what is the correct internal size for the Australian Cooper S fuel line which runs the length of the body.
Not sure so want to check before making a fuel line up.
Cheers.

Pretty sure it's 1/4" OD tube same as common minis.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Ditto for main fuel line.
The joiner pipe inside the boot is 5/16 inch OD and I found that 1/4 hose wouldn't fit.

Just trying to work this out..

Main fuel line is 1/4"
Right tank is 5/16"
What size is the left tank?
What size is the fuel pump?
What size are the three connections on the joining pipe?

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The SU pump used is very good at pushing and useless for sucking hence 5/16 on the suction side ( its really only gravity that feeds the pump) and 1/4" from the pump to the carburetors. I am pretty sure that the plastic ports on the pump are 5/16" in and 1/4" out. While I have never measured them I have never had any difficulty connecting both sizes of pipes

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Mk1 Cooper S
Mini K Rally car

If adding an extra tank and the joining pipe is this the right method?
Remove 5/16" hose from bottom of left hand tank and connect to joining pipe
Add short 5/16" from left tank to joining pipe
Add short 5/16" from right tank to joining pipe

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I added an extra tank, but ran USA fuel hose from RH tank to a brass tee just below LH tank. It ain't in an S though...

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Yes. And a longer 5/16 hose from joiner to pump inlet through hole in boot floor.
I found the tanks must be connected to the joiner with 5/16. They are in an awkward position and need to be easy to fit and clamp.
The SU pump itself has identical spouts for in and out but they are just under 5/16 OD, so take sizes.
The 5/16 hose from inside the boot fitted easily and the 1/4 OD hose from pump outlet to underbody supply line also fitted easily.
The factory may have used all the one size of hose if the rubber was really flexible.
All these hoses on Ss are braided but on single tank models, the hose out was a plain, thick-walled type.