No, 20 Nm looks too high! The nuts are probably a 6 mm metric thread
size. If yes, then torque should be about 108 inch-pounds = 12
newton-meters (Source page 24-2 Haynes VOLVO 240 Series, ISBN 1 56392
136 7). A torque of 10 Nm corresponds to the recommended maximum torque
of a M6 8.8 grade bolt/nut. Info about bolt torque can be found on the
internet or in any decent Mechanical Engineering Handbook.
The setting isn't critical, just replace the gasket and work your way
out from the middle with a ratchet wrench. More than a few guys have
gotten themselves in trouble by misreading the foot inch reading used
for many valve covers as foot pounds and setting a torque wrench
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