The table defines three main types of thread run-outs/undercuts, labeled Form A, Form B, Form C (or sometimes short, normal, long). However, the most common gola reference for general mechanical design is the undercut for external threads (e.g., for bolts screwed up to a shoulder).
It is vital to distinguish UNI ISO 4755 from similar standards:
| Standard | Name | Key Difference | | :--- | :--- | :--- | | UNI ISO 4755 | Undercuts for metric threads (long version) | Standard undercut for general engineering. | | DIN 76-1 | Undercuts for metric threads | Almost identical to ISO 4755 (Germany uses DIN 76-1). In practice, they are interchangeable for most sizes. | | DIN 78 | Undercuts for bolts with reduced shank | Different geometry; used specifically for shoulder bolts. | | ISO 3506 | Mechanical properties for stainless steel | Refers back to ISO 4755 for undercut dimensions, but property classes (A2-70, A4-80) have different fatigue limits. |
Tip for buyers: If a supplier says "conforms to DIN 76-1," they also conform to UNI ISO 4755 for 95% of metric sizes. However, always verify the specific a dimension if you have a blind hole with a specific depth. tabella gola uni iso 4755
UNI ISO 4755 is the Italian adoption of the international standard ISO 4755:1983 – Fasteners – Bolts, screws and studs – Undercuts for metric threads. It replaces older national standards (e.g., UNI 4749) and aligns with modern ISO thread systems (e.g., ISO 68-1, ISO 261, ISO 965).
The standard specifies:
Connecting rod bolts and cylinder head bolts almost always conform to an undercut standard (often derived from ISO 4755). When rebuilding an engine, mechanics look for this groove to confirm they are using OEM-grade fasteners, not cheap hardware store substitutes. The table defines three main types of thread
The following table extracts the mandatory dimensions for metric thread undercuts as defined by UNI ISO 4755. The table is organized by the nominal thread diameter (d) – specifically for coarse pitch threads (the most common standard).
Note: All values are in millimeters (mm).
| Thread Size (d) | Pitch (P) | Groove Diameter (d₁) max | Groove Width (a) nominal | Radius (r) max | | :--- | :--- | :--- | :--- | :--- | | M1.6 | 0.35 | 1.25 | 0.5 | 0.2 | | M2 | 0.40 | 1.60 | 0.5 | 0.2 | | M2.5 | 0.45 | 2.05 | 0.6 | 0.2 | | M3 | 0.50 | 2.50 | 0.8 | 0.2 | | M4 | 0.70 | 3.30 | 1.0 | 0.2 | | M5 | 0.80 | 4.10 | 1.2 | 0.3 | | M6 | 1.00 | 5.00 | 1.5 | 0.3 | | M8 | 1.25 | 6.80 | 2.0 | 0.4 | | M10 | 1.50 | 8.50 | 2.5 | 0.4 | | M12 | 1.75 | 10.20 | 3.0 | 0.6 | | M14 | 2.00 | 12.00 | 3.5 | 0.6 | | M16 | 2.00 | 14.00 | 4.0 | 0.6 | | M20 | 2.50 | 17.50 | 5.0 | 0.8 | | M24 | 3.00 | 21.00 | 6.0 | 1.0 | | M30 | 3.50 | 27.00 | 7.5 | 1.0 | | M36 | 4.00 | 33.00 | 9.0 | 1.2 | | M42 | 4.50 | 38.50 | 10.5 | 1.2 | | M48 | 5.00 | 44.00 | 12.0 | 1.6 | | M56 | 5.50 | 52.00 | 14.0 | 1.6 | | M64 | 6.00 | 59.00 | 16.0 | 2.0 | Note: Exact diameters depend on whether the undercut
| Standard | Content | |----------|---------| | ISO 4755 | Same as UNI ISO 4755 (international version) | | DIN 76-1 | Very similar – thread run-outs and undercuts for metric threads | | UNI 9460 (obsolete) | Older Italian standard, now replaced by UNI ISO 4755 | | ISO 3506 | Mechanical properties – references undercuts for fatigue life |
| Pitch P (mm) | Undercut width g (mm) | Undercut diameter (approx.) | |--------------|------------------------|-------------------------------| | 0.5 | 0.6 | minor dia - 0.2 mm approx. | | 0.7 | 0.8 | ... | | 1.0 | 1.1 | ... | | 1.25 | 1.4 | ... | | 1.5 | 1.6 | ... | | 1.75 | 1.9 | ... | | 2.0 | 2.2 | ... |
Note: Exact diameters depend on whether the undercut is for external or internal thread. The standard provides distinct values.