Guys, I think I may have hit a couple of snags with my rebuild and would like some advice. I never really paid too much attention to the fuel tank as to be honest it didn't leak, already had a sender on it and fit in the gap I needed it to go. So after finishing my chassis, I've mounted it direct to the chassis to be self earthing and welded in the back crossmember after fitting.
Things I've just noticed are, there are no vents in the tank, and the inlet neck is 47mm. Am I right in thinking I can just use a vented cap to get around pressurising problems? The inlet neck however is worrying me as I can only find fuel filler pipe in 51mm and 57mm, does 47mm even exist or is it a really weird size for an inlet. Not sure about clamping 51mm pipe down with a jubilee clip, but it's all I can think of at the minute! Any advice most welcome!

The normal way to mount a tank is to isolate it from the chassis as mounting direct to the chassis can often lead to fatigue cracks or chaffing.

Get a metal reducer that has an outside diameter the same as the internal of your filler pipe that flares out to the normal filler pipe diameter
You can secure this with fibreglass or I saw in B&Q a fibre bandage repair kit that claims to work on all sorts of pipe and go as hard as steel when it cures

Thanks guys, some rethinking to do on this, it's been mounted direct since 1992 and looks fine, but it is quite old, I'm tempted to just bin it off and get a new aluminium one :-)