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I need the drill size for a British 2BA tap.

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Harvey

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#1
I need to drill out a passageway plug and tap the hole in each of my two 1970 BSA 650 "Lightning's" Amal carbs. The opened-up passageways will then allow the carbs to be thoroughly flushed out for the carb's upcoming overhaul. The hole will then be tapped for a 2BA screw and you britbike guys should recognize this as being one of those odd* English thread sizes. I already have the tap, now I need to know what size drill to use.

Does anybody know the correct drill size for a 2BA plug (tapered) tap?

Many thanks,

Harvey

*My first car was a 1965 MG Midget and my first motorcycle was a 1971 Triumph Bonneville so I'm allowed to poke a little fun at the Brits. It also allows me to drink heavily! :wink:
 
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British Steel

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#2
http://www.engineeringsupplies.co.uk/threads-tapping-drill-sizes-i-22.html says 4mm...

BA, although British, are based on metric dimensions but have a strange algorithm for calculation, the pitch starts at 1mm for 0BA, then reduces by a factor of 0.9 for each increase in BA size, the major diameter starts at 6mm and follows the same pattern BUT is then rounded to two significant figures, then converted to inches, then rounded to the nearest thou"... A 2BA is almost the same a M5x0.8 at 4.8 mm Major, 0.81mm pitch, but the BA also has a thread angle of 47.5 degrees... Very scientific...

Dave H. (the other one)
 
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British Steel

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#3
PS - nice bike, I had a '67 Spitfire 650 for a while in my youth... I learned what BSA stands for though, Bastard Stopped Again!

There are some good mods to do on 'em, the reliability *can* be improved, just they were thought of long after I parted with mine - I was getting thin and worn out from all the pushing it home :D It was great when/if it ran though!

Dave H. (the other one)
 

Dinosaur Engineer

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#4
My UK "Zeus" book quotes 4.0mm ( or 5/32 s) tap drill for 2BA.
 

markba633csi

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#5
Tap a test hole first, if the carbs are alloy you might want to go a wee bit smaller on the hole. (maybe one number size)
Mark
 

Harvey

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#6
Thanks Guys!

Harvey
 
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