Two standard systems for fasteners are in active use in Norwegian heavy industry. The ISO standard and DIN standards dominate in European- and Norwegian-made equipment. The ASME standard and ASTM specifications dominate in American-made machinery, offshore wellhead equipment and compressor technology. At facilities with equipment from both origins, both systems exist side by side. The problem is not that both exist. The problem is what happens during maintenance, in the stockroom and during an unplanned shutdown when they are mixed without control.
What are the practical differences between the ASME standard and the ISO standard?
The ASME standard and the ISO standard use different units, different designation systems and different requirements for mechanical properties. Imperial fasteners to the ASME standard are stated with diameter in inches and TPI. Metric fasteners to the ISO standard are stated with diameter in millimetres and pitch in millimetres. A fastener from each system with approximately the same measurements is not interchangeable: the thread geometry, the flank angle and the pitch differ, and the threads will not engage correctly with each other.
The strength classes are also separate systems. The ISO standard uses designations such as 8.8, 10.9 and 12.9, defined in ISO 898-1. The ASME standard uses SAE Grades (Grade 5, Grade 8 and Grade 10) and ASTM designations (B7, L7) defined under SAE J429 and ASTM A193. Even where the tensile strength is approximately equal, the requirements for chemical composition, heat treatment and marking are not identical. A 10.9 ISO standard bolt and an SAE Grade 8 ASME standard bolt are not directly substitutable without engineering approval.
What does that mean for maintenance?
During maintenance, the consequence of mixing the ASME standard and the ISO standard is concrete and immediate. Tools calibrated for ISO standard fasteners do not necessarily fit ASME standard fasteners. Torque values calculated for ISO standard strength classes do not apply directly to ASME standard strength classes. A maintenance procedure that states torque in Nm for ISO 10.9 bolts, but which is carried out on ASME Grade 8 bolts, produces a preload deviation that is not visible at installation.
Identification of existing fasteners is the critical first step in any maintenance operation on older or mixed equipment. Without knowing whether an existing bolt is ISO standard or ASME standard, it is not possible to select the correct replacement, apply the correct torque value or document the installation in accordance with the applicable requirements. Visual inspection is not sufficient. The standard can only be established from the as-built drawing, the equipment data sheet or the marking on the bolt head.
What does that mean for the stockroom?
A stockroom that does not distinguish between ISO standard and ASME standard fasteners is not a contingency stock, it is a source of risk. Bolts from the two systems can look identical at overlapping diameters, but have different thread systems, different proof load requirements and different documentation requirements. An ASME standard B7 bolt and an ISO standard 10.9 bolt in the same diameter and length look alike in a stock drawer. They are not interchangeable in a safety-classified system.
Correct stock management for facilities with both standard systems requires that ISO standard and ASME standard fasteners are physically separated, clearly marked and documented separately in the stock system. Stock cards and goods receipt documentation must state standard, strength class and certificate type explicitly. A stockroom without this separation cannot be verified against the requirements of the installation in an audit.
What is the downtime risk?
The downtime risk from mixing standards is not primarily the cost of the wrong fasteners. It is the cost of the delay that arises when nobody at the facility knows which standard applies to the joint in question. During an unplanned shutdown, with the cost of lost production accruing by the hour, the maintenance team starts by finding the answer to what they should order, not by ordering it. On older installations with incomplete documentation, this clarification can take hours.
The second scenario is worse: the wrong fasteners are ordered and installed under time pressure. The installation is not documented as a deviation. The next maintenance interval inherits the problem. In an audit or after a breakdown this is traceable, and it is the responsible fitter's and the maintenance manager's problem to explain. Prevention is simpler: document the standard per joint point, keep ISO standard and ASME standard separated in the stockroom, and have a procedure that requires the standard to be stated on all purchase orders.
Which standards apply on the Norwegian continental shelf?
For pressurised piping systems and flange joints on the Norwegian continental shelf, NORSOK M-630 applies to material requirements for fasteners. Where the original equipment was supplied to the ASME standard, the ASME requirements apply for maintenance and replacement unless a formal engineering change documents a transition to the ISO standard. NORSOK M-650 sets requirements for supplier certification and full traceability for fasteners used in offshore and petroleum installations.
A purchase order that states only the dimension is not valid in a NORSOK or DNV audit context. Standard, strength class, surface treatment and certificate type (minimum 3.1 per EN 10204) must be stated.
Why does standard locking fail under vibration?
Where ISO standard and ASME standard fasteners are mixed in the same system without control, the risk increases that torque values have been applied to the wrong fasteners and that the initial preload is lower than specified. Standard spring washers and nylon lock nuts rely on friction to maintain clamping force, and friction is progressively reduced under dynamic load. When the initial preload is insufficient, this process is accelerated. NORD-LOCK wedge-locking washers maintain the clamping force mechanically through the wedge-locking principle, independently of friction in the thread surfaces, and provide an additional level of safety where mixing of standards occurs. INDUSTRISALG carries a broad range of NORD-LOCK wedge washers in stock.
Conclusion
Mixing the ASME standard and the ISO standard without documented control has direct consequences for maintenance, stock and downtime. For maintenance: wrong torque values and wrong replacement fasteners. For the stockroom: a range that cannot be verified against the requirements of the installation. For downtime: the delay that arises because nobody knows the standard of the joint that has to be fixed. All three consequences are prevented by the same measures: document the standard per joint point, keep the systems separated in stock, and require the standard to be stated on all purchase orders.
Checklist: handling the ASME standard and the ISO standard
- Has the existing standard (ASME or ISO) been identified from the as-built drawing or the equipment data sheet?
- Are ISO standard and ASME standard fasteners physically separated and clearly marked in the stockroom?
- Are the torque values in the maintenance documentation calculated for the correct standard and strength class?
- Does the purchasing procedure require the standard to be stated explicitly on all order lines?
Related reading
- DIN vs ISO in practice: when "the same M10/M12" means different wrench sizes
- Thread types 101: machine threads, thread-forming, thread-cutting and wood screws — complete guide
Get in touch
Send us the specification with standard (ASME or ISO), strength class, dimension and surface treatment, and we will confirm the correct fasteners and delivery time the same day. INDUSTRISALG carries ISO standard fasteners in strength class 8.8 to 12.9 and imperial fasteners in strength class SAE Grade 5, Grade 8 and Grade 10 and ASME-specified fasteners in B7 and L7 in stock, with full traceability documentation as standard. We deliver to offshore, energy and heavy industry, including on urgent orders.