Overview
ISO 4755:1983 - Fasteners: Thread undercuts for external metric ISO threads specifies the required form and dimensions of undercuts on externally threaded fasteners (bolts, screws and similar products). First published in February 1983 and developed by ISO/TC 2 (Fasteners), the standard defines when and how an undercut should be applied - typically where an undercut is needed to accommodate a mating component such as a plug - unless an applicable product standard specifies otherwise.
Key topics and requirements
- Scope and purpose: Defines the geometry of thread undercuts for external metric ISO threads to ensure consistent fit and function with mating parts.
- Dimensional data: Provides a tabulated set of dimensions in millimetres tied to thread pitch and nominal diameter. Designers and manufacturers use these values to size the undercut correctly.
- Form features: Specifies the shape and transition between the threaded portion and the undercut, including rules for transition angles (the document notes a minimum transition angle of 30° where relevant).
- Applicability: Recommended for bolts, screws and similar externally threaded products when undercuts are required (for example, to clear plugs or to provide clearance for sealing elements).
- Normative notes: Includes limits and special notes (for instance, provisions covering small thread diameters and related dimensional exceptions).
Practical applications
- Ensuring proper clearance for plugs, inserts, sealing parts or inspection gauges that interface with the end of an externally threaded fastener.
- Avoiding thread interference during assembly by providing a controlled run-out region at the end of the thread.
- Guiding machining and thread-rolling operations to produce correct undercut geometry and minimize rework.
- Specifying inspection criteria and acceptance limits in quality control and manufacturing documentation.
Who should use this standard
- Mechanical and product designers specifying fasteners for assemblies where end-fit or plug engagement is critical.
- Fastener manufacturers, thread-rolling and CNC shops producing metric ISO threaded parts.
- Quality engineers and inspection personnel verifying undercut dimensions and transition geometry.
- Procurement and standards specialists ensuring purchased fasteners meet established ISO requirements.
Related standards
ISO 4755 complements general ISO metric thread standards and product-specific fastener standards developed by ISO/TC 2. Refer to applicable ISO thread and fastener product standards for full mating-thread and product requirements.
Keywords: ISO 4755:1983, thread undercuts, external metric ISO threads, fasteners, bolts, screws, undercut dimensions, plug clearance, fastener standards.
