Dumb-stall double disc forks

Has anyone here by chance had any experience with the Dunstall double disc forks that have the calipers cast into the fork legs. I got this bike with this front end that I thought looked cool and was somewhat rare but I am questioning if that was a good thing or a bad thing. Pistons were stuck in calipers, Brake shoes…..no longer available, chromed discs flaking and rusted. The corrosion of the aluminum pistons in an aluminum bore made them almost impossible to extract. All is restored now with new O-rings polished internal parts, cleared orifices, re-drilled and tapped hub for larger bolts holding the ground discs. I even splurged on the Colorado Norton teflon bushing extended top caps with the double lip seals. Re-lacing the front wheel will make this whole mess an expensive part of the restoration.
Question is……….Are these brakes any good or will this just be good for show and tell? I look with envy at the brembo single disc conversions I see on high end restored bikes.

I have run the Dunstall discs for 56 years. Most recently due to a little corrosion in the aluminum calipers they have been bored oversize, and I have fitted stainless sleeves and I made oversize threaded piston plugs. Previously I had a very good brake, which was far superior to the original Norton brake, however I have used up my stock of Don asbestos pads. So I reduced the dia and thickness of standard Norton pads so that they fit the Dunstall caliper. Now the brake is not so good. I am going to let them bed for a while and unless they improve dramatically I will be looking for different compound pads. So the answer is yes the brake can be excellent with a suitable compound.

With regards to your brake did you use edpm O rings as most other rubber compounds are unsuitable for use with glycol based brake fluid. Any minor corrosion in the caliper cylinder will mean the O rings will not seal. That will mean boring and sleeving as I have done or boring and fitting oversize pistons.

David

David,
Thanks for the thorough reply. It’s good to hear it is actually a descent brake. My Ducati with 2 finger braking has spoiled me and (maybe unfairly) become the basis for comparison.
I did not think of boring the cylinders oversized. I can see how that could be necessary I hope I got the bores clean enough to provide a good seal. I did use EPDM o-rings. A fixture to hold that fork lower square to the boring bar would be interesting, Did you do the work? I was able to find a company that “re-lines” brake material so that the original puck backing plate can be bonded to new compound. I do not know if the master cylinder was part of the “Dunstall Upgrade” so the diameter might not be right for correct feel at the lever but that evaluation will have to wait untill the first road test which is still many months away. I love to hear from owners with years of ownership and direct experience with upgrading these bikes Thank you for taking the time to respond.

The std Norton master cylinder is more suited to twin calipers than single calipers. The magic ratio is 27:1 which you can calc from this site

kommando,
Thanks for the reference, I am on my way out to check the ratio right now. Let’s see……area of a circle is Pi X Radius squared? I better confirm that, memory ain’t what it used to be.

tinkertoy said:

kommando,
Thanks for the reference, I am on my way out to check the ratio right now. Let’s see……area of a circle is Pi X Radius squared? I better confirm that, memory ain’t what it used to be.

Click to expand...

You just need to know pistons diameters and use the table Kommando posted.

But be aware… nothing you do is gonna make this brake like your Ducati… unless you fit a Ducati front end !

I had a friend mill the calipers oversize for the sleeve. Halray Brake, a brake cylinder reconditioning firm in Lismore NSW Australia sold me the stainless sleeve and they provided me with the adhesive for the sleeve, Cyberbond RH99. I cut and fitted the sleeve, tapped the caliper for the oversize piston retaining plugs and I made the oversize plugs to suit.

The master cylinder that was fitted to my bike originally, (as far as I know supplied by Dunstall) is the same as the master cylinder originally fitted to a k1 Honda 750. The piston diameter is 14mm which gives a ratio of 25 to 1 so it is very close to the sweet spot and good feel on the road. In Australia, master cylinder seal kits for the Honda k1 master cylinder are available from Puds for Parts.

