1. Introduction
DIN938 stud bolt is a double-ended threaded fastener widely used in mechanical assembly, pipeline flanges, and equipment base fixing. As one of the most commonly specified German standard stud bolts, DIN938 features equal thread length on both ends and a plain shank in the middle, making it ideal for applications where one end is screwed into a threaded hole and the other end receives a nut.
NINGBO TOPBOLT METALWORKS CO., LTD supplies DIN938 carbon steel stud bolts in full size range from M6 to M52, with multiple strength grades and surface treatments available to match different industrial requirements.
2. What Is DIN938 Stud Bolt?
2.1 Standard Definition
DIN938 is the German standard for "stud bolts with equal thread length at both ends — product grade B". The standard defines the dimensional tolerances, thread length formula, material requirements, and marking rules for this type of fastener.
The key characteristic of DIN938 is that both threaded ends have the same nominal length, calculated as b = 2d (where d is the nominal thread diameter). This distinguishes it from DIN939 and DIN940, which have different thread length ratios.
2.2 Structural Features
Both ends threaded with equal length (b = 2d)
Plain unthreaded shank in the middle section
No bolt head — tightened by nut on one end only
Metric coarse thread as standard (fine thread available on request)
Product grade B for general industrial applications
3. DIN938 Dimension & Thread Length Specification
3.1 Size Range & Standard Lengths
DIN938 stud bolts are available in nominal diameters from M6 up to M52, with standard nominal lengths ranging from 30mm to 500mm depending on diameter. The table below lists common sizes and their corresponding thread lengths.
Nominal Diameter (d) | Thread Length Both Ends (b = 2d) | Standard Length Range (l) | Typical Application |
M6 | 12mm | 30–80mm | Small equipment, electrical enclosures |
M8 | 16mm | 40–120mm | General machinery, pipe clamps |
M10 | 20mm | 50–160mm | Automotive, light industrial |
M12 | 24mm | 60–200mm | Flange connections, equipment bases |
M16 | 32mm | 80–250mm | Pipeline flanges, structural joints |
M20 | 40mm | 100–300mm | Heavy machinery, pressure vessels |
M24 | 48mm | 120–400mm | Large flanges, industrial equipment |
M30 | 60mm | 150–500mm | Heavy structural, turbine casings |
M36 | 72mm | 200–500mm | Large pressure vessels, steel structures |
M42–M52 | 84–104mm | 250–500mm | Heavy industrial, power generation |
3.2 Thread Length Calculation Rule
For DIN938, the thread length on each end follows the formula b = 2d. For example:
M12 stud bolt → each end thread length = 2 × 12 = 24mm
M20 stud bolt → each end thread length = 2 × 20 = 40mm
M24 stud bolt → each end thread length = 2 × 24 = 48mm
This equal-length design ensures sufficient thread engagement on both the tapped-hole side and the nut side, providing balanced clamping force in standard assembly conditions.
4. DIN938 vs DIN939 vs DIN940: Key Differences
While all three are German standard stud bolts, they differ primarily in the thread length ratio between the two ends. Understanding these differences helps avoid wrong specification in procurement.
Standard | Thread End 1 (Screwed into Hole) | Thread End 2 (Nut Side) | Design Type | Primary Use Case |
DIN938 | b = 2d | b = 2d (equal) | Equal thread both ends | General purpose, standard tapped holes |
DIN939 | b = 1.5d (shorter) | b = 2d (longer) | Unequal — short end for hole | Thin-wall components, shallow tapped holes |
DIN940 | b = 1.25d (shortest) | b = 2d (longer) | Unequal — very short end for hole | Very thin flanges, limited thread depth |
Key Takeaway: DIN938 is the most versatile and widely stocked option. Choose DIN939 or DIN940 only when the tapped hole has limited depth and cannot accommodate the full 2d thread engagement of DIN938.
5. Material & Strength Grade Options
DIN938 stud bolts are commonly manufactured from carbon steel and alloy steel, with strength grades selected based on the application load and environmental conditions.
