Difference Between UNC and UNRC Threads

Both UNC and UNRC are coarse-thread types used in the Unified thread system.

The main difference lies in the shape of the thread root — the small groove between the thread peaks.

1. UNC (Unified National Coarse) threads can have either a flat or slightly rounded root.

2. UNRC (Unified National Rounded Coarse) threads are made with a smooth, rounded root by design.

This rounded shape helps reduce stress in the threads, giving better fatigue resistance — especially useful in parts that face vibration or repeated load cycles.

Even though the shape is different, UNC and UNRC threads can be used together, since their dimensions and pitches are identical.

Feature	UNC (Unified National Coarse)	UNRC (Unified National Rounded Coarse)
Thread Root Shape	Flat or optionally rounded	Must be rounded
Root Radius	Optional / not controlled	Controlled radius (˜ 0.108–0.144 × pitch)
Fatigue Strength	Standard	Improved due to rounded root
Interchangeability	Compatible with UNRC	Compatible with UNC
Use Case	General-purpose applications	Applications with vibration or cyclic loading
Designation Meaning	Unified National Coarse	Unified National Rounded Coarse

In short: UNRC threads are basically stronger versions of UNC threads because of their rounded roots. Both types fit together — no compatibility issues.