An alternative fuel tank mount for the late widecase fuel tanks is the old Renault 12 exhaust bobbin. Both threads are M8 and the bearing area between the tank and the frame bracket is larger than the standard bobbin. I coat the galvanised areas with a smear of copper grease before assembling and the last time I bought any was back in the 1990's. After a little hunting, I found that they are still available from any competent motor factors.
I attach tthree images, one of an oblique view of the bobbin, another giving the dimensions and finally the part number codes. Cost for a pair was just shy of £2-50 inc VAT.
Bill
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