stress fractures in 3gal. tank

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steve m
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stress fractures in 3gal. tank

Post by steve m »

up until now i have only had enough fuel in my tank to start the motor and ride around the block a couple of times to test things on the B50ss i bought six months ago.
last monday i road it to get the roadworthy cert. and then on to the rego branch, where they knocked me back -that's another story i will tell later.
when i got back home i had to take the tank off to do something and i felt petrol on the outside, when the fuel sloshed. turns out there are 2 stress fractures. one at the front, along the weld at the base of the cutaway. the other just above the top of the seat along the centre weld.
has this happened to anybody else? and ,are these locations where it happens? or is it a random thing?
all the rubber parts seem in reasonable condition.
i'm going to get it tig'd, but it has also had a sealing treatment done to it. will the welding destroy that?
i'd be grateful, if someone can help.
looks like i'll be driving the car as usual, to the Phillip Island Classic this week end.

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Post by BSA_WM20 »

Yes this is very common.
It is not a stress crack, it is a fatigue crack.
There are 2 main causes.
1) over tightening the center bolt
2) running without the tank strap on.
When you are riding the sides of the tank actually flap like a pair of wings unless they are restrained.
The net effect is the same as bending a nail back & forward.
If you do it for long enough it will break.
In the case of the BSA tanks it is made worse by the actual welding process.
Tanks that were made in the middle of winter, early in the shift were too cold to be welded and the coldness of the steel caused a bad microstructure ( parent metal chill) to occur at the weld / heat affected area interface.
By the time industry realized that things like your tank required some preheating prior to welding on very cold days BSA was well & truly closed down.
The down side is there is virtually no way of telling a good one from a bad one so it is a case of suck it & see.
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Post by steve m »

thanks trevor,

There are 2 main causes.
1) over tightening the center bolt
2) running without the tank strap on.

1)yes, i'm thinking now, that my rubbers are too hard - the nut does seem to tighten down solid.
2)ya got me here. does the b50 have a tank strap?

anybody able to help with the sealant /welding problem?
previous experience, etc.
i would imagine the heat transfer would destroy the layer. hard to see in there, maybe they (p.o) only sealed the bottom( the part i can see) so that would explain why it leaks through the cracks, which are up a bit higher than i can see.

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Post by B44R »

The tank strap is a strip of approx 1"x1/8" flat metal that is bolted between the 2 threaded holes on the underside of the tank at the front.

HTH

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Petrol Gas Fuel tank tie strap

Post by Chris Pellett »

Steve,
There is no provision for a tie strap on 1971 or early 1972 tanks.
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Post by steve m »

thanks all,
had a look at my '72 tank. also read brads book - again :roll: anybody who hasn't got it, should get it, it's very, very helpful.
my 71 tank hasn't got the lugs but, the 72 that i bought new from frank mussett in '74-75 has.
have got no idea what a tank strap looks like , but we have the desciption above.
looks like i'll be chasing some new rubber parts and fabricating the lugs to be welded on, before the respray.

here are the lugs

Image

Image

Image
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Post by b50root »

Never seen before :shock:

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Post by steve m »

brad's book, page 70 and 71
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Post by minetymenace »

B44R wrote:The tank strap is a strip of approx 1"x1/8" flat metal that is bolted between the 2 threaded holes on the underside of the tank at the front.
Yet steve m's pictures show a couple of brackets (not threaded holes).

I'm with Rickard, never seen this before. Was it one of the mods recomended on the service sheets, or just a common mod. that has only just made it to the forum?
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Post by ghislain »

Here's the real miracle : with BSA's there's always something new !!!!

Of course, as every one but his owner , I never saw such a tank !!!!!! :shock:
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Petrol tank tie strap

Post by Chris Pellett »

The tie strap for the 2 gallon tanks: part no. 83-4722 measures 5½" x ¾" x 3.45mm with paint - sorry about that last one but it seems a little more than 1/8".
the fixing holes are 4½" between centres and 5/16" diameter elongated to 7/16".
The strap is slightly bent midway to align with the slightly out of vertical threaded bosses on the tanks.
Clearly this tie strap would not fit the 3 Gallon tanks.

Steve,
What's the distance between the tabs on your 72 3 gall. tank?

Has anyone got a 3 Gall with tie strap?
Has anyone else got a 3 Gall with tie strap provision? Has it bosses or lugs?
Somewhere there must be a part number for the 3 gallon tie strap, and probably someone has a box full of them but doesn't know what they are for.

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Post by steve m »

chris,
from the centre point of the radius, to the other, as the crow flies. close approx. 9ins.
this is the most obvious crack, the one at the front.

Image

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tank strap

Post by Canberra »

Hope this helps - Photos of standard tank strap

Image

and the tankstrap end
Image

Hole centres are 9.25inch and holes 0.375inch. the tank lugs are same as Steve M.

Let me know if you want me to do a better drawing.

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Cracked tank

Post by Canberra »

Forgot to mention my original 71 3 gallon tank had one fitted and the replacement 3 gallon tank I got from UK after the 71 was stolen.

Mine cracked in the same place as Steves after a prang. It was welded up using a mig welder and has had no problems since.

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Post by steve m »

john,
thanks for that , excellent work. distance between centres on my tank was very close to 9ins. not 9.25 so i think thats why the slots in the lugs, some tanks have spread a bit, i guess. i bet Dave Beanham at Modak has a shed full.
so easy way is, make parts, bolt up, and tack then weld. that's is if David hasn't got one.
you say you haven't had a problem since - is that because you have a strap fitted? or is everything all right without?

steve (off to the Island classic - pic's on monday)
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Post by Canberra »

I should have said - The weld is OK and the tank does not leak.

I have not used the strap for sometime and it was not on in the prang. It was suggested that the tank would have probably not split with it on.

I aim to use it and had just pulled it out of my copious box of "bits left over" from working on the bike when this topic came up.

Hope the meet went well.

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Post by BSA_WM20 »

And I should add that the style & type or riding you do will make a big difference to wether your tank cracks or not.
If you run up and down playing a tango on the gear change then the tank is less likely to form & grow a crack than if you sit on constant revs doing ( boring) highway running which promotes resonant vibrations in all of the tin wear.
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Post by Canberra »

One thing I noticed when I fitted the tank strap - the angles of the plates on either end are slightly different. Therefore the strap only fits one way.

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Post by steve m »

thanks trevor, good observation there,-(note to self- learn to dance and don't be so boring on the road).
john,
thanks, will bolt lugs to strap before welding to tank, and adjust bends too suit.
the island classic was great - as usual, 3 days of good fast racing in a great setting. i'll get a few pic's done sometime today and post in the news section or should they go in the off topic section, yes, off topic i think- too many on unit bsa's, non-bsa's, etc.

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Post by BSA_WM20 »

I know tat I am probably going to get pilloried for mentioning this on the singles list , but I had an A 65 which I bought secondhand and the DPO had made up a rigid steel fuel line linking one tap to the other and a T soldered at the center to feed the carb.
I was told that it was the only way the owner could stop the rear of the tank splitting ( there were several welds on there already & it was starting to crack again.
I torch welded it then gave the area a good cooking to stress relieve the area & it never cracked again.
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Post by steve m »

sounds like maybe all that is required is that cracks in tanks be welded properly.
if i get it tigged this week all i will have to do is put it outside, to cook.....6:00pm and it's 46deg. under my back porch, in the shade! ! !
that is not relieving my stress, at all.
thanks trevor.

steve
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