Hitchcocks Forum

Can anyone confirm if this is correct?
When tapping a hole for 9/16 cycle thread the tapping drill is 13.1mm (33/64) for 20 TPI and 13.5mm (17/32) for 26 TPI. Thanks, Zippy

You are correct with the 33/64" hole for the 9/16" 20tpi thread but not sure about the 26tpi.is this A/F American Fine?

26 tpi is British Cycle Institute thread, also known as CEI or usually just Cycle thread. AF stands for Across Flats and just measures the head of the bolt, it has no relevance to thread. "American Fine" does not exist as far as I know!
REgards, Mark

26tpi is the usual, I doubt you'll find 20tpi much. You find various tapping drill sizes quoted but 13.5 seems a tad big. 13.3mm would perhaps be better. It rather depends what metal you're tapping.

It is to go through about 10mm thickness of stainless steel. Do different materials have different hole spec's?

I thought zippy meant motor/cycle threads and replied for a medium thread per inch.I`ve never worked on a so finer thread as Mark says but i`m going back 40years.Just remembered U.N.F U.N.C. B.S.F? Too many for me.

Keep in mind: if a drill is badly sharpened you can easily get say, a 13.0 mm hole with a 12.7 drill...
What comes to stainless steel drilled holes tend to be slightly tighter (smaller) than in mild steel. If tapping by hand you will feel if the hole is too tight. If it first goes well and then tightens you are not tapping straight.
(Just some observations acquired along my career in a machine shop...)

Hi ZIPPY----------- British Standard Cycle Thread (included angle = 60 degrees)}----------------- 9/16" at 26tpi is}----- Tapping drill is}0.5313 = 13.3mm = 17/32"........................At 20tpi Tapping drill is}0.5118" = 13mm = 33/64" Don't be tempted to use a BSF thread as this is has an included angle of 55 degrees. Stainless steel can be "Sticky" when machining/drilling ---- have a look at this }---- http://www.bssa.org.uk/topics.php?article=192 ----- I use Parrafin or candle wax (Similarly with Aluminium) and a High Speed Steel (HSS) tap for toughness..... No more than one to two turns before you reverse the tap each time...... blow-clear the hole ---- this stuff can be a bitch depending on which particular grade of S/S you have!!!!! Be lucky !!!

I think that 9/16" cycle thread is traditionally 20tpi but RE stuck with 26 with everything other than the timing cover screws which are whitworth.

Mr Villiers is to blame for a lot of these issues. They decided regardless of screw diameter they threaded 20 TPI. This is the same idea as 26 TPI as used in British Standard Cycle thread except Villiers decided on 20 TPI not the 26 as BSC. It was acceptable engineering practice of the day. Either BSW or UNC covers 1/4x20 and BSF covers 3/8x20 and both 7/16 and 1/2x20 are covered by UNF sizes The one thread that causes problems is the 5/16x20 and luckilly Villiers didn't use it a lot. Rotor nut on a Bullet it a Villiers thread