Fan Belt

I've just had a new (second hand) generator put in my 73 Super Bug and was wondering how tight the fan belt should be?

I am getting an occasional "rattle" from the rear end and was wondering if a tight belt would cause this?

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its not likely,
more like a rumbling/groaning from the bearings if its to tight

you should be able to get 15mm deflection between the 2 pulleys