UNF vs BSCy threads

[Greenfield]

I've a 1/2" x 20tpi BSCy thread that needs cleaning up. I've a 1/2" x 20tpi UNF tap but somewhere at the back of my mind I think there's a small difference in the thread which affects the strength if the two are mixed. Does anyone have an opinion on this?

[iansoady]

They're both 60 degree thread angle but one (can't remember which) male thread has a rounded profile while the other is truncated. In the real world I'd probably use the tap you have.

[DAVE BRADY]

Hi,

As it is a threaded hole rather than a bolt/stud I would get the proper tap.  The tap you need is readily available. Try Tracey Tools.  There are several others offering them just do a google search.

Dave.

[V500]

Agreed. Get the correct tap for the thread concerned.

[neil1964]

I have a decent range of UNF and BSC taps and dies.
I wanted a die to clean up the 5/8 20tpi BSC on of my BSA gearbox main shafts.
I ordered a BSC one from Tracey tools and received a 20 tpi UNS (a spec new to me).
I tried to run it down a known good shaft and it bound up. So I contacted the supplier and the advised that the did not have the correct die and that 9/10 it will work.
I'm guessing that it is the peak on the threads that makes the difference?

[Greenfield]

After a bit of research I think that the UNF is a squared off thread and the BSCy is rounded. If that's the case with everything else being equal, a 1/2"x20 BSCy bolt would be tight going into a UNF nut and if it's the other way around, it would probably be an easy fit and possibly weaker. Some of the taps advertised are simply marked 1/2"x20. I phoned one supplier to ask about this and they told me that they supply the UNF tap for both threads.
So, I agree with the above comments, always use the correct tools, and make sure you insist on it being correct.

[scifi]

I think it is the same for UNC and BSW.   I wanted to clean some 3/4 BSW threads, and our bolt supplier said they had only UNC.   The tap worked well, but the Die was a tight fit.

[V500]

Some of the larger BSCy threads confusingly can be 20 or 26 TPI. Unless someone lives 3000 miles from civilisation and they're desperate I'd buy and wait for the correct tap.

[cornishbob]

the difference between bsw and unc is the pitch one is. 55 degrees and the other 50 ....i think