Britbike forum Forums British Motorcycle specific Brands Bsa bulletin board Twins tank bracket question.

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

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.

Thanks Gary, that's exactly what I was looking for. I did look at a couple of online twins parts books......and could see what I thought was a pin. But there was a watermark right where it shouldn't have been.

Thanks again, Gordon

Now I have to decide if it's worth the trouble of adding a pin? 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. The tank is sitting on correct frame/tank rubbers and is connected by the center bolted attachment with the correct parts. There really is very little movement in the tank at the front. Right now, I have a unit single strap attached (82-9746) to the tank but not the frame (that strap wasn't designed for that?) Other than movement side to side (at the front) is there another advantage to the strap attaching to the frame?

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.

/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.[/quote]

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

Having experienced the result of running a BSA twin WITHOUT the tank strap, I don't run without one any more. 90% of the purpose of the thing is to tie the two "saddles" of the tank together so that they don't flex while running and fatigue and crack the weld at the top center of the tank. I really think that the pin and the rubber grommet that it goes into are just for a little extra security for locating the tank to the bike, although I can't imagine how a tank with the center bolt properly installed could come loose - you could pick the whole bike up using the tank when that rubber is compressed ....

Lannis

Lannis......hopefully I'll have that bike (Richrd's) at the beach with me. The tank is solid without that strap being attached to the frame. I'm not going to change anything.....for now, just curious.

I agree with you that if the strap is between the tank halves.......it's doing it's job.

I also agree with you about that center bolt attachment.........add the correct sized/material for the frame rubbers and it's a rock.

We've used your FB tank for measurements over the years.......but I never paid much attention to the strap.

Here's another question on the same subject. I don't like the metal to metal connection at the strap to tank. I've used rubber washers in the past between strap and tank. Might use red fiber this time around to see how it works. With the bolts tightened down there shouldn't be any rubbing (metal to metal).......I just don't like it on a tank. What ya think?

Gordon

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

Tridentman......what Gary explained was that the strap did indeed attach to the frame on some twins. BUT......it wasn't a bolted/hard connection. It was a pin and socket with a rubber bushing to isolate the two. You can see the bits in the parts book. I just didn't know how they went together.



Gordon

All A65 tanks have a strap attached to the frame, you can clearly see the pin on my A65 frame:



And the strap with a hole for a pin here:



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 .

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

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.

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

Gordon-my A10 with the large tank has a cross brace but in no way does it contact the frame anywhere.

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

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.

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.

Same with the unit single fiberglass tanks…..no strap.

Thanks Gary…….Gordon

My guess is fiberglass tanks are unlikely to crack down the center from vibration?

No METAL equals no "METAL FATIGUE?"

One further thing to consider is that the Unit singles carried over the from the Pre-unit Twins and singles, the anti-roll rubber stops on the side of the frame under the tank. I know these work very well on the VS alloy tanks.

The Unit twins did not fit these rubber stops but instead used the bottom cross strap with the rubber fitting to the frame.

Gordo

VERY good point!!! I have almost no experience with the unit twins but I think you cracked the code.

Gordon

The only tank I have intimate knowledge of is the '67 chrome tank for twins. On this tank, the piece that holds the tank onto the center bolt is a delicate bit that doubles as a trim piece. If the tank is allowed to rock from side to side by any amount, the piece I'm talking about will break; I know because I broke two of them, and that was only because, in one case, the rubber bushing worked its way off that pin on the underside of the frame, and in the other case, the bushing I used was too soft, and allowed a bit of movement. Once I installed a bushing of the correct consistency, no more broken center mount bits.

The narrower tanks as used on singles and those of different model years that did not have such a delicate center mount were less prone to rock from side to side. I believe they removed the pin on OIF twins.

take a look at the "richrd" bike. I added the frame brackets to give the tank a solid mounting

I love ya brother but that was ABSOLUTELY NOT how to mount a steel tank on a unit single frame.

I can bring those brackets to the beach run... if you want them back?

You'll see how BSA did it.... ....... ...no worries.

That said.......you did a LOT more right than wrong. I've said it to the folks that matter......it's the best running 441 I've owned.



Gordon ....who can't find the correct anti roll rubbers but uses the earlier ones (used with fiberglass and alloy tanks) with an added rubber shim. (the metal tank's tunnel is wider than the alloy and fiberglass and needs a thicker anti roll rubber)

Gordo cracked the code......the anti roll rubbers are the key to that set up. IMO if you have them, the strap AND the correct rubbers in the tunnel you don't need a connection to the frame like some A-65s and that's exactly the way BSA designed it!

It also helps to have an assortment of vibration damping material you've collected through the years. I've never minded cleaning up behind the elevator subs.......they got the best stuff. NOBODY likes to hear anything rattling around inside a moving elevator.