Thread help / ID please

[pptom]

Hello all, currently in the process of adding an internal pump / swirlpot to an alloy fuel tank (60's) car.
I've opted to use a 90's range rover assembly which drops in from the top and is clamped by means of a female screw down ring.
In an ideal world I'd like to make a 'receiver' to be welded into the top of the tank with a male thread, which will take the female 'nut/ring:
I'm trying to work out the thread form of the female ring, which is what I need help with
They look a bit Acme with flat peaks and troughs, a 6mm pitch gauge seems to match, as does another which is labeled '4G 2 1/4. 2 1/2'
Major diameter is around 131.5mm or 5.175"
Any ideas please?

 

[8ob]

Could be an s60 buttress thread like used on stillages and other water containers.

Bob

 

[pptom]

Thanks Bob. I'll have a look into that

 

[Agroshield]

A buttress thread is non-symmetrical (the leading flank is not the same as the trailing flank). Press some plasticine or blu-tak into the female thread and that will confirm or eliminate that possibility.

If you have a spare, cut it in half and look at the cross section. You could coat the cut surface with ink, stamp onto a piece of paper, scan into the computer and draw over it (or draw manually) to identify pitch, major diameter, minor diameter and flank angle(s).

Given it is plastic, it could be a non-standard thread, so rather than starting from assumptions, measure exactly what is there with no preconceptions trying to fit it into an existing chart and reproduce what you have.

A 3D-printed male to test fit would be quicker and cheaper than metal. A 50mm wide rectangle with curved ends would be enough for this (i.e. you do not need to print a full circle).

 

[m_c]

Given it is plastic, it could be a non-standard thread, so rather than starting from assumptions, measure exactly what is there with no preconceptions trying to fit it into an existing chart and reproduce what you have.

That was my initial thought.
Looking at the thread gauges, the angle looks more like metric 60deg, whereas imperial is usually 55deg (it means a metric thread can be filed with a standard triangular file, whereas the file angle is wrong for imperial).

The flattened thread form is likely just because that part of the thread doesn't add anything to the strength, so I'd probably guess it's somewhere near a standard metric 130x6mm thread, but with a bit additional clearance to allow for the fact it's plastic.

 

[Turbo]

Could you cut the threaded part off the range rover tank along with a section of the tank and rivet/bond that to your steel tank?
Be less work and a lot safer than trying to weld an old fuel tank!

 

[pptom]

Thanks all. My plan was to print something similar before going any further. It's definitely a symmetrical thread and I did think that the pitch fits the 60deg thread form well. Mr C's post sounds good to me.
I'll draw and print something up tomorrow and see how I get on. Liking the plasticine idea too
The tank it's going in is a pretty new Aluminium one, so no worries there. I don't have a rover tank.
If all else fails I'll just ditch the screw cap idea and make a sandwich flange which would probably be easier, but where's the challenge in that?

 

[Pete.]

Check that it's definitely a single start thread.