Bit the bullet and took the transfer cae off my Cooper S engine to see what idler washers that had in it (they are going back in)

Quite interesting to see the oil grove is quite a bit deeper and wider that than the early washers, but not as wide as the A+ ones

The nominal thickness of the genuine "red" transfer case gasket is approx 0.035".
The same problems of wrong thickness aftermarket gaskets apply to other critical areas such as diff side cover gaskets and the engine to gearbox gaskets.
The original thickness of these is approx 0.009". Many aftermarket ones are too thick. This means you have to reshim the diff bearings even if you haven't replaced the bearings and the backlash in the idler gear train can be too much.

Edit: Check the idler clearance with the gasket you intend to use, torque up the case to gearbox nuts, check the idler gear clearance and use the same gasket for final assembly.

RonR

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68 build Mk1 Cooper S
72 Clubman GT project. Total restoration required