Amazon fan belt tension
 

Can anyone advise please on the tension that my fan belt should be set to. I have a 1964 B18B with standard dynamo, in the past I had a dynamo fail as a friendly mechanic left the belt too tight and ruined the bearings, hence my caution.

My Haynes says: Deflection of .39 inches (10mm) with a pressure of 15.5 lbs (7kg).

Figures above are used on the longest section of the belt.
Not very scientific but have the belt as loose as you can get away with. If you can turn the engine over using the fan it might be a fraction too tight. If it slips a bit but you can turn it over if you press the belt in with your finger it's probably about right. This will depend on the smoothness of the pulley faces and the age of the belt. A few variables there. Regardless, make sure the belt is in good condition.