Threading for Gold Jewelry: 
External threading for Gold is a better choice
Customers don't want to lose those gold balls and will blame defective manufacturing.  

Keep in mind when it comes to allergic reaction to nickel, 14Kt contains nickel. The skin is not protected by a chromium oxide layer as Stainless Steel is so it is a wonderment when a customer reports that they experience an allergic reaction to nickel in 316L and therefore only use 14Kt.

History from Unimax.
Wes Wood Gauntlet Body Piercing Trained in 1995
In the run-up to modern USA piercing (1990-5) internal threading was standard because that's what the few piercing  jewelry manufacturers were making. There was no external threading to speak of.

External T3 threading was developed to be smoother and more refined and was a much less costly alternative, cheaper than internal.
By 1995  "Internal-only" enthusiasts looking for market share (encouraged by the internal-only manufacturers) began claiming alleged superiority of internal because it was "internal."  Internal is not superior. 
Before 1994 it was true, but by 1996 it was a fabrication  to promote sales.

We became a surce gold body jewelry from new manufactyrers.  Internal was the first kind sold sourced from Gauntlet, a pioneer. The problem of stripping appeared right away and a steel threaded male post was pressed into the ball with steel sleeve in the bar but slight over-tightening pulled the sleeve out and also produced discoloration. The steel post and sleeve caused discolorations in the gold so this was abandoned and external replaced internal.

Eventually a good externally threaded bar could be made that held pretty well with only a rare stripping problem.
If balls easily become loose that was a different problem: a length threading problem  that has nothing to do with internal vs. external. There is no rational debate and no one can rationally defend internal as superior.

Today a demand has been created for internal gold threaded jewelry which is now being produced with 10 Kt post and thread to help prevent the stripping.  A confusion obscures this issue because traditionally-produced internally threaded jewelry made in the USA has a higher finish level than most imported goods.  

We recognize and appreciate the high lustrous finish produced by USA manufacturers but not because it is internal, but because it is polished to a wonderful degree of perfection.  USA producers can make the finest body jewelry in the world.

Gold body jewelry is now being made by regular jewelry trade manufacturers who see a market for themselves to expand.

As a final word: -- and a concern expressed by some:

Internally threaded jewelry accumulates dirt and bacteria in that tiny hole in the internally threaded bar and cannot be cleaned adequately, even with an ultrasonic (except for larger gauges).  Externally threaded bars avoid this problem.  The hole in the threaded ball is larger and can be cleaned.

Internal is standard for large gauges (because the threaded diameter would be too large for external) and chosen specialty items such as prong settings or designs that have no ball.
Tongue and Body Gold Cleaning: 
Special considerations in the mouth.
                Foods containing salt, and smoking, will likely discolor the bar.
                There is nothing wrong with the gold.
                To remedy take it to a jeweler to have it buffed back to original finish.
                "420" smokers will leave a black tar on the bar.
                 Body fluids can discolor gold but they too can be buffed back.
 
Buyer Beware: Gold content can be compromised.
Anyone may make a business card claiming factories in other countries,
rent mailboxes, shared office space and phone numbers.

Some examples of scams:

The Scam:  9Kt 10Kt and 11 Kt substituted for 14Kt Gold

The Scam:  Hollow Balls and Bars. 

The Scam:  Gold Plating over brass or other metals.
---We actually caught an importer doing this for navels.
Test: (Hard to scrape or cut even with a razor.)

The Scam: Curves represented as a certain gauge measure less.
Test: (Check with a Micrometer, not your gauge wheel.)

The Scam: The balls are hollow.
Test: (Look inside the ball.)

The Scam: The ball is "loaded"
Test: (Spin the ball on a table and see if it wobbles like loaded dice.  Look at the threading inside the ball for a different color.)

The Scam: Bars are shaved, not round.
Test: (Roll a BB on a flat surface.) Look to see they are smooth.

Only buy your Gold from someone who has a reputation in the industry.  

Note from Wes Wood.
There is no tearing or micro-tearing during insertion
caused by external threading because
External Threading is smooth T3.
.
This argument was made-up to promote the sale
of Internally Threaded jewelry. Anyone caught teaching this
has not thought about "tearing" or has motivation to say such things.