I was wondering if there is any difference between an 850 left hand petrol tank and a mk1 cooper s left hand petrol tank?

The senders are different. You cannot get the 850 sender units anymore.

They screwed in, where as all others were a turn to tighten type washer thing that kept them in

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So therefore you can't put a cooper s sender unit in a mini 850 fuel tank?

So then are deluxe fuel tanks the same as cooper s?

Yes, identical.

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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.

I see why my fuel gauge doesn't work therefore I will go ahead and get a deluxe or later fuel tank with sender and that should fix my problem.
One last thing is that I need to vent both tanks. According to my readings the rubber pipe from each tank runs under the rear parcel shelf and is joined by a tee and then where does it vent out?
Pictures would be helpful

Thanks again

My understanding is they each run across the parcel shelf, but exit separately through the floor.
LH tank through RH floor grommet, RH tank through LH floor grommet.
This prevents fuel from the higher tank overflowing the lower one, when parked on an angle.

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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.

I can convert the 850 sender to work with a later fuel gauge and vice versa
The changeover date is sept 64 which means that early English cooper s 1275, 1071 and 970 along with 997 and most Aussie 998 are in this category
I run a clubman long range tank in my 997 and an even more obscure combo of two tanks in my mini marcos

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1960 Morris Mini Minor