I THINK the thickness is nominally 1/16".  

The rubber sender gaskets are something I keep a couple of in my spare parts collection.  One or two spares are always good to have.  

Thanks guy for the tech info.
i will check locally.

Still looking for the thickness.

looks like it may be easier and cheaper to purchase from host!

Agreed.  Use Viton or Buna-N (Nitrile) sheet.  Do not use the red stuff from the hardware store as it is typically for water service.  Also, never use silicone rubber or RTV sealant with gasoline.  

I would be afraid of the red rubber from the hardware store for a fuel sending unit gasket, no problem making your own but I would buy some Viton sheet stock, which is fuel and alcohol safe.

Thanks Willie

the seal/gasket you refer to is for an earlier mini.
looks like the cut over to the new unit occurred in FEB64 (Parnell).

the part I need is ARA1502.

rather than buy and pay postage I should be able to either source one locally or make my own.

so far I have made one from a red rubber gasket from a hardware store.
looks good so far.

i will also be using this gasket on the top of the rear subframe where it meets the boot.

Our host sells this gasket for early tanks.

https://www.minimania.com/part/2H1082/Classic-Mini-Fuel-Sending-Unit-Cork-Seal-Early-Model

I believe I may be OK with the orientation of the unit.  I noticed the metal guides (2 large, 1 small) as well as the matching notches on the fuel tank itself.

so... I only need the specs for the seal/gasket.

‘66 S with dual tanks.

I have the tanks out at the moment.

is there a proper way to install the fuel sending unit so the float is in the proper position?

does anyone have the measurements for the seal/gasket - inside and outside diameters and thickness?