Topic: Any good tricks for restoring the front brake disk on a CB750?

[markb]

I have a disk in nice shape but would like to improve the looks of the disk itself. I can chuck it on the lathe on the ID and I'm thinking maybe Scotchbrite or fine sandpaper. Any opinions before I try it?

[69cb750]

You can block sand on bench or on wheel, lathe not needed.

[calj737]

http://forums.sohc4.net/index.php/topic,175236.msg2043166.html#msg2043166

[markb]

You can block sand on bench or on wheel, lathe not needed.

True, but I figured the rotary motion would make a more uniform pattern.

http://forums.sohc4.net/index.php/topic,175236.msg2043166.html#msg2043166

That is a good looking disk. Do you know where True Disk is located?

[calj737]

https://truedisk.net

[MauiK3]

True Disk is the bomb, great work.
So far no squealing either.
And after you get it back you do this.
I used paint remover, wire brushes, elbow grease, patience.

[sixthwisconsin]

True Disk looks like they do great work. I notice from their web site that they will not thin the rotors beyond OEM spec. (6mm in our case)

I am running several bikes with Godffreys drilled & thinned (4mm) rotors and have not heard of any issues with them.
https://godfferysgarage.com

Good to have options.

[markb]

Those all look good but I don't think it matches the original grind pattern. This would be for my sandcasts so I looking for something closer to original. These others have more of a circular or swirl pattern similar to what you would get from a Blanchard grinder.

[calj737]

Those all look good but I don't think it matches the original grind pattern. This would be for my sandcasts so I looking for something closer to original. These others have more of a circular or swirl pattern similar to what you would get from a Blanchard grinder.

TrueDisk does Blanchard grind, for a reason. A AcotchBritr won’t do crap to the surface other than remove smootz. If you want originality (you asked for improvement) then best to buy a NOS rotor and re-rivet it to your carrier.

[seanbarney41]

Does anyone really know what these discs looked like before they were ever used?  50 miles of use is gonna establish a pattern that will stay forever.

[markb]

Sorry, I guess I should have been more specific in my first post. I wasn't trying to be argumentative. I asked for opinions and I appreciate all that I got.

TrueDisk does Blanchard grind, for a reason.

Just curious, what is the reason?

[markb]

Does anyone really know what these discs looked like before they were ever used?  50 miles of use is gonna establish a pattern that will stay forever.

Here’s what they looked like. I actually have a NOS K1 rotor (the hub is different on the earlier ones). The rivet idea might not be a bad way to go.

[MauiK3]

I totally agree about Blanchard vs. circular grind pattern. If absolute stock is what is called for, such as a very low number restoration, put the disc in a lathe or similar tool to put a circular pattern on it. In my case, I wanted them drilled so totally stock is not in the decision. I’m very happy with how they work, very light feel, no squeal (at least not yet), very predictable and progressive. Of course the dual discs feel a lot different than a single.
I have no scientific data but I wonder if the Blanchard grind makes the feel better. I sure like them.

[754]

 I will post more info later re original finish..
 But really sanding block on a lathe looks pretty close. And Blanchard does not .l

[calj737]

TrueDisk does Blanchard grind, for a reason.

Just curious, what is the reason?

Extreme tolerance of flatness of both sides (0.001). Not many other methods short of complicated setup in a lathe will get you there on a vintage rotor.

[seanbarney41]

Also, the stainless is too hard to be easily cut...thus the grinder.

Thanks Mark for the posted pic.  I can't imagine I will ever see a NOS rotor anywhere else.

[calj737]

Also, the stainless is too hard to be easily cut...thus the grinder.

Actually, stainless cuts easily with the correct tooling, though it is a different beast than carbon or cast.

[754]

 A few things, they are hard to surface or thin on a lathe due to resonance  or ringing or vibration.
 And while Blanchard grinding is efficient and fast,  it will never look stock  and that is a problem when you  restore a high end  bike, you need the right finish.

 Which I tried to set up for.. but I had to close down the shop.
 But I am  trying to setup a smaller place.
 And still have the grinding machine, and a very Skookum rotary table that we were trying to set up a drive  for... but nothing in the immediate future.

So basically picture a record player , the disc  is the record, the needle is a surface grinding wheel, it needs to grind say a thou or two per cut, and feed across the disc surface.. then it will look stock.

And I had the grinder working, the grindstone did a nice job on stainless, but I was turning a rotary table by hand, and feeding out... long enough to see that it could work... but it needs  a motorized drive.
 I hand cranked to see if it could work and I had the right stones to prove the concept.. but the tryout gave me tendonitis.
 

[scottly]

I've spun them in a lathe and used 150 grit emery cloth on a rubber sanding block. This won't remove much material no matter how long you do it, so any deep grooves won't clean up, but it might be closer to the finish Mark is looking for. Pics are a bit fuzzy..

[calj737]

I've spun them in a lathe and used 150 grit emery cloth on a rubber sanding block. This won't remove much material no matter how long you do it, so any deep grooves won't clean up, but it might be closer to the finish Mark is looking for. Pics are a bit fuzzy..

For a stock restoration, this would be my approach too.

[markb]

I've spun them in a lathe and used 150 grit emery cloth on a rubber sanding block.

I think I will give this a try. I have plenty of disks to experiment with. Thanks everyone for all the great info.

[markb]

Anyone know what the minimum thickness of the disk should be?

[Stev-o]

Anyone know what the minimum thickness of the disk should be?

6mm. Out of curiosity, I just measured mine and it was over 7...

[scottly]

Anyone know what the minimum thickness of the disk should be?

6mm. Out of curiosity, I just measured mine and it was over 7...

I thought it was 6.5mm? It's marked on the hub.

[Tom C]

Here is the disc from my 72 CB350 four with 6800 miles before I sent it off to https://truedisk.net/:


And here it is afterwards:


Very happy with the results! 

Hooray!  My first post on SOHC/4!