Before I make a start on the bottom bracket lug for the frame I’m building I was wondering if one of the frame builders out there could clarify the Ø I will need to make the bore pre-threading?

I’ll be using a standard 1.375″ x 24un BB and based on tapping drill sizes for ‘normal’ 24 UN threads it seems to be 1mm below thread Ø so:

1.375″ = Ø34.9mm, pre-threaded bore = Ø33.9mm. Does that sound right?

no

edit ….unless your thread milling

Use the thread root diameter. 1.375″ refers to the thread external diameter, so your bore needs to be 1.375″ – (2x thread height)which I expect is more than 1mm. Alternatively buy a threaded shell from CeeWay if its regular width?

unless your thread milling

That was the plan but don’t have right pitch TM 🙁

Alternatively buy a threaded shell from CeeWay if its regular width?

I’m machining a solid lug that will hold the BB, seat-tube and down-tube so can’t to that.

..of course you could just measure a regular BB shell ID?

Around 33.75mm.

..of course you could just measure a regular BB shell ID?

I was hoping someone might be able to confirm the size so I don’t have to take my bike apart 😉

Around 33.75mm.

Thanks Andy 🙂

You could go down to 33.6mm and have the benefit of 75% thread depth.

ore bore it straight through and glue a/the shell in and pin it

other method if you don’t have the correct threadmill, you can single point it and helix it (you can grind your own cutter)

For a 60deg form, multiply pitch x 0.6134 to get thread height, so 0.65mm.
Take this twice off your nominal 1.375″ dia to get you a minor dia of 33.625.
I’ve successfully used a single-point threading toolholder in a sidelock chuck in the past, starting at the bottom and helical interpolating out into fresh air. Z figure for each pass is your thread pitch. It works best if you can cut a groove at the bottom prior to threading so that the cutter has somewhere to go on initial engagement. Keep cutter overhang short as possible as this tends to be prone to vibration.
You’d also need to grind a bit off the back of the insert to stop it from rubbing and unfortunately a second, opposite-hand toolholder for the other side!
If you don’t have a 24tpi insert, a 1mm metric insert will do ok.

The thread form is an old ‘BSA’ standard (pre war?) anyway BS 6104 part 9 gives some more detailed dimensions for thread forms and diameters.

Basic dims:

Major dia- 34.925
Pitch dia- 34.238
Minor dia- 33.779
24 tpi

There’s some tolerance data but I am not rewriting the whole standard here.

Machinery’s Handbook states for a 1.375-24UNS class 2b internal thread, minor diameter is 1.33″ – 1.34″.

Thanks for all the info.
I’ll get one of our turners to do the threads for me so they can use a full form insert 🙂