Tech Tip: How to Cross-Drill your Rotors

by Al Peterson
Competition Coordinator

Have you imaged how your Z would look with race inspired cross-drilled rotors? But you just couldn't justify the added cost of them. This article will outline the steps that you can do for a few dollars and an afternoon of work to have those great looking cross-drilled rotors on your Z. There is a lot of debate on the value of cross drilling and/or slotting, it seems that everyone has an opinion about it. I'm not here to change anyones opinion, but to document the process that I went through to achieve the look for the rotors on my Z.

Developing a Pattern:
The first step is to develop a pattern for drilling the rotor. You will need to determine the overall diameter of the rotor and the height of the portion of the rotor that the brake pads rides against. From these numbers, you will determine the inside and outside diameter for the sweep area for the rotor. The outside diameter is equal to the overall diameter of the rotor and the inside diameter is equal to the overall diameter minus twice the height of the potion rotor that the brake pads ride against. For example, my front rotors measure 12" overall and a height portion is 2.125", therefore my outside diameter is 12" and the inside diameter is 7.75". Always check both side of the rotor, they may be slightly different on vented rotors. Now that you know the diameters of the rotor, you need to make a drill pattern for your rotors. This can be done many different ways; I drew mine on a CAD system on my computer, using an alternating pattern of four holes and three holes in a sweeping arc. The pattern is not important of us, just look at the number of differing patterns available on the market, everything from large number of small holes (over 100 holes) to small number of larger holes. (Less than 35 holes). I choose a pattern that I thought looked good. The two things that I learned in doing this was to keep the diameter of the hole between .187" to .250" and keep the holes near the edge at least 3/8" from that edge. The other method for creating the pattern is to Xerox a pattern from a book or on-line and using a copier to size it to match your rotors. Note you will need to do it in halves since most copiers are limited to 82 by 11" copies.

NOTE: I recommend that you have the rotors turned before you cross-drill them. Pep-Boys did mine for $6.00 each.

Making/Drilling the rotors:
Once you have a properly sized template you are ready to mark the rotors, cut the pattern so that you can tape it to the rotor. Use a center punch to mark each hole on the pattern. If this is a solid rotor, you are ready to start drilling, for vented rotors you need to turn the rotor over and repeat for the other side. Repeat process for all rotors that you plan to cross-drill.

I would suggest that you purchase one new drill bit for each rotor that you plan to drill. I used .187" drill bit in my drill press to drill the holes my rotor, you can use a hand drill, but make sure that you hold the drill straight. For vented rotors, drill the hole until you go through the first side, don't be lazy and drill through both sides at once. For solid rotors, you can drill through the rotor in one pass. Don't worry about the roughness of the rotor, the next step is to countersink the holes and this will finish up the rotors.

You will need a countersink bit for the next step, they can be purchased at any Home Improvement or Tool Store for less than $5.00. The purpose of the countersink is to take the sharp edge left by the drilling operation. Again, using the drill press drill with the countersink bit about .05". This is where a Drill Press is great, because you can set the depth of the countersink and you buy a small bench top Drill Press for less than two hundred dollars. The key to being successful at the step is that you make each countersink the same. So be very careful!

Conclusion:
Now that you have cross-drilled your rotors, lets finish with a few details. First pick up some High Temp Paint, VHT make a glossy version of their High Temp Paint that is great for painting your calipers. Remove any rust from the rotor with sand paper to prepare it for painting with High Temp Paint. I painted my rotors Flat Silver and my caliper a Gloss Red. This will help makes them stand out from behind your wheels.

preparation with pattern
Drilling
Camfer the Hole
Finished on the Car