Ba tap query

I recently bought a couple of 2BA taps as 2BA is used on my lathe in places. When I came to use them I noticed that they were actually marked 2BA x
4BA and in fact are 2BA diameter but 4BA thread. Anyone any idea what they would have been used for? Carburretor jet locations? Is it common?
Whilst on the subject of BA threads I have a large quantity of BA Die Box chasers from 0BA through 6BA. I do not have die boxes so will use them instead of single point threading on the lathe when the time arises (if I ever make anything apart from fixing things and tooling). Does anybody want any? 0.50 per chaser plus postage. If interested the email is valid with the provisos. If you think 0.50 is too high let me know, I do not want to "do" anybody.

Email address is valid but remove burrs before sending!
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.