I need to fabricate a part for my bike and the original part had a hole that took a 1/2" BSC bolt.

I therefore need to drill a hole and run a 1/2" BSC tap through it it.

Can someone tell me what size hole I need to drill.

I suspect it is a 11.5mm drill based on this page https://www.tap-die.com/contents/en-uk/d297_Tap_Die_Company_Tapping_Drill_Sizes.html

But that page refers to a 1/2" x 26 as a BSB Whitworth thread. But I assume it is close enough.

TBH I have never seen an 11.5mm drill bit, but apparently you can buy them.

Can anyone advise please?

Cheers

Al

Alun, is it 26tpi?
Here is the chart for 26tpi = 11.9mm drill.
https://www.tap-die.com/contents/en-uk/d198_Tapping_Drills_HSS_for_Cycle_26tpi___20tpi_taps.html

Ron

email (option): ronpier@talk21.com

I'd be interested to know what BSB Whitworth is...Can't say I recall that one...
As Ron says, the thing to do is check the tpi of the part that screws into the hole...It could be 26tpi or 20 tpi...Both are common on these bikes at the 1/2" size...From there you can look up the tapping drill you need...Ian

email (option): ian@wright52.plus.com

Thanks for that guys.

So, a 12mm drill it looks like, because I can't justify paying 30 quid, plus shipping to Australia, for a drill bit I will use twice.

Yes it is BSC 26TPI.

Bolt, fixing stand to fork end, 1 13/64" by 1/2" by 26
Old part number CF-38-12-F71
New Part number 010788

I'll do a test run on a scrap bit of metal to see how it goes first before committing to a hole in my soon to be fabricated part.

Cheers

Al

Alun 15/32" is near spot on to 11.9mm. Look for a drill bit from a local machine shop supplier or Ozzy ebay. Ron

email (option): ronpier@talk21.com

11.9mm isn't an unusual drill size...Metric drills are garaduated in 0.1mm increments...I found one in seconds on UK ebay for £5....You should be able to find one on ebay at your end I'd have thought....Ian

email (option): ian@wright52.plus.com

Again, thanks.

I'll be a monkey's uncle. Never knew. Just did an eBay search and found one.

All sorted.

Cheers again

Al

You won't find one at Bunnings "where lowest pries are just the beginning ,,,,, of your problems " but you you will find them at most good machine supply shops like Hare & Forbes for instance .
I get most of my odd size stuff from L & P Toolmakers in Victoria
They also sell on Evilpay
Virtually no tapping drill will be found in a set of Jobbing Drills that you buy from a hardwear store or auto store .
However 11.5 mm is a standard size in a metric drill set and unless you are using very expensive drills very few will drill a hole accurate to 0.4 mm ≠ 0.015" at that size and a good High speed steel taper tap will sort that 0.015" out ( first cut to USA readers )
Under sized holes are not a problem if you cut the thread slowly and use good thread cutting lube
Over sized holes are
FWIW
29/64" is 11.51mm ( so a pooftenth bigger than 11.5 mm drill
15/32" is 11.91mm so a poofteenth bigger than the nominal correct size of 11.9 mm
Both of the above are std inclusions in a 1/16" to 1/2" set of jobber drills.

Tapping drills are also offered in sets of exactly the right size and are made to a much higher tollerance ( thus expense ) and in particular sharpened much more precisely
Accurate holes are drilled then honed or bored Or drilled, bored then honed if you want them really accurate and of course have to be done with a lathe or mill