Petrol and oil tank threads

[Roger HulettParticipant @rogerhulett83124]

I am unable to purchase a/the spigot that is soldered/brazed into a motorcycle oil/petrol tank to hold the petrol/oil tap. I shall have to make one. My question is twofold. I know it is a 1/4" gas thread and it is only approx 1/4" long. Is it a tapered thread ? What change gear set up on my 9" South Bend (1939) do I need to cut a 1/4" gas thread ?

[Roger HulettParticipant @rogerhulett83124]

Thread cutting

[pgk pgkParticipant @pgkpgk17461]

You'r trying to make the bush in the tank that the tap threads into?. Looking up such things (not being into motorbikes) it looks likely to be a 1/4 BSP thread (although an american bike might be different). the obvious move is to buy the tap and measure it's thread or buy a tap with a known thread type. As to threading a 1/4 BSP on the lathe.. yeah it can be done if you make a tiny threading tool but the the nuisance and flex etc and multiple spring passes… well why not just buy a 1/4 BSP tap? For a through hole you can probably get away with just the one tap which are about £3 from here: **LINK**

[HopperParticipant @hopper]

It depends on the type of petrol tap you use. Some had a parallel thread with a flat flange and copper washer to seal with. Others had the tapered thread like a pipe fitting.

+1 on a tap being a much easier way to go, specially on a tapered thread, which would require either a taper turning attachement or offset tailstock in order to screwcut on an angle. (Tip, check the fit of your petrol tap into the boss thread before soldering the boss into the tank.)
 

Your 1/4 gas thread would be 19tpi, a bit of a mongrel that IIRC usually needs an oddball 38 tooth change gear on a lathe with an 8tpi leadscrew. (Which I think your Southbend has). I dont think the Yanks supplied a 38 tooth gear with their lathes because they were clever enough to avoid 19tpi threads.  But if you ever do find one somewhere, you could run a 40 – 38/20 – 50 gear train to get 19tpi.  1/4 NPT on the other hand is 18tpi, if you use an American petrol tap.

Edited By Hopper on 15/12/2016 12:00:25

Edited By Hopper on 15/12/2016 12:13:40

Edited By Hopper on 15/12/2016 12:14:55

[Chris Evans 6Participant @chrisevans6]

As Hopper says, most likely 1/4 19TPI. I mess with a lot of bike stuff and have come across BSP/NPT metric fine of many diameters all on petrol taps. If you are local to Lichfield I can lend you a tap but they are cheap to buy and easier to do the job. If you really want to make it try using a loose 20 TPI in a bit of scrap, it will probably work.

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