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From: Leon Fisk <lfiskgr@gmail.invalid>
Newsgroups: rec.crafts.metalworking
Subject: Re: Plumbing pitch puzzle
Date: Thu, 24 Apr 2025 10:12:13 -0400
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On Wed, 23 Apr 2025 23:57:40 -0000 (UTC)
bp@www.zefox.net wrote:

<snip"
>Perhaps the oddest thing is the rounded thread profile, almost
>like a Whitworth pattern. 

Wiki has a page for:

"Knuckle threads or round threads are an unusual highly rounded
thread form. The large space between the rounded crests and roots
provides space for debris to be shifted to not interfere with the
thread, making this form resistant to debris and thread damage."

https://en.wikipedia.org/wiki/Knuckle_thread

With a high tolerance for debris in the threads their use in this paint
application would make a lot of sense...

-- 
Leon Fisk
Grand Rapids  MI