BSPP and BSPT threads

[David Hill 5Participant @davidhill5]

Hello Guys, Is it possible to open out a female BSPTaper thread to accept a male BSPParallel thread using a male parallel tap? Is it acceptable workshop practice or a bodge?

Thanks, David

[JasonBModerator @jasonb]

I’ve certainly done it, will leave others to say if it’s a bodge or not.

[bernard towersParticipant @bernardtowers37738]

Can’t really see a problem with that.

[David Hill 5Participant @davidhill5]

Thanks JasonB and Bernard, I’ll give it a go.

[Martin ConnellyParticipant @martinconnelly55370]

The only issue may be that the widest part of the taper may be too wide to engage with the thread of the part screwing into it. Without a step gauge for the taper thread it can be hard to know if it has been over cut.

We used to buy in taper thread bosses to weld onto manifolds and ended up having to buy a a gauge to check them before putting them into the stores. The problem with the overcut parts was that the mating part could bottom out before the thread was fully engaged when the boss was positioned over the pre-drilled manifold. We had to make the hole larger than the matching pipe bore but smaller than the thread OD to allow the inspectors to be sure there was material under the boss for welding and the weld was therefore not dodgy. This visible edge of the hole was what the part being screwed in could come up against. Eventually we changed the boss design to have a counterbore where it was in contact with the manifold as this allowed the use of standard size drills for the holes so it was not such a problem anymore.

Martin C

[noel shelleyParticipant @noelshelley55608]

Like Jason and Bernard I have also opened them out where need be ! They have different purposes mainly how they will seal, on the thread or on the face. Noel.

[Bo’sunParticipant @bosun58570]

Hello David, as Noel has eluded to, and not wishing to teach you to suck eggs, a parallel thread/shoulder will need to seal on the face of the housing, or whatever it’s screwed into.  In which case, the sealing surface needs to be smooth and perpendicular to the thread axis, especially if a bonded (Dowty type) or copper sealing washer is to be used.

[JasonBModerator @jasonb]

Sealing will really depend on your specific use. I use BSP elbows and fittings quite a bit on the hit and miss engines particularly the exhausts and may just use a bit of liquid sealant Nore to stop things unscrewing than anything as the joint is not under pressure.

Also done plenty of plumbing with parallel/parallel without shoulders and they are under lowish pressure. Valves into radiators spring to mind, just PTFE to seal.

[Anonymous]

It’s not a bodge, but why would you want to do it?

There are four possibilities:

External parallel to internal parallel: Common; not self sealing, will need tape or paste to get a seal

External parallel to internal taper: Not common; may be self sealing or may not

External taper to internal parallel: Fairly common; should be self sealing

External taper to internal taper: Common; is self sealing

Andrew

[Author]

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