First time checking rotor runout-- As I turn the rotor, the gauge dips below my 0 mark as much as .005 and then goes past the 0 mark as much as .004. I can understand shimming at the lower spots but how do you adjust for the higher ones?

 

If you shim the low spot by .009, you should be good to go.

 

You never shim by the amount you are out. If the total runout is .009 you would shim about 1/2 that amount, check and adjust as necessary.
At the same time the wheel bearings must be also set up properly or they will add to the runnout.

 

I would move rotor around the hole patern till you get the least runout.
Then shim, dont forget to mark it, if you ever pull the rotor in the future.

 

Both good tips. I drill little holes about 1/8th deep and fill them with red paint. It makes future assembly easy to put it back to the best possible fit.

 

Help a simple guy like me out.I thought when a rotor has run out and has been moved around the lug nut pattern dosnt it then get turned?How do you shim a rotor?

 

The way I do them is after I rebuild a spindle and it has new bearings set to .001-.002" I tap the rivet holes 3/8-24 and use a 3/8-24 x5/8 flat head to bolt the rotor on. Doesn't matter if they are used or new rotors out of the box they're going to have runout. Many times the spindle flange has a few thousands in it that adds to the total.

When they were riveted and faced in a lathe they were concentric with the spindle centerline. When the rotor is removed or replaced this is lost many times. If you are reusing originals then clean off both mating surfaces and start at that point,no shims. Snug the flat heads down uniformly and check. It is most important that you use a good indicator that doesn't stick and a solid base. The mag bases alone will not work in most cases they'll move enough to throw you off. I use a c clamp to hold them on solid.

Your "feel" is also important, if you turn it too fast or jerk it then your indicator will give you a false reading. I very lightly rotate the rotor while watching the indicator. I use a china pencil and mark the lowest point nearest a stud. That's where I start.

I have a full SS shim set with hole punch. The shim stock comes in .001" increments from .001-.010, then .012 .015, .020" I'll punch a 1/2" hole in the shim so it doesn't bind on the stud threads. In this case I would see if the runout tapers from .009" to .004to.002- .000" or does it drop right out? If it tapers then I would split the difference and shim at the 2 studs. If it drops out then look for a burr, rust, something causing it at that spot.