Hi Hamish...Empty the tank out, remove the tap and give it a good flush through with water using a hose pipe. Make sure you completely fill it and empty it a few times...You'll find that will be sufficient..My rigid B33 tank has a leak at the moment, where they usually go by the mounting lugs at the front. I'm holding out on repairs hoping a good tank will come up for sale but I suspect I'll have to do it eventually as you don't see these earlier tanks so much these days....Ian
email (option): ian@wright52.plus.com
Soldered this 16H tank a couple years back, adn its still ok.
Never bothered to completely remove all petrol, have a 600 watt soldering iron, and that worked very well, just make sure the metal is very clean.
You could hear the petrol boiling a bit, but nothing happened.
But I'm not saying it's a safe method!! hate to put water in petrol tanks though, and will do anything to avoid that!
Cheers,
Lex
email (option): welbike@welbike.net
I had to repair my B33 tank earlier this year same as Ians fractured on front mounts.I removed tap and cap and left a airline running into tap mount for around five minutes took tank outside then laid on a table making sure the tap hole was clear at bottom then flashed a gas blow lamp over hole for cap sometimes you get a slight woosh of flames but mostly it will light remains of petrol fumes then go out.I have done several tanks this way never had a problem, but only do this if you are confident, my old boss showed me this when I left school for my first job as a bike mechanic he had been doing it for years must admit I stood well back,but once done you can do what you like to your tank Dave 
...Hi Dave...I've had the 'whoosh' genie a few times as well...
...As you say it's not as bad as it sounds...Ian
email (option): ian@wright52.plus.com
Well thanks for all the advice, the tank is now fixed,back on the b31,and filled with petrol. I used an ancient soldering bolt and tinmans solder. Kept the genie at bay by flushing tank with water and keeping naked flame away from tank.No accidents and i still have my eyebrows and hair! Regards Hamish.
email (option): fuzzyempire@hotmail.com
Hi Hamish...Are you the Hamish I see regularly at the April Stafford Show?....Ian
email (option): ian@wright52.plus.com
I was close then... ...Ian
email (option): ian@wright52.plus.com
I really like these early B group BSA's. Here a photo of mine, I bought it in 1978 for 75 Dutch guilders.
Henk
email (option): ahum@quicknet.nl
Yes, very nice. These early models were the lightest so best suited to the B31 engine. Although the changes made on later rigid models increased durability, particularly of the gearbox, they also added more weight...All later versions after that increased the weight further, making the B31 very slow and even taking the edge off the B33s performance...
B31s up to 47 and B33s for 1947 were really the way the designers intended them to be as 'stand alone' models that didn't have to share parts with others and due to their ligfht weight they went surprisingly well.
It shouldn't be forgotten that these models were originally sold as sports singles and not 'ride to work' machines....Ian
email (option): ian@wright52.plus.com
Hi,Peter I dont advise other people to try this unless they are confident IE; know what they are doing.Yhe airline will dry most of the petrol out of tank and remove the fumes.I have also repaired diesel tanks on my trucks without washing out tank with no problem.My mate worked at a local garage and had to repair his mini tank so as the river wensum ran at the bottom of the yard he staked out the tank after he had submerged it and left it all morning till his lunch break then tried to repair it after removing all water it blew the seams out as the remaining fumes could only escape through a small filler pipe hence the reason you need to remove taps as well as caps.If I told you of things my old boss got up to you would soon be running for your hard hat and copy of health and safety manual . I must agree with Henk and Ian these early BSA B31 /B33 are the nuts.my own B33 is a early 1949 plunger but I once had a 1948 B33 which would hit 85 MPH easily in STD form wish I still had it. Dave...