How did GM balance 69 rotors?

[Bullshark]

I pulled out an old rotor that I have been storing since my brother launched his 69 upside down in a creek back in 74. I am pretty sure this is just how it looked back when we parted it out. My question is, did GM balance this rotor by drilling holes in the center hub section....see pic. None of the other rotors on my 69 or the two 70's have rotors like this one. The other two remaining rotors off the this same parted out car did not have holes either. I have not seen this before, Is this stock?????? Bullshark

[Red69]

The rivits in the roter indidate that it came from the factory....

I cant explain the holes....

Mt factory 69 did not have any holes...

[eileenthevette]

My '69 has 3 holes in the side of this rotor...



These 2 rotors don't have any holes in the side, but they have extra holes in the face. Should be 10 - 5 lugs + 5 rivets





And the last on doesn't have any holes, side or face.

And I know these rotors had never been off...because I drilled out every G@#D%*# rivet myself!

[mayberg]

The extra holes are for adjusting the park brake mechanism. I thought there was only one - maybe there is supposed to be two.

So, 5 for lugs, 5 for rivits, and 1 or 2 for adjusting the park brake on the rear.

[eileenthevette]

You're right. I went back and checked my tags, and those were both the rear rotors. Sorry, this is my first frame off and I have carefully tagged everything, guess I should read those tags every once in a while. Duh!

So back to Bullshark's question...He has 1 rotor out of 4 with 4 large holes, I have 1 rotor out of 4 with 3 large holes?

How could 3 out of 4 rotors balance so well that they need no holes, and 1 rotor out of 4 is so far out of balance that it needs so many large holes drilled into it?

[JohnZ]

Originally Posted by eileenthevette

How could 3 out of 4 rotors balance so well that they need no holes, and 1 rotor out of 4 is so far out of balance that it needs so many large holes drilled into it?

There's a lot of part-to-part variation in wall thickness in sand castings.

[corvetteload]

Good luck with those rear emergency brakes, virtually impossible to get at. I left them off my 75.......................B

[eileenthevette]

That's why I never used the holes to adjust the parking brakes. It's tough to adjust something that is rust welded together.