Just to rubber tin . 
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A 1950 EARL'S COURT MOTORCYCLE SHOW 1950/51 MANX NORTON FRAME,
Frame No F11M 30787.
So STRAIGHT tubes for the main apparatus , should do the trick . Id ' TACK ' ( spot weld / braze ) & get out the levels and string lines & check diagonals etc thoroughly , Before FINAL welding .
The IDEA is its SACRIFICIAL , hopefully the rest dosnt get bent , if someone overcooks something .
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Had a home made alloy oil tank , on ine . old Rover 75 oil filler neck & screw it cap at seat nose .
Wondered for a moment what was going on when the cap fell off , once . Checked tight afterward.
' Manx Norton Oil Tank ' in google pic's will get zillions , Domi etc etc . youve missed one for 35 quid . !
BUT , if your not somewhere rocky , the rear face of a custom tank , concave curve , can serve as the rear
guard at the tyre front . Getting the tank as large & aft as possable . ALLOY is way preferable to the insular fibreglass .
3/4 Gal oil 1/4 air - for one Capacity ( Imp . ) for a Triumph . 2/3 gal usual 1/3 spare . fill 3/4 for 24 hour races . 
to small and itll run hot , overheat , and fall to pieces .
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