Unacceptable bodge or not

I have an air line fitting with a 1/4" BSPT female thread and I want to make a mating part. BSPT dies are for a one off job are quite expensive and I do have a BSPP die already. It obviously wedges into the female thread. Since I don't need to take it apart will a drop of loctite be adequate or alternatively can I tap out the taper thread with a parallel plug tap for a better fit, overall size and TPI are the same so it doesn't look as though too much material needs to be removed?

Regards

Martin

Martin .Tap parallel and use loctite or If the job will allow it a bonded seal or Dowty washer will make an air tight seal. Noel

You could use PTFE tape or plumbers mate.

I have a commercial set of airline accessories that includes various 1/4" BSP adapters and couplers. Taper threads on the males, parallel on the females.

So I guess the mix is acceptable. Not had leaks yet using loctite, plumbers mate or old fashioned heave it up good'n tight methods.

OK it's a cheap set from LiDL but I've found LiDL airline components to be of more than decent quality and excellent value. Not terribly impressed by the airline supplied but at the kit prices I'm still saving serious money over the usual affordable suspects even if I never use the airline.

Clive

The standard rule is you can fit a straight or tapered male into a tapered female. So your arrangement is not a bodge. I would use a suitable sealant.

Robert.

If it will do the job, safely and reliably, and is not subject to frequent disassembly, it should be an acceptable solution.

Taper into parallel is often used, so there is a seal, where the two threads intersect.

Taper into taper becomes, effectively, a parallel thread arrangement, since there is then no obvious interference to make a seal.

Howard

Bodge jobs for air pressure lines can work for a while and then fail quite spectacularly

At sea I learned never to cut corners with high pressure air, the fittings they made were very precisely designed not cheap and did a perfect job

Depends upon the air pressures you are dealing with IMO

The arrangement is not a bodge. Any of the four combinations of BSP parallel and taper is acceptable, provided one understands the limitations.

BSPP = parallel thread

BSPT = tapered thread

BSPP external to BSPP internal - not regarded as pressure tight, ie, the seal is made by the threads themselves. Mainly used for non-pressure glands and fittings, or where pressure tightness is achieved by other means such as tapered cones on the pipe and recesses

BSPP external to BSPT internal - not regarded as creating a pressure tight joint using the thread alone, not much used

BSPT external to BSPP internal - can provide a pressure tight joint using the thread alone. It is relatively easy to create a tapered external thead compared to a tapered internal thread, so this combination is a good compromise. Of course some additional sealing can provide belt and braces.

BSPT external to BSPT internal - provides a pressure tight joint using the thread alone

Andrew

Simple Question you can ask yourself

 Would it be called a bodge job if it was done on a boiler?

If it's not a bodge job on a boiler then its not a bodge job on high pressure air.

Edit:

It's deffo worth picking cheap Lidl air line stuff up if you are a user but I haven't seen any for quite a while

I'm still getting welding and plasma bits appearing

Edited By Ady1 on 23/07/2023 13:52:16

Howard; Not sure I understand your thinking on tapered (male) into tapered (female) threads being effectively parallel thus no interference. My understanding is that taper to taper is the ideal self sealing situation because it equates to a rotary corking / wedging action along the whole mating area, including thread roots & crests. In fact, I believe the only reason PTFE tape on taper to taper is used is to reduce friction because of the high metal to metal contact area. Whereas taper male into parallel female only wedges at the female entry edge and parallel male into taper female only wedges at the tip.

If you do need to fill taper to parallel thread 'voids', thread sealing cord works well ( Loctite 55 for example).

For the more woke amongst us, please read male & female as external & internal.

Edited By ALLAN QURASHI on 23/07/2023 13:49:06

Alan,

Two identical tapers become effectively parallel.. Full engagement produces a face to face situation (Think Morse tapers as an analogy, or the dual conical faces of nipple on pipe fittings)

On Internal Combustion engines, the angles of the poppet valve and its seat differ minutely, by maybe half a degree, to produce a line contact and high interface pressures, to produce a seal, without lapping. Lapping merely lengthens the leak path.

The thread, being parallel to each other, merely increase the area available for the face to face seal, by increasing the length of the leak path.

The actual interface pressures are likely to be lower than a taper/parallel situation.

Excterrnal taper into parallel internal produces an interference, so that the closing force is concentred on a small area, producing a high sealing pressure.

Hope that this explains it for you

Howard

Edited By Howard Lewis on 23/07/2023 14:01:50

Ignore

Edited By Martin Shaw 1 on 23/07/2023 17:30:56

Thanks very much chaps, it would appear that there is a diverse opinion so, these are the bits,

the small spigot is an interference fit into the brass bush and sealed by soldering, the BSPP bush will jam in the BSPT fitting and with a drop of thread seal probably not leak. For the occasional test of one of my locomotives at 60psi it will probably be adequate. Many thanks for the most interesting discussion.

Best wishes

Martin

For safety I would make the spigot currently an interference fit as a clearance and silver solder so you get full penetration of the joint. As an interference with a solder “caulk” the solder will add no strength. If you silver solder so you get penetration nothing will break it asunder!

Paul.

I think you're right Paul and when I assembled it a niggling thought lurked at the back of my mind. I have decided to make a mk 2 version where everything is threaded together.

Regards

Martin

In Anglian Water we used just PTFE thread tape, just a couple of turns and its fine. Done it that way for years with no problems, but be careful not to overtighten.

During my apprenticeship in the shipyard some used hemp and "boss white" jointing paste ...

Regards

Derek

The joint is only for 1/4 pipe and possibly up to 100 psi ,taper and parallel fittings mate ok for some services,s if I went outside to make this joint connection now ,I would screw it together with either hemp and boss white or ptfe tape, for long term joints I still prefer hemp and boss white, if the joint may need to some apart i would use ptfe tape, I still stock both materials as I restore stationary engines and the cooling pipework has a lot of joints. Too many people nowadays get concerned about all the so called risks,just think on a compressor the suzie flexible coiled airline pipe is only plastic and that holds 100 psi air without anyone getting nervous, different story when dealing with steam,another instance the popular Ifor williams twin axle trailers with small wheels,working pressure for those tyres is 95 psi and four of those tyres are rated to support 3.5 tonnes . with steam more care has to be taken . I remember talking to a foundry engineering manager about fifty yearsago and at that time he still preferred hemp and white lead, it use was banned around this time, though he said that to get really good seals on the large threaded pipe work white lead was still best, it must have been good as later that day I was walking round the foundry and leaning up against a wall was a Stillson pipe wrench the biggest I ever saw close to 5 foot long with the handle bowed like a banana, I commented that they must have had a long bit of pipe to extend the handle,his reply was when you use white lead and hemp on 4 inch pipe work for the foundry services thats what needed to break the joints.

As stated previously, BSPP male into BSPT female is NOT a bodge. It is accepted practice in industry. Thead sealant is required for leak free joint. either liquid or tape.
Glad you threaded the small part. Another option would have been to countersink the inside end an flare it out so the piece can't blow out.

Robert.