David

Ando, Yes, tanks for that. Just confirmed my M.C. piston diameter is 14mm., probably fitted with the Dunstall setup. I knew that master cylinder looked familiar. I have a 1972 Honda 550 with the same part on it. Do you think I am I asking for trouble by continuing to run the aluminum pistons in the aluminum bores of the calipers?

If there was no pitting of the cylinder I do not see a problem. Any brake fluid leak due to pitting will quickly appear on the disc or fork leg, so if there is no evidence of brake fluid then no problem. Hope it goes well for you.

David

From the responses here, the consensus seems to be the Dunstall dual disc setup can be made to be a decent brake. The question to answer is, is it worth the effort. My experience with the Dunstall brake is it is a poorly engineered, poorly manufactured item. As has been stated, the aluminum pistons in an aluminum bore is one example. The relatively small diameter disc and round o-rings for piston seals is another. Commercial brake piston seals are square section for a reason.
For the same or less money you can buy a kit such as the one Don Pender supplies or CNW. With a little ingenuity you can put together your own system with a Nissin or Brembo caliper and proper sized master cylinder and a larger full floating disc that will stop better than the you can ever make the Dunstall dual setup work.
I ran a Dunstall disc on a cafe racer for years before removing it to build my own single disc unit that put the Dunstall to shame. In fact the stock disc with a 13mm master cylinder and Ferodo or EBC pads is better than my Dunstall twin disc ever was.
As with most Dunstall Commando items it was designed more with appearance in mind than functionality.

The name junk-stall seems fitting for a lot of his products.

bill said:

The name junk-stall seems fitting for a lot of his products.

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Yes, I am in agreement. I thought Dunstall meant hi-performance racing parts rather than poorly executed, half baked ideas about how to improve performance. The whole mess looks like shed built tinkering to me.

tinkertoy said:

Ando, Yes, tanks for that. Just confirmed my M.C. piston diameter is 14mm., probably fitted with the Dunstall setup. I knew that master cylinder looked familiar. I have a 1972 Honda 550 with the same part on it. Do you think I am I asking for trouble by continuing to run the aluminum pistons in the aluminum bores of the calipers?

Click to expand...

I just upgraded my axial 13mm Brembo MC on my CNW kit to a Brembo 15RCS radial to very good effect.



You may want to look at a radial MC.

When upgrading from an axial master cylinder to a high-leverage radial configuration, a common rule of thumb is that the radial bore size needs to be roughly 1mm to 2mm LARGER to achieve an identical hydraulic-to-mechanical feel.

Because the twin-disc Dunstall setup utilizes dual-opposed calipers (2 pistons per caliper, totaling 4 pistons across the whole system), it matches the total volume profile of a classic 14mm axial configuration.

The Brembo 15RC might be the sweet spot for this Dunstall build. Due to its radial lever pivot geometry, it performs exactly like a premium 13.5mm–14mm axial cylinder:

14mm Axial Surface Area: 153.9 mm² (The baseline target)
Brembo 15RCS Surface Area: 176.7 mm² (+14.8% fluid displacement)

The slight increase in piston area is perfectly balanced by the extra mechanical leverage of the radial lever layout. Additionally, the Ratio Click System (RCS) allows for easy fine-tuning depending on rider preference:

1. Setting it to 15x18 shortens the pivot, maximizing mechanical leverage to yield a deep, highly progressive modulation that perfectly emulates the target K1 Honda feel.
2. Setting it to 15x20 increases response for a crisp, short-travel vintage race bike feel.

Food for thought.

I have a Lockheed AP Racing caliper on my Mk3. It's really good, never use the rear brake and mostly 2 finger braking. Andover Norton do a kit. Mine has a billet adapter, with the original fork leg. The kit has a rather neat fork leg. Pricey though, but the braking is good.

Mart UK said:

I have a Lockheed AP Racing caliper on my Mk3. It's really good, never use the rear brake and mostly 2 finger braking. Andover Norton do a kit. Mine has a billet adapter, with the original fork leg. The kit has a rather neat fork leg. Pricey though, but the braking is good.