Strength Grade | Typical Material | Tensile Strength (min) | Yield Strength (min) | Common Applications |
4.8 | Low carbon steel (C1010/C1015) | 400 MPa | 320 MPa | Light assembly, non-critical joints |
8.8 | Medium carbon steel (C35/C45, quenched & tempered) | 800 MPa | 640 MPa | General machinery, standard flanges |
10.9 | Alloy steel (42CrMoA, quenched & tempered) | 1000 MPa | 900 MPa | Heavy machinery, high-load joints |
12.9 | Alloy steel (42CrMoA / 35CrMoA) | 1200 MPa | 1080 MPa | Ultra-high load, critical structural |
6. Common Applications of DIN938 Stud Bolt
DIN938 stud bolts are used across a wide range of industries where a bolt head is not practical or where disassembly from one side only is required:
Pipeline flanges: One end screwed into the flange body, the other end clamped with a nut and gasket
Equipment base fixing: Screwed into threaded holes in machine bases or concrete embed plates
Pressure vessel covers: Studs permanently installed in the vessel body, nuts removed for maintenance access
Automotive engine components: Cylinder head studs, exhaust manifold studs
Structural steel connections: Where through-bolting is not possible due to access limitations
Valve and pump assemblies: Body-to-bonnet connections requiring repeated disassembly
7. Installation Notes & Best Practices
7.1 Thread Engagement Depth
When screwing DIN938 into a tapped hole, ensure sufficient thread engagement for the base material:
Steel base material: minimum 1d engagement (e.g., M16 → 16mm minimum)
Cast iron base material: minimum 1.5d engagement
Aluminum base material: minimum 2d engagement
Since DIN938 provides 2d thread length on each end, it generally meets the engagement requirement for steel and cast iron without issue.
7.2 Torque Application
Apply torque only to the nut side, never to the stud itself. Over-tightening the stud into the tapped hole can cause thread galling or strip the internal threads. Use a calibrated torque wrench and follow the recommended torque values for the specific strength grade and lubrication condition.
7.3 Anti-seize Compound
For stainless steel or high-strength alloy steel stud bolts, apply a suitable anti-seize compound to the threads before installation to prevent galling and facilitate future disassembly. This is especially important in high-temperature or corrosive environments.
8. Surface Treatment Options
Surface Treatment | Corrosion Resistance | Typical Salt Spray | Suitable Environment |
Plain / Black Oil | Low | — | Indoor dry, short-term storage |
Black Oxide | Low | — | Indoor, lubricated assemblies |
Zinc Plated (Blue/White) | Medium | 48–120 hours | General indoor, mild outdoor |
Zinc Plated (Yellow) | Medium-High | 120–240 hours | Outdoor, moderate humidity |
Hot Dip Galvanized | High | 500+ hours | Outdoor, corrosive, coastal |
Dacromet / Geomet | Very High | 500–1000+ hours | Severe corrosion, automotive |
9. Frequently Asked Questions
Q1: Can DIN938 stud bolts be used for flange connections?
Yes. DIN938 is commonly used for standard flange connections where the flange body has tapped holes. For through-hole flanges, through-bolts or full-thread studs may be more appropriate.
Q2: What is the difference between DIN938 and a fully threaded rod (DIN975)?
DIN938 has a plain unthreaded shank in the middle with threaded sections only at both ends (each b=2d). DIN975 is fully threaded along its entire length. DIN938 provides better shear strength in the shank section, while DIN975 offers adjustable nut positioning.
Q3: Can I customise the length of DIN938 stud bolts?
Yes. NINGBO TOPBOLT METALWORKS CO., LTD offers custom length DIN938 stud bolts within the M6–M52 size range. Custom lengths, non-standard thread ratios, and special materials can be produced according to customer drawings or specifications.
Q4: Which strength grade should I choose for high-temperature pipeline flanges?
For high-temperature applications (above 200°C), grade 8.8 or higher with alloy steel material (such as 35CrMoA or 42CrMoA) is recommended. Always verify the material temperature rating against the specific operating conditions.
Q5: How do I prevent stud bolts from loosening due to vibration?
For vibrating environments, use lock nuts (DIN985 nylon insert or DIN6923 flange nut), spring washers, or thread-locking adhesive on the nut side. Ensure the stud is firmly seated in the tapped hole before applying the nut.
