Template Drilling Front Brake Disc

I am looking for a template to help me drill the brake disc on my 1972 Commando. I am partial to the look of the hole pattern on the Old Britts site: https://www.oldbritts.com/14_061885.html

I know many others have done this: drilling-norton-front-brake-disc-t25008.html

It looks like everybody makes their own drilling template. Something I am capable of. But does anybody have an electronic file that when printed produces a drilling template?

A bit of research produces two design considerations:
1. the size of the hole should be not larger than the thickness of the rotor, so about 1/4 of an inch.
2. the holes should be distributed so that the pad "sees" holes across the length of its face, so as to create even disc and pad wear.
Interestingly, it seems the "Old Britts pattern" meets neither of these design considerations.

Having read most of the threads on this, I appreciate that some folks think that cast iron discs should not be drilled. Noted.
And some folks like to share photos of their cool stainless lacey after market discs. Noted.
I hope we can use this thread primarily to share various drilling templates for the original rotors.

Stephen Hill

I don't have a file or even a template, but I can recall how I (and a friend with a milling machine!) did it.
The machine was set to "click-stop" every 30 degrees, to make 12 holes per full rotation. Marked the disc to keep all holes within the "pad band".
I punched the outermost holes (looks like we used a 19/64" or Letter M or maybe 7.5mm bit? I can't remember), moved the index "forward" 1/2 inch and inward the same 1/2 inch, punched 12 more, repeating till I had 4 holes per group, 48 total (rather than the 36 total of the Old Britts pattern). Then recalled the advice to chamfer them all, and went about the whole routine again with a "countersink" bit, just barely biting the surface.
It wd probably have saved some time if we had switched out the bits 4 times and done the chamfers with each indexing? I don't know, but it wasn't very hard to recover the original placements for the chamfering. And of course, we had to flip the part to relieve the back side of the holes, too.
I don't think my design meets either of your criteria either, but it's done now, no going back.

Send your rotor to DBR. Gary resurfaces the swept area and can drill any pattern you like. Worth the peace of mind to know it will turn out as desired. I like the idea of drilling larger holes in the carrier portion of the rotor to reduce weight.

Cast steel not cast iron

I got one right here in my hot little hands.

The large hole near the center are 7/8 and the brake surface hole are 5/16th.

This process is stupid easy and the cast steel drills like butter or cast iron (no need to spit hairs in this regard). You slip and tape the template over the hub side, transfer with a center punch, start with an 1/8 drill and work up. To reduce chatter on the big 7/8" hole, do not step up too close to the finish size, say 1/2" maybe 5/8". DE burr thoroughly.

For what it is worth, the RGM floater is 9/32", roughly 7mm. I am sure 1/4" would be fine but I know when I did the 5/16" it work a treat.

Thanks Pete,
I saw your template in an earlier search. What I was hoping for was to create a thread with a number of files that could be used to print full size templates. Such a thing exists on the XS650 and CB750SOHC sites.

Your template pretty much replicates the Old Britts pattern. Including its flaws, if you don't mind my saying so. Apparently if the disk is .250 thick, the holes should be no larger, and your pattern calls for 5/15. And that pattern leaves portions of the disk pad not "seeing" any holes, which leads to uneven wear resulting in ridges. I spent some time on the draughting board trying to come up with a pattern that met these criteria, and the smaller holes look less interesting, and it ended up calling for a heck of lot more holes, which raises other issues, like time, strength, reduction in surface area, etc. This is still a work in progress.

gripper, I beg to disagree. I strongly believe the disk is made of cast iron, not cast steel. I have a chunk of a broken disc, from a friend's Norton who t-boned a taxi. The disk shattered into fragments, no bending whatsoever. I have also made some test drillings in the material, and it drills exactly like cast iron, you can almost see the graphite in the chips which come out as a powder; the material drills nicely without lubricant. This has been discussed elsewhere, and I recall LAB pronouncing it cast iron as well.

Stephen Hill

Oh, ok.

Stephen Hill said:

This has been discussed elsewhere, and I recall LAB pronouncing it cast iron as well.

Click to expand...

Well, that has been my understanding, and it is referred to as "cast iron" in the factory manual (section H).

Pete,

Did you notice any difference once you drilled the rotor? Stronger braking? In the wet? Brake pad wear? Rotor noise?
Aside from the obvious cool factor, of course.

Stephen Hill

Stephen Hill said:

Pete,

Did you notice any difference once you drilled the rotor? Stronger braking? In the wet? Brake pad wear? Rotor noise?
Aside from the obvious cool factor, of course.

Stephen Hill

Click to expand...

I admit, a lot of it is aesthetics, however, accompanied with a good after market master cylinder it was noticeably better.
that old OEM caliper was the weak link in my opinion.

This works very well, only holes through the disc are close to the centre.
Used a nubbing tool
Regards Mike

L.A.B. said:

Well, that has been my understanding, and it is referred to as "cast iron" in the factory manual (section H).

Click to expand...

When you drill into it, it comes out as chips, which is a sure sign its cast iron.
And you'd think/hope/trust the manual knew the difference. !

And it RUSTS like cast iron too !! (joke).

Here's mine - I had a CAD file for it but I've changed jobs since then & may have left it behind...



Cheers, Jon

H & R Tool & Mach Co  
Machine shop in Caseyville, Illinois
Address: 19 W Scates St, Caseyville, IL 62232
Phone: +1 618-344-7683

I believe the above information is correct.

Gary will grind the chrome off keeping the faces parallel and drill in any number of patterns. Getting the chrome off combined with a 13mm sleeve in the master cylinder and ferrodo pads made my brake quite usable. The swept drill pattern and lightening holes contribute to the cool factor if nothing else.

Price was very reasonable and the service was quick.

As soon as I get the other one from GPZ, I'll be taking two to Gary's shop which is just a few miles north of here. Just to flat grind the one off the flood bike was 50 bucks.

For those who didn't see it in another thread, it was mentioned that Gary Hollowich of DBR is looking to sell his business and retire.

I don't know the particulars, but you'd best send him your discs right away!

The surface grinding / face matching that he does is referred to as "Blanchard" grinding.

Plain

Drilled

Boxerfan said:

Here's mine - I had a CAD file for it but I've changed jobs since then & may have left it behind...

Cheers, Jon

Click to expand...

I like your disc job a lot, significant weight reduction plus holes overlapping drilled (unlike some other home made templates)
Could you try to dig up your CAD file to share it :?:

Remember, the chamfers on the holes are decorative only. :idea:

concours said:

Remember, the chamfers on the holes are decorative only. :idea:

Click to expand...

sorry, but this is a mistake
the chamfers are alright in the dry , but can be lethal in the wet , why?

imagine a layer of water on your disc , and the diameter of the hole(chamfer ) is , let say 4mm
the ,as you apply pressure on the pads , they close up the gap between pads and disc and trap the water in the drilling/hole
but if your drilling has not enough room to take the water because it is only ,lets say 2mm , than the trapped water actually prevents you to put pressure on the pads to the rotor
the 4mm and 2mm is only to make clear what i mean , and only to avoid people to put a NICE chamfer on your drillings
far better to grind your rotor after drilling

another thing one has to consider is that cast iron is not the ideal material to drll liberally , avoid drilling to close to the edge to avoid cracking,
although brembos of the 70s were drilled and cast iron , i have seen a few broken ones

i drilled and made mine floating(brembos on the commando) and found one of them had cracked right across, scary