservation measures following World War I. The larger hexagon pattern was standardised as BS190, whilst the reduced sizes became widespread after 1924. Interestingly, during World War II, the smaller hexagon sizes were adopted even more widely to conserve metal, making the term "pre-war" stick in common usage.
Q: What's the difference between BS190, BS191, and BS193 standards?
BS190:1924 specified the original larger hexagon dimensions for BSW (British Standard Whitworth) fasteners - what we now call "pre-war" sizes. BS191 covered BSF (British Standard Fine) fasteners with their finer thread pitches but still using the original larger hexagon dimensions. BS193:1929 introduced the "AutoWhit" or "Auto-Whit" series, which used Whitworth threads but with smaller hexagons identical to BSF sizes of the same bolt diameter. This was primarily adopted by the automotive industry to save weight and material. All three standards are now withdrawn but remain critical references for restoration work.
Q: How much larger are pre-war hexagon sizes compared to modern equivalents?
Pre-war hexagons are typically one spanner size larger than their modern counterparts. For example:

• A pre-war 1/2" BSW nut has an A/F of 1.010" (requiring a 1.010" spanner)
• A modern 1/2" BSW nut has an A/F of 0.920" (requiring a 0.920" spanner)
• The difference is approximately 0.090" or about 2.3mm

This pattern continues across the entire size range, though the exact differences vary slightly by nominal size.
Q: Can I use a modern BSW nut on a pre-war bolt?
Whilst the threads are identical (both being Whitworth standard with 55-degree thread angle), the hexagon sizes differ. A modern BSW nut will thread onto a pre-war bolt perfectly, but you'll need different spanners for the nut and bolt head. This creates practical problems in assembly and can look incorrect on restoration projects. For authentic restoration, matching pre-war nuts and bolts should be used throughout.
Q: Why did Morris and MG use metric threads with Whitworth hexagons?
An interesting historical quirk: when Morris Motors took over the French Hotchkiss engine works in Coventry in 1919 after WWI, they inherited metric threading machinery. However, metric spanners weren't readily available in Britain at the time. The solution was to manufacture fasteners with metric threads but Whitworth-sized hexagons, allowing British mechanics to use their existing tools. This hybrid system continued from 1923 to 1955 in Morris and MG engines.

Railway Stay Nuts & Crown Stay Nuts

Q: What are stay nuts and why are they critical for boiler safety?
Stay nuts work in conjunction with stay bolts to hold the flat or curved surfaces of a locomotive firebox in position against tremendous steam pressure - up to 13 tons per square foot. The firebox is essentially a pressure vessel within a pressure vessel, with water surrounding it on all sides. Without stays, the firebox sheets (typically only 3/8" thick for efficient heat transfer) would collapse inward or blow outward catastrophically. Stay nuts secure the threaded ends of stay bolts that span the water space between the inner firebox and outer wrapper sheet.
Q: Why do all stay nuts use 11 TPI regardless of diameter?
The standardisation on 11 TPI (threads per inch) across all diameters from 5/8" to 1.1/2" was a practical engineering decision. It allowed railway workshops to minimise tooling inventory - fewer taps, dies, and thread gauges were needed. This standardisation also reduced the risk of assembly errors and made field repairs more straightforward. The 11 TPI pitch provides adequate strength whilst allowing reasonable assembly torque without risk of thread stripping in the relatively soft boiler plate materials.
Q: What's the difference between rigid stays, flexible stays, and radial stays?

• Rigid stays are solid threaded rods, typically under 8 inches long, screwed through both firebox and wrapper sheets and upset (riveted over) at both ends. They provide maximum strength but no allowance for differential expansion.
• Flexible stays incorporate a ball-and-socket joint allowing limited movement to accommodate the differential expansion between the hot firebox (exposed to fire) and cooler wrapper sheet (exposed to atmosphere). The ball end sits in a cup-shaped sleeve welded to the wrapper sheet, whilst the threaded end screws into the firebox. This flexibility prevents stay breakage from thermal cycling.
• Radial stays are longer stays (over 8 inches) installed on radial lines around curved firebox surfaces, particularly the crown sheet. They're typically 3/16" smaller in diameter than the thread size to allow for expansion and have enlarged ends for better load distribution.

