69 2 piece rotors

[DONCZ28]

My June built Z28 has it's original 2 piece rotors . There are 6 holes in each casting.  I have seen many Nos 2 piece rotors with no holes. These holes are cast in, not drilled and equally spaced on 1 half.  How could they be for balancing?  Could they have been for cooling?  When did they appear or stop?

[DONCZ28]

My rotors

[ZLP955]

I was under the impression the hub-and-rotor assemblies were balanced as an assembly, and that's when the holes were made. I'll have to have another closer look at mine. Could it be light surface corrosion built up on the holes that makes them look like they weren't drilled?

[bertfam]

Can you post a close-up picture of the inside of one of the holes? From what you've posted, they appear to be drilled. Holes that have been cast usually aren't as smooth as those.

Ed

[DONCZ28]

The holes need to be cleaned up. Why are there 6 holes ,on both rotors . All the same diameter.  We need to see if there  are other rotors with a different number of holes. If the holes vary in number, they were for balancing.

[x66 714]

Although that rotor was used on most of the GM cars, except Impala, I have only observed those on Chevelles/El Caminos. I doubt that means anything...Joe

[KurtS]

I'm pretty sure those are balancing holes and are drilled, not cast.

[Dusk Blue Z]

I will agree with Kurt and others those holes are for balancing the rotor assembly. One of my original 28K mile rotors did not have any balance holes and had this paint mark on it. The other side had three holes and no paint. My guess would be they were all checked, and anything out of spec got balanced, and any that were in spec got a paint mark to show they were checked.

[jdv69z]

Here's some pics of my originals.

[DONCZ28]

After measuring the distance between the holes,  the gaps are inconsistent  i agree they are for balance. The rotors had to be drilled independent of the hub or they could have  hit the  bolts that  hold the rotor in place. The bolt heads are right in front of the balance holes.

[Petes L48]

Were the rotors sold separately as a service part, or did you have to buy a complete (and balanced) rotor-hub assembly?   

[ZLP955]

Just found the driver's side rotor, it has 3 such holes, each spaced 2" apart from the next. The edges of the holes have a rough appearance, but it looks like surface corrosion and maybe some brake dust accumulation. Haven't found the passenger side rotor yet.

[bertfam]

Were the rotors sold separately as a service part, or did you have to buy a complete (and balanced) rotor-hub assembly?

It came as an assembly, P/N 3901098.

Ed

[169INDY]

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