I just spent a bit of time trying to find a thread I'd read sometime in the past discussing rotor thickness. Eric mentioned that he has turned them below spec. due to their somewhat over-engineered original thickness. I just couldn't find the thread.

One of the sedans I'm driving has an annoying pulsation on stopping that I'm pretty sure is due to a warped front rotor. I haven't yet measured them, but wanted to either find that thread or get opinions about how much beyond the spec thickness I can turn down to.

I have rarely turned either drums or rotors, and try to avoid it, but if I get the wheel off and get excessive runout I am willing to turn them if, it's possible.

The shop manual lists .500" to.504" as new and .480" as reconditioned, but no minimum, so the question is what's the minimum we're comfortable with? Also would one turn them both to match or just the offending one?

thanks

Brake rotors seem like a bad place to experiment. If Eric says .455 is accceptable then I'd take that as gospel. Here's my shade tree way of checking for obvious warp: after visually inspecting for grooves or scoring, I lay the rotor on a flat machined surface. (I used my Rockwell unisaw table before my cousin gave me a truly flat granite stone of the type a machinist would use). Bad warps will be obvious like the scoring, suble differences might be found with a feeler gauge.

Setting up a surface gauge while the rotor is still on the car might work except they are so sensitve that it would probably just lead to confusion.

I finally found a minimum thickness by reading a Swedish version of an original green book Volvo manual that I found online that was originally published in 1974.

The official Volvo specification is 11.56 mm which is .455".
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Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502

To me, it depends on how many miles you plan to drive on them, how carefully you intend to check them, how you drive - and, more importantly, will someone else drive the car?

I would also consider adding shims to the pads to make up the space and make the throw of the pistons stay within normal limits.

Seal failure would be my biggest worry.

I would want my emergency brake to be 100% also. no rusty cables or parts.

The bad news is that turning a warped rotor seldom fixes the problem. It won't hurt anything, but if the warping isn't caused by overheating there are hard spots or other 'structual' issues. If the runout is caused by overheating, one of the calipers needs attention and the rotor still might be junk...

The Volvo manual lists difference in thickness as 0.001". Haynes lists the minimum as 0.480". After that, I think you are on your own.

Greg