It's not uncommon to have different thread sizes on inverted flare tube nuts for brake systems.
Are you sure the tube nuts are the same diameter with different pitches? That sounds odd to me.

These are the only standard ones I know of:

1/4" line,
tube nut with 9/16-18, 1/2-20, or 7/16-24 thread

3/16" line,
tube nut with 9/16-18, 1/2-20, 7/16-24, or 3/8-24 thread

Grigg

Last edited by Grigg; 05/09/2010 4:55 AM.
•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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