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[MattiFin]

Which one exactly of these are are you referring to? https://www.mcmaster.com/disc-springs/

IIRC recommended torque for the M6 thread on the soft aluminium terminals was 4Nm.
4 Nm translates to 3.6kN clamping force on M6 thread (slightly lubricated thread)

Values for metric coarse threads VDI 2230

The table lists maximum permissible tightening torques and the resulting maximum preload for hex cap screws and socket cap screws.

www.bossard.com

3.6kN is approx 800lbf so the disk spring should be somewhere between 50% compression and 100% compression with that force.
Metric DIN 6796 standard washer would be obvious choice but these are too stiff for the extremely low toque the aluminium terminals are able to hande.
For example this looks suitable:

McMaster-Carr

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.

www.mcmaster.com

or this https://www.mcmaster.com/9713K412/

 

[rhino]

Does that mean you couldn't use a torque wrench if you are using a Belleville disc spring?