I'm in trying to replace a bolt for an old singer sewing machine, it has an outside diameter of 6.9 mm, and 28 threads per inch. I initially thought it was a slightly oversized quarter inch UNF, but these are far too small. It doesn't match BSF, or any fine thread pitch metric threads. Any ideas?
Practical machinist has a thread on this.
www.singersewinginfo.co.uk has some very useful thread tables, apparently Singer created their own standards.. sound familiar?
Too late!
Probably a proprietary part - threads vary by diameter, pitch and angle - you need a thread gauge to help identify which one.
Whitworth comes in fine and course threads ( IIRC the fine one is called BSF?) You also get cycle threads which look very similar to BSF. There is also BA.
I think it unlikely its a propriety thread form but more likely one of the obscure british ones
Practical machinist has a thread on this.
Please tell me that was an intentional gag.