Here is what I do ...There is more to it than 3/8 . Its the number of threads per inch if you need to fit a replacement drain tube to a 3/8 hole . I use a 3/8 by 32 threads per inch tap . This works with the drain tube threads that most model A parts venders handle. This is probably an uncommon tap . I got mine from Kodiak cutting tools
www.amazon.com or
www.kodiakcuttingtools.com . Brattons lists the drain tubes on page 44 and yes I have performed this proceduer many times .