Q: What are crown stay nuts and how do they differ from regular stay nuts?
Crown stay nuts support the crown sheet (top) of the firebox - the most critical area as it's directly exposed to intense heat and most prone to failure if water level drops. Crown stay nuts are typically:

• Longer/taller for increased thread engagement
• May feature castellated tops for safety wire installation
• Often drilled for inspection access
• Sometimes incorporate extended collars for additional bearing surface
• Made from higher-grade materials due to the critical nature of crown sheet support

Q: What's a "tell-tale hole" in a stay bolt?
Stay bolts are drilled with a 3/16" diameter hole approximately 1.1/4" deep from the outer end. If the stay bolt cracks or breaks internally (usually near the outer sheet due to bending stress), water or steam will leak through this tell-tale hole, alerting maintenance crews to the failure. This is critical because a broken stay isn't visible externally but represents a serious safety hazard if multiple adjacent stays fail.
Q: How many stay bolts does a typical locomotive boiler have?
A medium-sized locomotive boiler typically contains 300-900 stay bolts, depending on firebox size. Large locomotives can have over 1,500 stays. For example, the Chicago & North Western #1385 restoration required over 900 stay bolts. Each must be individually fitted, as the spacing between firebox and wrapper sheets varies throughout the boiler due to manufacturing tolerances and thermal distortion.
Q: What materials are used for stay nuts and why?

• EN1A (230M07) - Modern bright mild steel, common for general stay nuts. Excellent machinability, adequate strength for most applications, but requires some expertise to weld.
• EN3B (070M20) - Standard mild steel, most common for general stay nuts. Good machinability, adequate strength for most applications and it's weldable.
• EN8 (080M40) - Medium carbon steel for higher strength requirements, particularly in high-pressure boilers or critical locations.
• EN16T (605M36T) - Through-hardened alloy steel for extreme service conditions, often used for crown stays or marine boiler applications.
• Copper - Sometimes used where corrosion resistance is paramount or to match copper firebox stays, though less common for nuts themselves.

Q: Can broken stays be repaired or must they be replaced?
Broken stays must be completely removed and replaced - repair isn't acceptable for safety reasons. The process involves:

1. Cutting out the broken stay from both ends using an acetylene torch
2. Cleaning and re-tapping the holes (often requiring oversize taps if threads are damaged)
3. Installing a new stay bolt (possibly oversize to match re-tapped holes)
4. Upsetting (riveting over) both ends to seal

Modern welded boilers may use different repair techniques, including welding up holes and re-drilling/tapping, but this varies by jurisdiction and inspection requirements.

Technical & Application Questions

Q: What are the torque specifications for pre-war nuts?
No formal torque specifications exist for pre-war fasteners as torque wrenches weren't commonly used in the 1920s. Assembly was typically done by feel, with mechanics judging proper tightness by spanner length and effort. For modern restoration work, we recommend using standard Whitworth torque values reduced by 10-15% to account for the larger bearing surface of pre-war hexagons. Always consider the application and material grade when determining appropriate torque.
Q: How do I identify whether I have pre-war or modern Whitworth fasteners?
Measure the across-flats dimension and compare to standard charts. Pre-war nuts for a given thread size will measure approximately one standard spanner size larger. For example:

• 1/2" BSW pre-war: 1.010" A/F
• 1/2" BSW modern: 0.920" A/F

Additionally, pre-war fasteners often show different manufacturing characteristics - less precise hexagon forming, coarser surface finish, and evidence of hot forging rather than cold forming.
Q: Are left-hand thread stay nuts common?
Left-hand thread stays are used in specific locations where rotation during service could cause loosening. They're particularly common on reversing engines or where vibration patterns favour anti-clockwise loosening. We regularly manufacture LH thread stays in any size, though they represent perhaps 5-10% of total stay nut production.
Q: What's the difference between stay nuts and coupling/connector nuts?
Stay nuts are specifically designed for boiler stays with standardised 11 TPI threading and dimensions suited for boiler plate engagement. Coupling nuts are general-purpose extended nuts for joining threaded rods or mixed-thread applications. Whilst both are "long nuts", stay nuts have specific dimensional requirements for boiler code compliance whilst coupling nuts follow general engineering standards.
Q: Can you manufacture stays to American boiler codes?

• American boiler stays typically use different specifications:
• ASME Boiler Code requirements
• Often 12 TPI Whitworth standard threads (not 11 TPI)
• Different material specifications (ASME grades vs British EN standards)
• May require different tell-tale hole specifications

We can manufacture to any international boiler code with appropriate documentation.

