Reliant Rebel 700 1967 Workshop Manual page 21

Fan Belt
A single "V" belt is used to drive the dynamo/alternator and water
pump from the crankshaft pulley.
Correct fan belt tension is important otherwise the belt itself could be
damaged or undue strain placed on the dynamo/alternator or water
pump bearings. If the belt is too slack poor charging of the battery
could result.
There is provision for fan belt adjustment by pivoting the
dynamo/alternator on its mountings. It is vital that this adjustment is
released when a new belt is fitted otherwise any attempt to strain the
belt over the pulleys can easily damage the belt.
To adjust belt tension
The correct tension of the fan belt is such that if the belt is deflected
at a point midway between the dynamo/alternator pulley and the
water pump pulley the total movement should be 13mm (1/2")
(Figure 6). Figure 6 Fan belt tension
Page 21