Thread angle, 55 degrees v 60 degrees

I didn't know, so I asked ChatGPT "What are the pros and cons of a 55 degree thread angle versus 60 degrees?"

This what it said . . .

The 55° and 60° thread angles are primarily used in machine screw threads and pipe fittings, and the difference between them impacts strength, ease of manufacturing, sealing performance, and compatibility. Here's a breakdown of their pros and cons:

🔩 55° Thread Angle (e.g., British Standard Whitworth) ✅ Pros: Better fatigue resistance: The rounded roots and crests of Whitworth threads reduce stress concentrations, improving resistance to fatigue and vibration — ideal in older or dynamic machinery.

Sealing capability: Often better for sealing (especially with tapered forms like BSPT), as the thread profile allows for tighter engagement.

Historical compatibility: Common in British and Commonwealth legacy systems and older equipment.

❌ Cons: Less efficient load distribution: The shallower thread angle reduces axial load-handling efficiency compared to 60°.

Harder to machine with modern tools: Modern tooling is typically optimized for 60° threads, so manufacturing 55° threads can be slower or require custom tooling.

Obsolete in many industries: Not typically used in new designs; mainly for legacy compatibility.

What that summary doesn't say is by how much difference in fatigue resistance or sealing capacity, and with the latter how much when used in conjunction with a jointing compound or tape.

The machining is somewhat of a red herring as if 55 degrees were the more common thread the tooling would be optimised for that.

The WW thread form is a miniature work of art with the precise radiusing. Knocks V forms into a cocked hat on that score alone.