Obsolete & Withdrawn International Standards

Q: What other obsolete fastener standards should restoration projects be aware of?
Beyond British standards, numerous international standards have been withdrawn:
German DIN Standards (largely withdrawn April 2001 via DIN 918):

• Replaced by ISO standards but still referenced in older machinery
• DIN 931, 933, 934 were globally dominant before ISO adoption

French AFNOR Standards:

• NFE series largely superseded by EN/ISO
• Still found in vintage French automobiles and machinery

Japanese JIS B1180-1186 (older series):

• Modernised but older patterns still found in pre-1980s equipment

American Standards:

• ASA (American Standards Association) - predecessor to ANSI
• SAE patterns before unification with ISO

Q: Why weren't BA (British Association) threads affected by the pre-war changes?
BA threads were a separate metric-based system using a 47.5-degree thread angle, primarily for small instrument and electrical work. They had their own distinct hexagon sizes unrelated to Whitworth standards. The pre-war dimensional changes only affected Whitworth-form threads (BSW, BSF), not BA, BSP (British Standard Pipe), or other thread systems.
Q: What happened to BS84:1956 and why is it still referenced?
BS84:1956 consolidated and replaced the earlier separate standards for BSW and BSF threads, including BS84:1940 and earlier versions. Whilst officially withdrawn and replaced by BS84:2007 (which covers ISO metric threads), BS84:1956 remains the definitive reference for Whitworth thread specifications. Many engineering handbooks still reference it as it contains the complete dimensional data needed for manufacturing Whitworth threads.
Q: Are cycle thread (BSC) fasteners related to pre-war standards?
British Standard Cycle (BSC) replaced the Cycle Engineers' Institute (CEI) standard and used a 60-degree thread angle (not Whitworth's 55 degrees) with very fine pitches - 26 TPI regardless of diameter. BSC fasteners had their own unique hexagon dimensions unrelated to pre-war changes. However, vintage bicycles and motorcycles from the pre-war era might use BSW, BSF, or BA threads depending on manufacturer and component.

Manufacturing & Sourcing Questions

Q: Why is it so difficult to find pre-war nuts commercially?
Pre-war fasteners represent a tiny niche market with limited demand, making mass production uneconomical. Most fastener manufacturers have long since scrapped the tooling for these obsolete sizes. The hexagon bar stock in pre-war dimensions is no longer routinely produced, requiring either:

• Custom hexagon bar production (minimum quantities often 1 tonne)
• Machining hexagons from oversized bar (labour intensive)
• Polygon turning (we can do this relatively quickly and cheaply)
• Hot forging with special dies (requires quantity to justify tooling)

Q: Can you match fasteners from a sample without drawings?
Yes, we routinely reverse-engineer fasteners from worn or damaged samples. Send us your sample and we'll:

1. Measure all critical dimensions
2. Identify thread form and pitch
3. Determine appropriate material based on application
4. Often improve on original specifications using modern materials
5. Produce exact replacements or advise on suitable alternatives

Q: What's the typical lifespan of stay nuts in service?
Stay nut lifespan depends heavily on:

• Water treatment quality (poor water causes rapid corrosion)
• Operating pressure and temperature cycles
• Maintenance practices (regular inspection and replacement of failed stays)
• Installation quality (proper torque, thread engagement)

Well-maintained stays in good water conditions might last 20-30 years. In poor conditions or with irregular maintenance, failure can occur in under 5 years. Heritage railways typically inspect stays annually and budget for 5-10% replacement yearly.
Q: Do you supply matching bolts for pre-war nuts?
Whilst our speciality is internal threads (nuts), we can manufacture matching bolts, set screws, and studs to complete restoration requirements. Pre-war bolts require the same larger hexagon heads as the nuts, maintaining dimensional consistency across the assembly. We can also supply square heads if originally specified.
Q: What surface finishes are appropriate for restoration work?
Historical finishes include:
Black oxide - Common for industrial/railway applications
Natural/self-colour - Machinery often left unfinished
Cadmium plating - Now restricted but was common pre-1960s (we don't offer this)
Nickel plating - Decorative applications, vintage vehicles
Hot-dip galvanising - Structural/outdoor applications
Phosphating - Military vehicles and equipment
Modern alternatives like zinc plating can be artificially aged for authentic appearance.
Q: Can you manufacture to Imperial sizes not in your standard range?
We manufacture any Imperial size from #2 through 2.1/2" diameter in:
Standard coarse threads (BSW, UNC)

• Fine threads (BSF, UNF)
• Extra fine threads (UNEF)
• Special pitches to drawing
• Left-hand versions of any thread

All available in pre-war or modern hexagon dimensions.

Industry-Specific Questions

Q: What other industries besides railways use stay-type fasteners?

