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Brass or cycle thread ?

What do boiler makers use: BSB or BSC ?

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michael rietveld18/11/2018 20:38:43
2 forum posts

I wasn´t aware that 1/2" 26tpi thread could be BSB (brass) or BSC (cycle) thread when I bought the taps and dies. I see that what I have has BSC or BSCY stamped on them. I want to make fittings for a commercially made Simplex boiler, so I want to know what is normally used for boiler fitting threads. The difference is in the angle, 55 or 60 degree of the thread. I will try finding out from the boiler builder. The drawing does not specify which should be used.

Mike R.

Paul Kemp19/11/2018 13:22:38
383 forum posts
18 photos

Well if you make your male fittings (assuming all threads on the boiler are female) BSB (55 degree) they will fit either. They are not going to fall out or be significantly weaker for the sake of 2.5 deg on the flank angle!

The only way to be sure what is in the boiler is as you suggest, ask who made it.


Bazyle19/11/2018 13:38:39
4905 forum posts
195 photos

If you can find or make known threads you can test with engineer's blue which is the best fit.

Clive Brown 119/11/2018 13:49:34
320 forum posts
7 photos

I would be extremely surprised if Martin Evans had cycle thread in mind when he specified 26tpi threads.

Brass thread was widely specified in ME in the previous century.

Bazyle19/11/2018 13:50:06
4905 forum posts
195 photos


Edited By Bazyle on 19/11/2018 13:50:24

JohnF19/11/2018 13:57:35
919 forum posts
114 photos

Gentlemen, please do not mix incompatible threads and in particular where pressure/stress is involved. Yes they will fit together but they WILL be much weaker.

Threads with different angles will screw together providing they have the same pitch but they will bear on the crest and root and not on the flank thus they will not perform as they should. You are compromising safety.


michael rietveld26/11/2018 20:31:36
2 forum posts

Thanks for your replies. The boiler maker confirms it was BSB as expected.

It just shows you have to be careful when buying taps & dies at shows, to look not only at the size and pitch.

I see that this was pointed out by houstonceng in another posting on 10/02/2018.

So I will buy some BSB taps/dies. It is amazing how prices vary between suppliers!enlightened

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