Andover Norton - FRONT BRAKE IMPROVEMENT KIT CP2195 (AP RACING /LOCKHEED) CALIPER LEADING c/w LEFT SIDE SLIDER - 13.1909

FRONT BRAKE IMPROVEMENT KIT CP2195 (AP RACING /LOCKHEED) CALIPER LEADING c/w LEFT SIDE SLIDER

andover-norton.co.uk

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Mark,
After looking at the options I think you have the best configuration as far as the right vintage look. I like the look and function of a competent single disc, especially if a floating disc is compatable.

gortnipper said:

I just upgraded my axial 13mm Brembo MC on my CNW kit to a Brembo 15RCS radial to very good effect.

Post in thread 'Muttster: A long, slow '74 resto-mod'

Jul 17, 2026

The Dupetruyn's contracture in my right hand has gotten worse in the past few years. This is fully stretched open.



I have to twist my hand thumb down 45 degrees to pick up a glass.

It had gotten to the point where I haven't be able to use my middle finger effectively enough when braking to give me high confidence. I had adjusted the levers down a bit and gotten some shorty cammed levers for my CNW Brembo kit, and that worked for a number of years.

So, I started using mainly my index and ring fingers to brake. But with the setup I had I found I just wasn't getting enough...

• gortnipper

• 

You may want to look at a radial MC.

When upgrading from an axial master cylinder to a high-leverage radial configuration, a common rule of thumb is that the radial bore size needs to be roughly 1mm to 2mm LARGER to achieve an identical hydraulic-to-mechanical feel.

Because the twin-disc Dunstall setup utilizes dual-opposed calipers (2 pistons per caliper, totaling 4 pistons across the whole system), it matches the total volume profile of a classic 14mm axial configuration.

The Brembo 15RC might be the sweet spot for this Dunstall build. Due to its radial lever pivot geometry, it performs exactly like a premium 13.5mm–14mm axial cylinder:

14mm Axial Surface Area: 153.9 mm² (The baseline target)
Brembo 15RCS Surface Area: 176.7 mm² (+14.8% fluid displacement)

The slight increase in piston area is perfectly balanced by the extra mechanical leverage of the radial lever layout. Additionally, the Ratio Click System (RCS) allows for easy fine-tuning depending on rider preference:

1. Setting it to 15x18 shortens the pivot, maximizing mechanical leverage to yield a deep, highly progressive modulation that perfectly emulates the target K1 Honda feel.
2. Setting it to 15x20 increases response for a crisp, short-travel vintage race bike feel.

Food for thought.

Click to expand...

Gortnipper,
Not sure what your “handle” means but still think it is pretty funny! I too have a compromised right hand from a nasty tank slapper get-off from two years ago. Tore the ligament that holds the three bones together at the base of my wrist. That allows them to rotate and get in the way of nerves. I can accomodate the problem by treating my hand much more carefully but still have pain after a longer ride.
I have never owned a bike with a radial MC but I understand the benefits. All my bikes are pretty old, only modern features on all are electronic ignition and the fuel injection on my 2006 Ducati which I can now appreciate after a long process of getting sensors, balance, idle mixture and performance ECU to work as they should. This restoration has me constantly asking myself how to balance a vintage look with increased reliability and safety. My other bikes are heavily modified but I like the look of the Commando so much I don’t want to spoil it with parts that are not period correct. Are there other modifications you have made that you would reccomend?

tinkertoy said:

Gortnipper,
Not sure what your “handle” means but still think it is pretty funny! I too have a compromised right hand from a nasty tank slapper get-off from two years ago. Tore the ligament that holds the three bones together at the base of my wrist. That allows them to rotate and get in the way of nerves. I can accomodate the problem by treating my hand much more carefully but still have pain after a longer ride.
I have never owned a bike with a radial MC but I understand the benefits. All my bikes are pretty old, only modern features on all are electronic ignition and the fuel injection on my 2006 Ducati which I can now appreciate after a long process of getting sensors, balance, idle mixture and performance ECU to work as they should. This restoration has me constantly asking myself how to balance a vintage look with increased reliability and safety. My other bikes are heavily modified but I like the look of the Commando so much I don’t want to spoil it with parts that are not period correct. Are there other modifications you have made that you would reccomend?

