View Full Version : What's the trick? welding thick to thin

09-24-2011, 12:06 AM
I am building a fuel surge tank which needs female NPT bungs welded on. The actual bungs I have are here (http://www.summitracing.com/parts/FRA-596701/). They are larger than I expected, at 0.65" thick, and 0.80" in diameter.

My problem is the tank's wall thickness is less than 0.030", and I'm not sure about the technique of welding something as thick as the bung to thinwall shell. Any tips? Like aim the torch at the bung mostly? Any preheating of the assembly?

09-24-2011, 12:26 AM
That is going to be a challenge. It sounds like you are bronze welding (brazing) mentioning a torch but the principal is the same whether you are brazing or welding. Aim the torch at the larger material and let the molten filler material carry the heat to the thinner material. You will have to take great care not to let the torch play on the thing material otherwise it will overheat and at worst melt and almost certainly burn so badly the flux will not be able to keep it clean.

I suggest you get some scraps of material and practise your technique. This is going to take a bit of finesse.

09-24-2011, 12:11 PM
Thanks for the comments, Geof.

While editing my text to make it more readable I removed a few important details. I meant the TIG torch. So far I've welded a patch over the holes and attached some mounting tabs, but those parts were out of 18ga sheet and were easy to weld to the tank. I am out of practice with TIG and with the bung's thickness I'm paranoid about burning holes in the tank.

I may just try brazing the bung to the tank, with some practice on scraps first. Thanks again.


09-24-2011, 01:56 PM
To some extent it is easier with TIG because the heat source is so localized. The principle remains the same. Heat the heavier part and let the thin stuff heat and melt by the puddle of molten metal from the heavier section flowing onto it.

Actually once you have a bit of practise you will find that TIG is the ideal way to joind very different sized material.

09-24-2011, 06:18 PM
Ah, then I just need more practice before I do this particular task. When I welded the patches on, I burned through once, then after that directed the heat into the patch, which completed fine. Thanks!

09-24-2011, 08:34 PM
lets see pics of your surge tank :)