Petrol Taps

[shaunstaples]

I’ve been reading about these for a while, I haven’t taken one out to check the thread size into the tank, but was reading tat a couple of People had opened the boss up to 1/4” bsp from 1/8” standard when their internal thread had become loose or stripped. Just wondered if anyone had done this recently and how much of the boss is left when it’s increased to 1/4” bsp?

I don’t want to start welding or brazing a boss on, but if there was enough left I could make collars to shrink on without damaging any paintwork, if the tap was a taper thread?

The choice of tap at 1/4” seems to be much better and room for a reasonable size filter is what I’m after

[shaunstaples]

Also, while thinking about tanks, I sent for a new set of bolts, washers and rubbers that were missing. Should’ve checked the threads first because all the threads seem tired but one has the first 1/8” or so missing.
I read that 5/16 unf exhaust bobbins we’re a good work around, but the ones I got next day in their Prime weren’t up to the little force needed to screw them in, as the stud spun with very little force being screwed in by hand…
So back to the drawing board and I found some M8 one with a stud one side and a threaded hole the other. So I’m going to make some studs up that are M8 one end and 5/16 cycle the other that will reach the bit of thread I have left.

[Groily]

Not sure from individual tank to tank how it would go on the fuel tap front Shaun. But probably OK.
The major diameters are 9.73mm for 1/8 BSP and 13.15mm for 1/4.
If you look at your bosses it should be reasonably obvious if there's enough metal around the hole to go that way. Certainly you'd get a bigger filter up a bigger hole - but the gauze ones on 1/8th Ewart taps aren't so bad (I don't think myself).

Now and then I've bought or made 1/8 to 1/4 BSP adaptors (male / female or male / male) to be able to use a 1/4 tap on one or other bike I've had- but can't recall now what I did for filters. An in-line external one between tank and carb, somewhere unobtrusive, isn't a bad plan. Also easy to clean or replace without spillage - and therefore a rational departure from 'proper' I reckon.

Ref your second point about the threads for the tank mounting bolts on bikes like yours (and my '54 twin), there's a fair bit about this on here. Quite a bit of ingenuity on display too. It's a common problem. viewtopic.php?p=242530&hilit=petrol+tan ... ts#p242530 was one such 'thread', pardon the pun, from last year. There's a more recent one too, I think, but couldn't find it.
The thread depth is extremely shallow, and the bolt lengths and washer thicknesses etc have to be right - or you end up winding them into the tank itself, behind the little threaded lugs/plates. People do things with M8 and so on, and I've bodged in the past with the best of them.
Most durable fix for these critters though (in my humble) is correct-size nuts brazed or welded on to the lugs.

[shaunstaples]

I would be worried mostly if the boss is reduced and let into the tank then brazed, maybe that would be the weakest bit?
I can make an adapter up as you say, but would be nice to do it if possible, both taps are mismatched and think one is poor quality.
It was an old post where the holes had been increased successfully, I just wondered if anyone else had done it

Think there would be enough thread in the tank mounts to make a permanent repair, that won’t be disturbed in future.
Thanks Bill think that thread is where I saw unf mini bobbins, but the ones I got won’t be fitted on anything that might have to come off in the future!

[Groily]

Well, I wouldn't want to be making braze-in inserts to go up the fuel tap bosses into the tank. I'd far rather drill and tap to the larger size - if it's safely possible.
Or see if a new male 1/8th BSP thread would hold better and be leak-proof in the female thread that's there already (if that is part of the problem).
I don't think the advantage of a bigger in-tank filter, on its own, outweighs the potential hassle factor!

[shaunstaples]

No I would make a threaded adapter and the tap unfortunately be lowered.
I meant the original boss, if it was reduced like a spigot let into the tank when made. That’s how I would do it rather than just sweating one on to the surface.
That’s the size that I’d be guessing and getting close to not just the outside diameter of the boss. It could be that there’s none of the spigot left after it’s opened up, leaving the boss weak?
I want to replace and improve them if possible, to try and make it more tolerant of this rubbish we’re served up at the pump?
Could do with having a look inside the tank, probably won’t see much as it’s lined.