IS 3757 vs EN 14399 HR Bolts: Which HSFG Bolt Should a Designer Specify?
IS 3757 and EN 14399-3 (system HR) both give you a high strength bolt for friction grip joints, in the same property classes and with the same large hexagon. On a drawing they look interchangeable. On site they behave differently, mainly because of the thread length and the nut. This note is for the engineer who has to write one of the two on the drawing.
Two different ideas
IS 3757 is a standard for the bolt. It covers large series hexagon bolts in property classes 8.8 and 10.9, from M16 to M36, with short thread lengths. The nut comes from IS 6623 and the washer from IS 6649, and tightening is covered by IS 4000.
EN 14399 is a standard for the assembly. Bolt, nut and washers are supplied as one set by one manufacturer, and the set is tested together for how it tightens (EN 14399-2). Part 3 of the standard is system HR, in classes 8.8 and 10.9, from M12 to M36. A second system, HV, exists in 10.9 only; this note is about HR.
Difference 1: thread length
This is the difference that matters most on site.
| Size | IS 3757 thread length (mm) | EN 14399-3 HR thread length (mm) | ||
|---|---|---|---|---|
| Bolt up to 100 long | Bolt over 100 long | Bolt up to 125 long | Bolt 125 to 200 long | |
| M16 | 31 | 38 | 38 | 44 |
| M20 | 36 | 43 | 46 | 52 |
| M24 | 41 | 48 | 54 | 60 |
| M30 | 49 | 56 | 66 | 72 |
IS 3757 keeps the thread short so that the plain shank, not the thread, sits inside the plates. That is good for the joint, but it leaves very little spare thread. If the actual grip (plates plus washers) turns out smaller than the grip the bolt length was chosen for, the nut reaches the end of the thread before the joint is tight.
The HR bolt has 7 to 17 mm more thread in these sizes, so one bolt length covers a wider range of grips. That margin is the practical reason to choose HR.
Difference 2: the nut
The HR nut is lower than the IS 6623 nut.
| Size | IS 6623 nut height, max (mm) | EN 14399-3 HR nut height, max (mm) |
|---|---|---|
| M20 | 20.7 | 18.0 |
| M24 | 24.2 | 21.5 |
| M30 | 30.7 | 25.6 |
| M36 | 36.6 | 31.0 |
One more point for the fitter: in M20, the IS 3757 bolt and its nut are 34 mm across flats, while the HR bolt and nut are 32 mm. The spanner is different, so do not mix M20 bolts and nuts from the two standards.
Difference 3: coating on 10.9 bolts
IS 3757 carries a note that property class 10.9 bolts should not be hot dip galvanised, because of the risk of hydrogen embrittlement. Its table of bolt and nut finishes accordingly pairs the 10.9S bolt only with a black finish. If a job needs 10.9 bolts with heavy corrosion protection, hot dip galvanising under IS 3757 is not the route.
For 10.9 HR bolts we use zinc flake coating. Zinc flake is applied without acid pickling and without electroplating, so the coating process does not put hydrogen into the steel. Under EN 14399 the coating is the manufacturer's responsibility as part of the assembly, because the coating and the lubricant on the nut decide how the set tightens.
Class 8.8 bolts can be hot dip galvanised under either standard.
Difference 4: washers and DTI
An IS 3757 set uses plain hardened washers to IS 6649. EN 14399 has its own plain washers and, in Part 9, the direct tension indicator (DTI): a washer with raised protrusions that flatten as the bolt is tightened, so the remaining gap shows that the preload has been reached. DTIs are made as H8 for 8.8 bolts and H10 for 10.9 bolts.
Can you mix the two?
For DTI washers, yes. Indian designers already do it: drawings now call for IS 3757 bolts with EN 14399-9 DTI washers. RDSO's guidelines for railway bridges preferred DTI washers over plain washers even when the bolts were to IS 3757. RDSO has since gone further: its specification for HSFG bolting assemblies with DTI washers is written around EN 14399-3 HR bolts.
Two cautions when mixing:
- Match the DTI to the bolt grade: H8 with 8.8, H10 with 10.9.
- Buy the bolt, nut, washers and DTI as one set from one manufacturer, so that one party answers for how the set tightens.
Do not mix bolts and nuts between the two standards. The threads may fit, but the nut heights, and in M20 the spanner size, are different.
Which one to specify
| Your situation | Specify |
|---|---|
| The grip at every joint is known and will not change | IS 3757 |
| Grips vary, plate thickness may change in detailing, or packing plates may be added | EN 14399-3 HR |
| 10.9 bolts with corrosion protection | EN 14399-3 HR with zinc flake coating |
| 8.8 bolts, hot dip galvanised | Either |
| Railway bridge work | Follow the current RDSO specification: HR assemblies with DTI washers |
| IS 3757 bolts, but preload must be visible at inspection | IS 3757 with EN 14399-9 DTI washers, if the client accepts |
The short rule: if you are sure about the thread length, go for IS 3757. If you are not, go for HR.
What to write on the drawing
- Standard: IS 3757, or EN 14399-3 system HR
- Size and length, with the grip at each joint
- Property class: 8.8 or 10.9
- Coating: black, hot dip galvanised (8.8) or zinc flake
- Nut and washer standards, and whether DTI washers are required
S R Forgings supplies HSFG bolt sets to IS 3757 and to EN 14399-3 HR in classes 8.8 and 10.9, with DTI washers to EN 14399-9. Send us the size, grip, grade and coating and we will suggest the bolt lengths.
Standards referred to: IS 3757:1985 (bolts), IS 6623:2004 (nuts), IS 6649:1985 (washers), IS 4000:1992 (tightening), EN 14399 Parts 2, 3 and 9, and RDSO report BS-111. Dimensions are quoted from the standards for comparison. Check the current edition and the project specification before ordering.
Useful tools on our export site: Bolt Weight Calculator and Bolt Torque Calculator.