Does anyone out there know, if it is common to purchase front rotors for a volvo from aftermarket makers i.e. zimmerman, to find that in the machining process, a new rotor is found to have a 5 to 6 inch grooved out area between the two inner surfaces of the rotor? the outside is normal, but the inner area has this crescent shaped moon area cut out? I purchased two zimmerman rotors from a reliable source and for the second time, one was completely normal and the other was missing some metal in the shape I indicated. I was told by the dealer this is completely normal, that they find this on other brands as well (ate) and would not affect the function of the rotor. Any comments would be appreciated as always. thanks!

My experience with a variety of rotors on several 940s..Zimmerman have performed superbly, as have ATEs and Brembos.

As others posted, the inexpensive rotors from China/Taiwan are, IMHO, a waste of time and money.

just my $.02

Ther are a few ways a mfgr can remove metal from the heavy spot to properly balance the casting. Drilling holes is one way - disc grinding is another (it leaves the rounded pattern you described). The cheap Chinese mfgrs may not balance rotors or drums at all.

It sounds like you are describing an area which was ground off at the end of manufacturing to balance the rotor. This is common and is the sign of a quality brake rotor.

John