In reply to AlanLittle:
There is no real answer, there are quite a few different types of bolts out there and with some it matters the nut is tight and with some it doesn´t. Some you can overtighten and some you can´t.
Standard Euro sizes are 10 & 12mm so a 17mm/19mm spanner is normal. Tighten them enough so the hanger doesn´t move easily, the different bolts require different torque but if you don´t know what they are anyway it´s a problem!
The entire bolt may spin in the hole in which case you can´t tighten it and fairly often the nut is seized onto the stud and won´t tighten either, some people are even misguided enough to Loctite the nuts as well preventing any maintenance.
Personally my advice is if you don´t know then leave them alone and try not to fall off.