Fuel Tank Fun.

It’s important (especially on an Interstate) that the front mounts are not bearing the weight of the tank. What I did was to sit the tank level on the bike making sure it was fully down on the frame tube, then measure the gap between the tank where the studs come out, and the top of the frame bracket. Then I just stacked the rubber washers to a tad under that measurement, so as to ensure the tank was pulled down slightly when the nuts were nipped up.
 
Well, I have committed myself to @dobba99 's fix, so used vise-grips to break the remaining welds on both bosses.
Surpises me how small the bosses are - max ID 19mm.
The top bit is only 9.4 mm - the stud is nearly 8mm FFS. Norton must have really been scraping the fiscal barrel!
Anyway, the holes in the tank are nice and round.
Will choose an appropriate thread - fine and 3/4" ish major diameter.
Fuel Tank Fun.
 
Please keep us posted Rob 👍

Our youngest works as an aerospace engineer and so I'm going to ask him his advice and possibly what he can do for me along the lines of the 'Dobba fix'. At the very least, he'll be able to knock up an accurate diagram to work from. Getting hold of him is the tricky part - he has a hectic social life 🤣

I'm wondering if (referring to Dobba's sketch) it would be wise to have the 7/8 UNF thread & nut as a left hand thread to avoid it turning on itself (against the right hand thread 5/16 UNF stud mount) in the bottom of the tank? If that makes sense?

Also wondering whether an appropriately size copper compression washer (like the one in the sump screen) would be better than a normal flat copper washer? 🤔
 
robs ss said:
Surpises me how small the bosses are - max ID 19mm.
The top bit is only 9.4 mm - the stud is nearly 8mm FFS. Norton must have really been scraping the fiscal barrel!
Click to expand...

Yes, I was amazed at how small the bosses are too.
 
Bonzo said:
Please keep us posted Rob 👍

Our youngest works as an aerospace engineer and so I'm going to ask him his advice and possibly what he can do for me along the lines of the 'Dobba fix'. At the very least, he'll be able to knock up an accurate diagram to work from. Getting hold of him is the tricky part - he has a hectic social life 🤣

I'm wondering if (referring to Dobba's sketch) it would be wise to have the 7/8 UNF thread & nut as a left hand thread to avoid it turning on itself (against the right hand thread 5/16 UNF stud mount) in the bottom of the tank? If that makes sense?

Also wondering whether an appropriately size copper compression washer (like the one in the sump screen) would be better than a normal flat copper washer? 🤔
Click to expand...
There’s no need for a left hand thread as the boss should be tightened solid. And the stud or bobbin screwed into it should not.

I’d use the rubber bobbins as their flexibility will help the bosses stay sound. There is no need for the bobbins to be any more than hand tight when screwed into the bosses, and I’d use anti seize.

I would also argue against the crush type washer. They create a ‘soft’ joint but this is a load bearing joint (think about slamming the brakes on with a full interbarge tank). To handle that kind of stress the boss needs to be a solid joint, and very tight, IMO.
 
Fast Eddie said:
There’s no need for a left hand thread as the boss should be tightened solid. And the stud or bobbin screwed into it should not.

I’d use the rubber bobbins as their flexibility will help the bosses stay sound. There is no need for the bobbins to be any more than hand tight when screwed into the bosses, and I’d use anti seize.

I would also argue against the crush type washer. They create a ‘soft’ joint but this is a load bearing joint (think about slamming the brakes on with a full interbarge tank). To handle that kind of stress the boss needs to be a solid joint, and very tight, IMO.
Click to expand...

Thanks for that, Nigel. Appreciate the sage advice 👍
 
Perhaps a stainless rivnut/nutsert of suitable size, plus a crush washer on inside, would do the trick here? I believe there are types with a blind threaded hole.

EDIT: called a"sealed" rivnut.
 
Last edited:
Bonzo said:
I in fact bought four of those a couple of years ago. Mini exhaust bobbins (from Wood & Pickett Ltd)...

Fuel Tank Fun.
Click to expand...
I would be very careful with rubber bobbin selection if I were you.
I just did an experiment stacking together 4 OEM tank rubbers onto the end of a rubber muffler bobbin and put them in compression in a vise.
The OEM rubbers were more compliant across the range of forces - the last on being typical - 2.5mm compression of the tank rubbers vs 0.6mm on the muffler rubber. The "rear cotton reel exhaust mounts" I bought from Mini Spares a couple of years ago were stiffer again.

I would check for better softness/compliance before fitting them to my bike.

Edit: The only advantage the bobbin type would have is some compliance fore and aft.
Cheers
 
Last edited:
Everything which can break needs to be rubber mounted. If I had a tank which bolted down, I would replace the bolts with studs and pull the tank down onto rubber with a strap over the top.
 
Fast Eddie said:
There’s no need for a left hand thread as the boss should be tightened solid. And the stud or bobbin screwed into it should not.

I’d use the rubber bobbins as their flexibility will help the bosses stay sound. There is no need for the bobbins to be any more than hand tight when screwed into the bosses, and I’d use anti seize.

I would also argue against the crush type washer. They create a ‘soft’ joint but this is a load bearing joint (think about slamming the brakes on with a full interbarge tank). To handle that kind of stress the boss needs to be a solid joint, and very tight, IMO.
Click to expand...
👍 👍 you must've typed that as I was doing the edit above.
I would still check for fore/aft compliance - the mini ones are much stiffer than the Norton muffler ones.
Cheers
 
Compression cushioning can be enhanced with bobbins by using a rubber washer twixt bobbin and frame, and as mentioned by someone prior, by drilling them… I’ve never tried this, but might give it a go.
 
Draft Front Boss Replacement Procedure
Okay - I think I've come up with a version of @dobba99 's fix that anyone with a vise/hacksaw/file can do.
I've elected to use brass, as its good in fuels and also free machining (easy to work)

Get a length of brass threaded rod (I've chosen M20 x 1.5 because I can't get 3/4" UNF - they both have similar pitch) and cut a 25mm piece and flatten the ends
Get 2 thin locknuts to suit
Get a short 5/16 UNF grub screw (preferably brass but doesn't matter if its steel)
Get 2 thin M20 copper washers (annealed) - just to cherry red and quench or not,

Next drill and tap 5/16 UNF down the centre of the 25mm piece (you can do a blind hole if you like but a through-hole is easier and cleaner).
File one of the lock nuts down to, say, 5mm thick.

Now to assemble, using loctite (567, 569 or 577), wind the thick nut onto the piece so that there is about 7mm of thread projecting from one end.
Again with loctite put the grub screw in the other end so that there is 17mm thread depth (existing boss number) for the OEM stud (only 10mm needed).

After the loctite has gone off - say 30 minutes - you're ready to fit to the tank.
First, ensure that the "boss hole" is a clean 20mm diameter and the threaded rod will slide through. Also that it's reasonably flat.
Put a copper washer onto the 7mm threaded projection
Next, snugly thread the OEM stud into the new threaded boss, having first locked 2 nuts together near the other tip.
Tie some braided (50-80ib) fishing line firmly near the tip (above the nuts) of the OEM stud and use a stick/wire (?) to poke the other end of the line through the fuel cap and through the boss hole.
Turn the tank upside-down, put some loctite on the copper washer and thread. (it is now important the job is finished within 5-10 minutes)
Pull the tip of the stud through the boss hole and use the the stud/nuts to pull the new boss into place.
Put the second copper washer and the "thinned" locknut onto the 7mm projection, with some loctite (i'd be liberal here), and tighten the locknut using the 5/16 nuts to counter the torque. Have not come up with a torque value yet - but schnisentite!

I am yet to get my bits but will report back after fitment.

Many thanks to Peter for his great idea!
Cheers ...and sorry if I've bored you all to death! (idle minds and devils you know!)
 
Last edited:
robs ss said:
Draft Front Boss Replacement Procedure
Okay - I think I've come up with a version of @dobba99 's fix that anyone with a vise/hacksaw/file can do.
I've elected to use brass, as its good in fuels and also free machining (easy to work)

Get a length of brass threaded rod (I've chosen M20 x 1.5 because I can't get 3/4" UNF - they both have similar pitch) and cut a 25mm piece and flatten the ends
Get 2 thin locknuts to suit
Get a short 5/16 UNF grub screw (preferably brass but doesn't matter if its steel)
Get 2 thin M20 copper washers (annealed) - just to cherry red and quench or not,

Next drill and tap 5/16 UNF down the centre of the 25mm piece (you can do a blind hole if you like but a through-hole is easier and cleaner).
File one of the lock nuts down to, say, 5mm thick.

Now to assemble, using loctite (567, 569 or 577), wind the thick nut onto the piece so that there is about 7mm of thread projecting from one end.
Again with loctite put the grub screw in the other end so that there is 17mm thread depth (existing boss number) for the OEM stud (only 10mm needed).

After the loctite has gone off - say 30 minutes - you're ready to fit to the tank.
First, ensure that the "boss hole" is a clean 20mm diameter and the threaded rod will slide through. Also that it's reasonably flat.
Put a copper washer onto the 7mm threaded projection
Next, snugly thread the OEM stud into the new threaded boss, having first locked 2 nuts together near the other tip.
Tie some braided (50-80ib) fishing line firmly near the tip (above the nuts) of the OEM stud and use a stick/wire (?) to poke the other end of the line through the fuel cap and through the boss hole.
Turn the tank upside-down, put some loctite on the copper washer and thread. (it is now important the job is finished within 5-10 minutes)
Pull the tip of the stud through the boss hole and use the the stud/nuts to pull the new boss into place.
Put the second copper washer and the "thinned" locknut onto the 7mm projection, with some loctite (i'd be liberal here), and tighten the locknut using the 5/16 nuts to counter the torque. Have not come up with a torque value yet - but schnisentite!

I am yet to get my bits but will report back after fitment.

Many thanks to Peter for his great idea!
Cheers ...and sorry if I've bored you all to death! (idle minds and devils you know!)
Click to expand...

Not bored me Rob.

Looking forward to plenty of pics ;)
 
Mine was not fuel tight when i bought it. seems you can hide a lot of $hit with bondo. I had a shop make weld on replacement plugs. problem fixed with a mig, it is really thin steel.
 
I've been using exhaust bobbins on my Interstate since 1983 when it cracked the bottom of the tank. I didn't need to worry about the paint because the full tank went up in flames so I brazed it and decided against replacing the stud. Never a problem since and still running the same tank
 
Bonzo said:
Yeah, I'd dearly like to keep the paintwork intact as it's original & has patina, and so if there was any way around it avoiding welding, that would be my first option.

I'm trying to get my head around your stainless boss method dobba 🤔
Click to expand...
After washing it out thoroughly with water, submerse the tank upside down in water to within 1/2" (13mm) of the repair location and braze it on keeping the flame away from the edge of the bottom of the tank. The mount has more bulk than the metal of the tank so concentrate the heat there. Of course, spotlessly clean metal is required for brazing. MIG or TIG might work but electricity and water don't play well so be very careful if you try.

BTW, if worried after washing out the tank, with all openings open, stick a propane torch in the fuel filler and move it around to slightly warm the tank. At most there will be a little pop - usually nothing - definitely won't explode!
 

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