did the search, came up dry ... any help?

[chinrest]

can xs and xj tanks be interchanged as is, or do they need adapting? thanks

[CatatonicBug]

They will not interchange. In fact, the XS Standard and Special tanks will not interchange with each other either (without some modifications anyway).

[Bigfoot]

Yes, you can put an XJ tank on an XS, but it does take a little modification.

[fredintoon]

- - - the XS Standard and Special tanks will not interchange with each other either (without some modifications anyway).

Hi 'Bug,
to clarify that, The Standard and Special tanks themselves will interchange without being modified at all.
To get the Special's seat on with a Standard tank you have to crop the seat's nose or at least notch it to fit the Standard tank's fuel cap pocket drain tube connection.
I'd suppose putting a Special tank on a Standard would leave a gap there?
The fuel senders need to swap but only if you want the security of a gauge or a warning light because the senders work in different ways.
The gas taps have to stay with the tanks because they have different bolt centres.
Which means the Standard owner will have to either find a spare octopus or remember to turn the gas off.

[CatatonicBug]

Exactly... "Some modification". It's not a simple cut-and-paste.

[Bigfoot]

can xs and xj tanks be interchanged as is, or do they need adapting? thanks

Refer to the original question. They will work, with some adapting.

[Jerry]

In addition to the mounts, the fuel sending units are different and would need to be changed. Petcocks are not the same either, as the later ones are vacuum-operated in stock configuration and not all XSs have the "octopus" that the XJ and later standards(?) use. This can be changed / bypassed but does require some tinkering. Just a couple more things to consider.