Screw thread identification No.2

Hi Folks,

Back with my K&B Hornet 15 model aero engine. Thanks to JasonB & Nick I've resolved most of the screw thread queries but the last one really has me puzzled.

The Spraybar of the Needle Valve Assembly is a brass tube with an external thread. The major diameter , as measured, is 0.138" or 3.54mm but I've been unable to ascertain the pitch as non of my thread pitch gauges cover it. It appears to be between 36 & 40 tpi or 0.6 - 0.7mm. These measurements seem to indicate 4BA but whilst a 4BA nut is a slack fit on the spraybar, the locknut from the NVA is too small to fit on a 4BA machine screw. Also, what would a BA thread be doing on a 1950s American engine? Unless of course my dad swapped the original NVA for a British one.

Anyway, I hope somebody can cast some light on the dark shadows of my ignorance. I'll be glad when I get onto the British engines, at least I'll recognize the threads there (I hope!)

Cheers

Bev

Oops! Its a K&B Torpedo not a Hornet.

Bev

Sounds like 6 x 40 UNF, I would check the pitch very carefully sometimes the male thread can become "stretched" thus not giving a true pitch indication. Lots and lots of this on old fishing reels, old guns etc. Just try a 6 x 40 tap or screw.

John

BA or Thury would be the only two between 36 & 40tpi around that diameter but as you say unlikely if its the stock needle, I'd go with 6-40 UNF