Mis-threaded

[oldbilbo]

My installation of some new through-hulls and seacocks recently - each manufactured to British Standard Pipe ( BSP ) thread standards - was frustrated by the sale of tapered-thread components. Exploration uncovered that fact that there are TWO BSP thread standards - Parallel and Tapered.

This won't come as any surprise to the injineers and shipwrights among us, but it did to me - and to several of the chandlery staff I queried. It emerges that certain trade wholesalers make the differences explicit in their online catalogues by use of BSPP and BSPT labels, together with some explanation. Others - which STILL do not seem to differentiate between DZR Brass and the likes of Tonval - are rather less open.

Of course, there are those who maintain that it doesn't much matter. I take issue with that. This, from the web....

TAPERED THREAD VS PARALLEL BSP THREAD

Note:
Do not use with tapered thread valves or fittings. The thread type on all Tru-Design Skin Fittings is a British Standard Pipe Parallel thread (BSPP).

The thread is a mechanical fastening with sealing provided by tape or one of the above sealant methods. These methods give a secure mechanical joint between the Skin Fitting and connected components such as Ball Valves.

A tapered thread cannot provide this strong connection. Mixing tapered and parallel threads can result in damage to either of the components.

 

[vyv_cox]

Tapers need very careful treatment. Applying the same torque that would be used for parallel threads can result in massive force applied to the joint. This is probably the reason that so many people struggle to get propellers off - it was tightened as if there was a shoulder to butt against.

 

[rob2]

Many fittings have parallel threads screwed down till they bottom out, so the actual skin fitting has to be the last to be positioned to get it correctly orientated. The great advantage of parallel threads is that a backnut should be used to lock it in the chosen orientation, so a perfect fit can be obtained. Unfortunately the caking nut is generally the hardest part to find. I admit to having used a mix of parallel and tapered thread fittings at times, when I couldn't source the correct ones, and only above the seacock! Often a case of a "temporary" solution that usually doesn't get put right.

Rob.