Click to expand...

I have had this Norton since about 1989. It was pretty bone stock until I started my rebuild thread and is now, like the title says, a non-stock Mutt. I would recommend most of my mods! LOL

Seriously tho, the ones I recommend most are the ones around stopping and handling:

1. Front brake mods - the full Brembo setup I have now feels like my Ducatis' braking. Get some better than stock sintred pads - EBC, SBS, etc. The radial MC is decidedly better and more adjustable than the standard 13mm axial mc. this is one place "vintage be damned". You want to stop. Period.
2. IKON rear shocks - fully adjustable and you can configure the "correct" look. Wont really break the bank.
3. Front fork damping kit - either the Lansdowne kit (which I have) or Kenny's NYC Norton kit. Makes a shit-ton of difference. completely hidden.
4. Vernier isolastic kit. New isos are completely needed if they havent been done.
5. Kegler mod
6. WM3 or WM4 rims. The stock rims are too narrow for modern tires. You want more rubber on the road.
7. RGM MZ Gold rear shoes. Bed them in with some sandpaper in the drum. Best rear brakes on a Commando.
8. Progrip 842 gel grips. Super comfy. No hand buzz fatigue.

gortnipper said:

I have had this Norton since about 1989. It was pretty bone stock until I started my rebuild thread and is now, like the title says, a non-stock Mutt. I would recommend most of my mods! LOL

Seriously tho, the ones I recommend most are the ones around stopping and handling:

1. Front brake mods - the full Brembo setup I have now feels like my Ducatis' braking. Get some better than stock sintred pads - EBC, SBS, etc. The radial MC is decidedly better and more adjustable than the standard 13mm axial mc. this is one place "vintage be damned". You want to stop. Period.
2. IKON rear shocks - fully adjustable and you can configure the "correct" look. Wont really break the bank.
3. Front fork damping kit - either the Lansdowne kit (which I have) or Kenny's NYC Norton kit. Makes a shit-ton of difference. completely hidden.
4. Vernier isolastic kit. New isos are completely needed if they havent been done.
5. Kegler mod
6. WM3 or WM4 rims. The stock rims are too narrow for modern tires. You want more rubber on the road.
7. RGM MZ Gold rear shoes. Bed them in with some sandpaper in the drum. Best rear brakes on a Commando.
8. Progrip 842 gel grips. Super comfy. No hand buzz fatigue.

Click to expand...

Gortnipper, That is a useful list, thanks. I am going to set up the front brake as I received it and see how it works, A radial MC might be in my future. Do you have a specific reccomendation on one? The few I have looked at were pretty pricey, in the $450.00 range. I have the scintered pads which should help the bite and removed the chrome off the funky Dunstall discs.
I am especially interested in the Ikon rear shocks, the RGM rear brake pads and the fork damping kit. I am not sure what the “Kepler mod” is but will look into it. I did get the vernier isolastics and am lacing up alloy rims albeit in the stock sizes. I have found that the narrower wheels and tires on vintage bikes are lighter and give a more nimble feel but do look weirdly spindly by todays standards. I also got a set of brand new Avon Roadrider tires with the bike so I wanted to use them.

Tinkertoy,..... What year is your bike? I didn't see you mention that, but maybe I missed it... As far as certain modifications and potential upgrades go, the year of your bike tells the people here what your stock parts are, and if you have "early model" parts or "later model" parts. Sometimes the recommended fix, like Kegler clamps are not necessary on later models because Norton fixed the problem by correcting the stock parts so the "fix" isn't necessary...

