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Thread Like Summary
BSA_WM20, Gordon Gray
Total Likes: 13
Original Post (Thread Starter)
#874521 03/15/2022 2:56 PM
by Gordon Gray
Gordon Gray
What does (if anything) the tank bracket for a 70's twin attach to? (68-8062, 68-8176) I know it's bolted to the tank at threaded inserts but does it also attach to the frame?

I'm a singles guy. Owned one twin and that was a long time ago. I'm replacing fiberglass with steel and wonder if I need to do anything extra if mounting a twins tank to a single? I used a 82-9746 strap on my 67's Indian copy and as far as I know it wasn't designed to attach to the frame on a single. That said, I've never owned the later models with a steel tank either......so I'm a bit clueless.

Gordon
Liked Replies
by kommando
kommando
Originally Posted by Gordon Gray
Best I can tell, the smaller and larger metal tanks used on the unit singles didn't attach to the frame (please correct me if this is wrong) but did use a strap bolted to the tank's inserts. I've got what I believe to be a twin's tank on one of my singles.

Gordon

Can only comment on the Unit Singles, the steel 69/70 tank has the cut outs under the tank for the twin carbs so is common to the twins and singles. On the singles the under tank front strap is plain (2 bends just to give some clearance) and only bolts to the tank and not the frame. On the singles the strap stops the 2 halves of the tank parting ways under vibration.
2 members like this
by Gary E
Gary E
The frame has a large pointed pin facing downward on the lower center frame tube. The tank bracket has a pocket facing up. The pin goes in the pocket.
With a rubber tube gromet (piece of rubber hose) around the frame pin and inside the bracket pocket the tank is held sort of secure.
1 member likes this
by Tridentman
Tridentman
On the twins the strap is there to prevent vibration.
It was originally fitted to the A7 and A10 pre unit twins and carried over onto the unit twins.
If you imagine a vertical cross section of the tank at right angles to the spine of the bike then it looks a bit like an inverted tuning fork.
And that is how it behaves without the strap.---it vibrates and causes leaks.
This was a particularly acute problem on the large UK (4 Imp gallons-- 4.8 US gallons) tanks on the pre units.
The strap just ties the two halves of the tank together to prevent vibration.
It should certainly not be attached to the frame---otherwise the isolation of the tank from the frame with rubber mounts is compromised.
HTH
1 member likes this
by Tridentman
Tridentman
Sorry, Gordon---my experience has been on the pre unit twins and I assumed (obviously mistakenly) that the tank mounting on the pre units carried across to the unit twins.
However I believe that the principle of not having any metal to metal contact but supporting the tank on rubber--- is a very sound one.
1 member likes this
by Gordo in Comox
Gordo in Comox
The A65/A50 cross strap has a welded on cup to fit over the rubber sleeve that slides up onto the pointed rod that is welded to the underside of the frame tube (as in Adams photo). Both the big 4 gal tanks and the smaller 2 gal tanks have a strap. The 2 gal strap is flatter than the one used on the 4 gal tanks.

In both case the rubber sleeve isolates the tank from contacting the frame.

Without the strap the tank would be quite unstable when full with only the single top bolt holding it.

Gordo
1 member likes this
by Adam M.
Adam M.
All A65 tanks have a strap attached to the frame, you can clearly see the pin on my A65 frame:

[Linked Image from i.postimg.cc]

And the strap with a hole for a pin here:

[Linked Image from i.postimg.cc]

However in your situation Gordon with smaller tank on a single I wouldn't bother with frame modification. I've never seen any evidence of movement between a strap and metal of the tank when everything is properly installed. But if it bothers you why not to use those fiber washers smile.
1 member likes this
by MikeG
MikeG
Gordon-my A10 with the large tank has a cross brace but in no way does it contact the frame anywhere.
1 member likes this
by Gary E
Gary E
Originally Posted by Adam M.
All A65 tanks have a strap attached to the frame, you can clearly see the pin on my A65 frame:

Clarification: A65/A50 2 gallon fiberglass tanks (Spitfire Hornet, Wasp, Hornet, Spitfire, Firebird Scrambler) do not utilize the bottom strap. The pin is still there on the frames though.
1 member likes this
by kommando
kommando
Originally Posted by Gordon Gray
Thanks fellows.....

Adam, I believe the 1969 and 1970 Shooting Stars (441cc) used the same tank as some of the twins. The "smaller" version was used on the 1969 Starfire (250cc).

The actual tank I'm using that got me to thinking about this is a LARGE version of the scalloped tank. Not sure of it's actual capacity but it measures wider than my other scalloped tanks.

BUT that said.........I'm using the singles tank strap and it's not attached to the frame. The tank is rock solid (IMO) even when full and I've put a thousand miles + on it so far.

Gordon

There are 3 versions of the Scalloped tank.

1. Smallest fitted to US Starfire and US Firebirds (plus UK ?)

2. Middle sized fitted to UK Starfire and all B44 Shooting Star

3, Largest fitted to some A65's (UK ?)

Difficult to give precise capacities as its a mix of UK/US gals and L's in the hand books but roughly its Type 1 2 gal, Type 2 3 gal and type 3 4 gal.
1 member likes this
by BSA_WM20
BSA_WM20
Originally Posted by Gordon Gray
/quote] Can only comment on the Unit Singles, the steel 69/70 tank has the cut outs under the tank for the twin carbs so is common to the twins and singles. On the singles the under tank front strap is plain (2 bends just to give some clearance) and only bolts to the tank and not the frame. On the singles the strap stops the 2 halves of the tank parting ways under vibration.


kommando.....Thank you for confirming that. I believe the 69'-70' tank used on the Shooting Stars (441) was larger than the the one on the '69 250 and was shared with some of the twins??? That is as much a question as a statement.

When I had my '67 VR glass tank copied (in steel) I had them add inserts for the strap.

This makes my third swap out from a fiberglass tank to steel. This particular tank is larger than the other's I have here. Different cap too. Using 69/70 SS tank rubbers the tank is as stable as any I've had before. I have had 3-4 different scalloped tanks on the shelf....not 100% sure what they came off of.

I'm just trying to wrap my head around why BSA used a different system for the scalloped tank on a twin......or was it just a holdover from the Lightening's tank design?

Gordon[/quote]

Top pressing is the same bottoms are different
One way it is no problem & the other way you need to put a right angle throttle cabe in because the tank fouls on the carb cables
From memory twin on single was the problem child
1 member likes this
by Irish Swede
Irish Swede
My guess is fiberglass tanks are unlikely to crack down the center from vibration?

No METAL equals no "METAL FATIGUE?"
1 member likes this
by Richrd
Richrd
take a look at the "richrd" bike. I added the frame brackets to give the tank a solid mounting
1 member likes this
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