IT WORKED FOR ME: 

How To Drill Disc Brake Discs: 
Author: Mutt / S&Y Rychener

I need to first credit Bill Unger for showing me some easy steps to the layout of the drilled disc. With knowledge of my own, we have put together an easy how to article for first time disc drillers. This is an easy project, but not a "no brainer". Remember we are dealing with brakes, which can become a life and death matter, using this information is very much at your own risk. If you have no brake repair experience, save this task for later. There are other methods I'm sure, so this article is not all inclusive of drilling.

Yamaha RD discs with the removable center are by far the easiest to layout work on. More difficult is the non-removable center disc and gang drilling double discs. You may decide to omit steps to meet your application. The first photo shows the tools I use for the layout of the hole spacing. The giant degree wheel is an enlarged photocopy of a degree wheel laminated in plastic. The crooked straight edge is a flat aluminum bar stock, bent to fit over the center hub. I have an old RD axle stub that I faced flat and center drilled for the trammel pivot point. The rest are standard tools.

I start by coating a clean disc with Dykem. By placing the disc evenly centered on the degree wheel with two mounting holes centered in line with TDC and BDC, I scribe my first reference line. Being careful when using the bent straight edge, I make sure I never move the relationship of the disc to the degree wheel and I always scribe on the same side of the straight edge, as to not be one width off on the scribed lines. By indexing the straight edge every fifteen degrees on the degree wheel, the result is a series of evenly spaced radial lines on the disc. This can be seen in the second photo and figure #1.

Next in the process I use a front RD wheel with a wheel bearing only on the brake side. I place the axle stub through the bearing so the center drilling is facing up. One trammel point will pivot on that center. Then I place the disc on the wheel. I determine how many rows of holes and the size of the holes, before using the trammel to scribe the circles. Use of the trammel is seen in the third photo.

NMRA, the Sanctioning Body that I drag race my Yamaha, which races under AMA/ProStar rules, has specific safety rules on drilling discs. The important rule to this article is; Maximum hole size 1/2" with all holes countersunk. No two holes closer that 1 1/4", center to center. If you are going to race, check your safety rules too. Street bike owners have a little more freedom, but don't "swiss cheese" your disc until web cracking and lack of brake friction surface becomes a problem. You assume that risk if you drill.

Three rows of 1/2" holes will not work for me on 1 1/4" centers, either the outer holes are too close to the edge, or the inner holes are into the center webs. Two outer rows of 1/2" holes and an inner row of 3/8" holes work out nicely. I scribe a point 3/16" in from the inner edge of the braking surface at the junction of a radial line. Here is where I scribe my first circle with the trammel point. Next I measure 1 1/4" diagonally to the next radial line; this junction point forms the measurement for scribing the middle circle. Again using the same diagonal 1 1/4" to the next radial line forms the outer circle. It is at these circle and radial line junctions, that I use a center punch for marking the holes. Shown in photo four. One radial has an inner and outer punch mark the next only has one in the middle. So on around the disc. When finished I check that all the punch marks are staggered. Mathematical wizards can figure out more or less rows and holes, you can change the process to suit your skills, desires and tools at hand. I then bolt the dual discs together, saving duplicate layout work, then moving on to the drill press.

Before the shavings fly, allow me to state that brake discs do not drill like mild steel. Sharp, high quality, high-speed steel drill bits fill the order. Drill bits made of "unobtainium" are nice but not needed and carbon steel bits can't make the cut. A standard 118-degree included angle drill point works fine, if you desire this angle can be reduced but not increased. A one horsepower or larger press with very low speeds works the best, Small, fast, 1/2 horse bench presses and hand drill motors loose the battle to bit chatter. The ability to accurately re-sharpen bits is also key. You will probably experience some bit chatter as the bit breaks through old glazing on the disc surface, slow the feed. If you are using a gear drive press be sure to clamp the work piece. I feel comfortable holding the work on my press, as the belt will slip if the bit grabs. This occasionally costs me a broken bit, and then I re-grind the shanks as miniature boring bars for my lathe. Use your best judgment on mounting the work piece.

I set the drive pulleys for 420 RPMs and use a 3/16" bit to drill pilot holes at all the punch marks. I use WD40 as a lubricant and one bit sharpening after drilling half the pilot holes. Then I step the drive down to 290 RPMs and chuck in a 3/8" bit to drill the inner circle of holes. The larger bits require re-sharpening after drilling every four to ten holes. The next drive step to 170 RPMs and a 1/2" bit completes the drilling as shown in the fifth photo. The shavings are hot and razor sharp, I always avoid a nasty bite.

Using a 5/8" countersink bit, I give all the holes a light chamfer on both sides of the disc. Lacking a countersink bit you could just use a much larger bit to cut a chamfer, though the finish will probably show chatter marks. The chamfering is shown in photo six. To wrap up the project, I clean the WD40, shavings and Dykem off the disc with brake cleaning spray.

As a few parting notes photo seven is the final result; waiting to be installed on my new drag bike chassis. Next to the wheel is my trusty street/strip RD350lc. This bike uses RD400 wheels with an aluminum center disc with two rows of holes on the rear, the front has one steel center disc and an lc caliper. Gang drilling allows a perfect match, including imperfect mistakes, so as I said this project is not a "no brainer". I am lucky enough to drill them without error. You can gang several discs to the depth of a drill bit.

I like the discs so well, I wish I had ganged on a third disc. I use one drilled disc, along with transfer punches to easily center punch and drill the disc for the front of my lc. The only drilling change is 7/16" holes on the inner row of holes. Drilled discs allow the use of padlocks or disc locks. Make a thief carry the whole bike not just pick up the back and roll it away! May this never happen to you. Using an inaccurate bath scales, I find that each drilled disc looses about one pound of weight or gains one pound more horsepower! The final photo is Bunger's street RD400 café racer. The holes in the disc are just a bit too close for racing legal, but this bike has had some hot canyon carving miles. The disc works great without cracking or bluing. Good drill job Bill Unger, thanks for your educational help. Thanks also to my wife Yvonne, for taking good photographs. As always, have fun!

Mutt

Ps: You may contact Mutt at <desertsage@worldnet.att.net>