Also, if you post a picture, guys here will see stuff that you don't yet see as a problem, and also see parts that need replacing that you should upgrade instead of replacing. For example, if you need new rims, then you can get lightweight aluminum wider rims, so you end up with lower wheel weight, less sidewall flex, and mulitiradius modern tire shape, besided just getting a greater contact patch area...

tinkertoy said:

Gortnipper, That is a useful list, thanks. I am going to set up the front brake as I received it and see how it works, A radial MC might be in my future. Do you have a specific reccomendation on one? The few I have looked at were pretty pricey, in the $450.00 range. I have the scintered pads which should help the bite and removed the chrome off the funky Dunstall discs.
I am especially interested in the Ikon rear shocks, the RGM rear brake pads and the fork damping kit. I am not sure what the “Kepler mod” is but will look into it. I did get the vernier isolastics and am lacing up alloy rims albeit in the stock sizes. I have found that the narrower wheels and tires on vintage bikes are lighter and give a more nimble feel but do look weirdly spindly by todays standards. I also got a set of brand new Avon Roadrider tires with the bike so I wanted to use them.

Click to expand...

The kegler mod is documented here - https://www.inoanorton.com/docs/swingarm.pdf

You can buy nice clamps from NYC Norton or make your own from clamps from McMaster Carr. (which I cant do cuz they wont ship overseas)


All the modern tires recommend a wider rim than a WM2 for anything bigger than a 90/90. If you have the ability, I would suggest moving up to at least WM3, especially for the rear.


As far as a Radial MC goes, I spent a lot of time researching what to do with mine. A lot of that was questioning via AI and picking thru the errors to get to a point where I was confident in the recommendation.

The Brembo RCS 15 is where I ended up, and that is a nice unit as it comes with a clip in microswitch for the brake light, which is super useful. It is perfect match for the four pot Brembo P4 30/34 caliper I have with the CNW kit, which used a PS13 Brembo axial MC. Brembo specifically designs and officially lists the 15RCS master cylinder to be paired directly with one 4-piston caliper (a single 4-pot configuration) to hit the performance target.

For your Dunstall setup, the universal formula for determining the correct radial master cylinder size the Total Hydraulic Area Ratio Formula. To achieve the ideal "sweet spot" for motorcycle brake modulation and safety, you aim for a target hydraulic surface area ratio between 23:1 and 27:1.

I just did a quick analysis (you should check 'my' work for yourself):

Step 1: The Core Formula​

The mathematical relationship is calculated as:

Hydraulic Ratio = Total Working Caliper Piston Area/Master Cylinder Piston Area

Because a dual Dunstall kit features two opposing-piston calipers, you calculate the area of one side of the pistons per caliper, multiplied by two calipers (meaning you calculate the area of 4 pistons total).

Piston Area = 3.1416 x (Piston Diameter / 2)^2

Step 2: Plugging in the Dunstall Twin-Disc Math​

The vintage Dunstall dual-disc caliper uses (4) 1.5-inch (38.1 mm) pistons.

Total Caliper Area = 4 x 3.1416 x (38.1 / 2)^2
Total Caliper Area = 4 x 3.1416 x (19.05)^2
Total Caliper Area = 4 x 1,140.09
Total Caliper Area = 4,560.36 mm2

Step 3: Finding Your Perfect Radial Master Cylinder​

Using the target 27:1 performance sweet spot standard for classic motorcycle vintage racing applications

Target MC Area = Total Caliper Area / 27
Target MC Area = 4,560.36 / 27
Target MC Area = 168.90 mm2

Step 4. Check the 15mm Radial Master Cylinder Area:

15mm MC Area = 3.1416 x (15 / 2)^2
15mm MC Area = 3.1416 x (7.5)^2
15mm MC Area = 176.71 mm2

Final Ratio = 4,560.36 / 176.71 = 25.80:1

The Verdict​

The exact correct size radial master cylinder for a dual-disc Dunstall setup is a 15 mm radial master cylinder (such as the Brembo 15RCS). It matches the unique, large-volume requirements of those four classic 38mm Dunstall pistons perfectly.


A note - you will want to use SS brake lines, whether you get a radial MC or not. You would also need a reservoir kit with the RCS, as they don't come with one. It mounts directly to the MC.