A bolt including its thread is quite a complex modelling operation to do manually. However, since it’s easily manufactured by a machine, so likewise you can create a bolt in Blender that can be easily generated parametrically. We’ll use the Bolt Factory add-on supplied with Blender to make it:
Instructions to install an addon in Blender can be found in the FAQ here
To insert a bolt, use the standard 
combination to add an object and click Mesh –> Bolt:
Once you have set your parameters, click in the viewport to complete the object.
Changing the parameters can have varied results:
If we re-set the defaults and change other parameters we get:
To finish off the newly generated bolt, we would shade smooth:
