On 23/05/2025 22:17, Roger Mills wrote:

On 23/05/2025 21:52, David Paste wrote:

Which is technically better?

Just wondering!

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.


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On 23/05/2025 22:17, Roger Mills wrote:

<snip>

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

Pretty much all my plumbing fittings use a 55 degree angle courtesy of
the British Standard Pipe thread.

Of course the US NPT thread is 60 degrees, and sometimes a different
pitch to BSP for the same diameter.

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On 23/05/2025 21:52, David Paste wrote:

Which is technically better?

Just wondering!

Most article I have read say the narrower angle is stronger. From the
increased depth of thread.

Counter intuitively fine threads have a higher tensile strength than
coarse but there isn't much in it. Yet coarse thread are better for
brittle materials like cast iron where they are less likely to strip.

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On Fri, 23 May 2025 22:17:03 +0100, Roger Mills
<[email protected]> wrote:

On 23/05/2025 21:52, David Paste wrote:

Which is technically better?

Just wondering!

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.

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.

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