Could Gordon at G-Mac help with this?
Just looked at some charts.
BA4 has od 0.142" approx, 3.6mm. So going less than this we come to UNF No 6 at 0.138" (40tpi) or No5 at 0.125" (44 tpi)
I think these are correct.
Cheers, Phil
Could Gordon at G-Mac help with this?
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All of this measuring seems a bit much why not just email the American stores for a quote using regular US mail.I recently ordered a co2 cover for a Crosman sa6 off an American guy a similar weight to a spring & the postage was under £3. There is a huge difference between UPS & regular mail.