• Marine Engineering: Ship boilers, similar stay bolt requirements
• Stationary Boilers: Industrial plant, power generation
• Traction Engines: Agricultural, fairground, road rollers
• Brewing/Distilling: Pressure vessel stays in process equipment
• Mining Equipment: Boiler-equipped machinery
• Sugar Mills: Process boilers in plantation equipment
• Textile Mills: Steam-powered machinery from Industrial Revolution era

Q: Are there special considerations for museum-quality restorations?
Museum restorations often require:

• Documentation of all replacement parts
• Matching of original manufacturing methods where possible
• Use of period-appropriate materials (even if inferior to modern alternatives)
• Preservation of original maker's marks or recreation thereof
• Sometimes deliberate use of inferior finishes to match aged appearance
• Retention of original fasteners where safely possible

Q: What about vintage aircraft restoration fastener requirements?
Vintage aircraft present unique challenges:

• Often use AN (Army-Navy) or AGS (Aeronautical Standards) specifications
• May require certified materials with traceability
• Close-tolerance threads (Class 3 fit)
• Special locking features (castle nuts with cotter pins)
• High-strength materials whilst maintaining period appearance
• Compliance with CAA/FAA regulations for airworthy restorations

Q: Do you support vintage military vehicle restoration?
Yes of course we do! Military vehicles often use:

• Unique thread combinations (BSF body with BSW head dimensions)
• Special materials for armoured vehicle applications
• Left-hand threads for track components
• Castellated nuts for safety-critical applications
• Often require matching of specific manufacturer patterns (not just thread standards)

Modern Applications & Alternatives

Q: When might modern fasteners be acceptable in restoration?
Modern fasteners might be acceptable when:

• Hidden from view in non-critical locations
• Safety requires upgraded materials
• Original specifications are genuinely unavailable
• Cost constraints on non-museum restorations
• Temporary assembly for testing/transport

However, visible fasteners should match original specifications for authentic appearance.
Q: What about metric conversions for international projects?
We don't recommend metric substitutions for Whitworth threads as:

• Thread angles differ (55° vs 60°)
• No direct metric equivalent pitches exist
• Metric fasteners look obviously different
• May not meet strength requirements

Better to source correct Imperial fasteners from Trojan Special Fasteners Ltd even for export projects.
Q: Can modern CNC machinery produce accurate pre-war threads?
Modern CNC lathes produce superior thread quality compared to 1920s equipment:

• More precise pitch control
• Better surface finish
• Consistent thread depth
• Accurate thread angles

We use CNC for all precision work whilst maintaining dimensional standards of the era.
Q: Are there modern safety improvements applicable to stay nuts?
Modern improvements we can incorporate:

• Enhanced tell-tale holes using EDM drilling
• Improved thread root radius for fatigue resistance
• Modern heat treatment for consistent properties
• Better material traceability and certification
• Non destructive testing of critical components

These don't affect authentic appearance but improve safety.
Q: What about environmental considerations in restoration?
Modern environmental requirements affect:

• Cadmium plating (now restricted/banned)
• Lead-based materials (historical bearing metals)
• Asbestos gaskets (common in vintage equipment)
• Hexavalent chromium finishes

We offer environmentally acceptable alternatives that maintain period appearance whilst meeting modern regulations.

Ordering & Commercial Questions

Q: What information do I need to provide when ordering?
Essential information:

1. Thread size and form (e.g., 3/4" BSW)
2. Pre-war or modern hexagon size
3. Quantity required
4. Material specification or application details
5. Surface finish requirements
6. Any special features (left-hand, drilled, slotted)
7. Required delivery timeframe

Q: Do you hold stock of common pre-war sizes?
We maintain limited blanks stock of frequently requested sizes but primarily manufacture to order due to the vast range of possible specifications. Common stock items include:

• 3/8", 7/16", 1/2" BSW pre-war nuts
• Standard stay nut sizes (3/4", 7/8", 1" x 11 TPI)

Contact us for current availability.
Q: What about certification and material traceability?
We provide:

• Material certificates to EN 10204 2.2 and 3.1 standard
• Dimensional inspection reports
• Thread gauge certification
• Proof load test certificates where required
• Full traceability from material source to finished product
• Compliance statements for specific standards

Q: Can you support restoration projects internationally?
We regularly export worldwide for restoration projects:

• Heritage railways in Australia, New Zealand, USA, Europe
• Military vehicle restorations globally
• Marine heritage projects
• Museum restorations requiring British Imperial fasteners
• International vintage machinery collectors

Q: What's your policy on prototype and development work?
We welcome prototype work for:

• Experimental restoration techniques
• Recreation of extinct fastener patterns
• Development of improved replacement designs
• Special tools and fixtures for restoration work

Single piece prototypes through to small production runs.
This FAQ represents accumulated knowledge from over 50 years of manufacturing specialist fasteners for restoration projects. For specific technical queries not covered here, please contact our engineering team.