Breechplug taps, 70s and now

[mountainman70]

Here is a question for y'all. I was working on some barrel pieces ,practicing drilling and tapping for 5/8 18 plug threads.Dang new bit is expensive but sure does cut well.
In digging thru my taps,I found one from my first days at this back in 1979-80s,and it is marked 5/8 20.It has been ground on the end to do bottoming work.
In my tang and plug collection, one I have I THOUGHT was 5/8 18, wouldn't thread into my newly threaded hole.
I laid it against the 5/8 20,and it seemed the same. my question is, has the industry changed what was natnl fine or is this a fluke?
Inquiring minds want to know.Thanks a heap. Dave 

[Clark Badgett]

A 5/8-20 (?) will not go in a 5/8-18 tapped hole

[Flint62Smoothie]

18 is the national fine thread for a 5/16" thread ,whereas 11 is the coarse thread. 5/16x20 is an oddball size that is close to the metric size of M8x1.25pitch. Note the other extra fine US thread in 5/16" size is 24tpi.

[Clark Badgett]

11 & 18 TPI is the standard for 5/8" too, but what is 5/8-20?

[RichG]

thompson center used/uses 5/8-20 threads.

[mountainman70]

Thanks Rich G,I thought about that last night. Now for the 69.00 question, wonder why I had it in the first place. Don't remember any T/C projects back then,but musta been one in the woodpile.hHAHAHAHA,.nUTTIN LIKE gEEZIN !!!tHANKS GUYS. I IMAGINE IF THE PATTENT PLUG AND TANG I HAVE SURPLUS IS indeed 5/8 20, now I can possibly use it. Best regards, Dave F

[Bob Roller]

It's a SPECIAL thread like my 5/8x32 or the 1/4x80 I sent to Taylor for
the tang sight adjustment on the pistol grip Hawken.I have a 7/8x16
also but 7/8x14 is the U.S.National standard fine thread.Industrial supply
catalogs are loaded with these and the dies to match.They are not too
costly until you get into larger diameters like 1 and 1/2x8.
When the lead screws on my Atlas 618 lathes wear out I make new ones
from 1/2 inch 12L14 and use another special thread tap,1/2x16 for the
job and it works fine.They are supposedly Acme threads but I am too lazy
to make the tools for that thread.

Bob Roller

[Goo]

Off topic but Indian Motorcycle company (1953 and earlier) used 5/8-20 for certain fasteners and some of those were left hand threads

[mountainman70]

Goo, did Harley maybe use this size on 56 panheads? I had one before I got into m/l hobby. Maybe a biker bro gave it to me. Thanks, Dave

[Goo]

Goo, did Harley maybe use this size on 56 panheads? I had one before I got into m/l hobby. Maybe a biker bro gave it to me. Thanks, Dave

Hi Dave -.  Sounds plausible I don't know much about HD . Indians are much simpler to work on HD are more sophisticated and beyond my skill set.