There are a number of ways to improve the disc brake on a Commando.
1) Fit a 1/2" mastercylinder from another brand of bike.
2) Buy an aftermarket Lockheed or similar calliper and/or disc.
3) Sleeve your mastercylinder to 1/2".
4) Get a 11mm conversion from the US or a 1/2" sleeve job from the UK (See the bottom of the page)
I would recommend Vintage Brake in the US for interesting info, on drum brakes as well.
An approach to 3) follows.....
I need to make a point regarding liability. I am not an engineer. I have no qualifications or experience of engineering. If you are interested in following this modification you must get it checked by a qualified engineer who can certify that it is safe. Do not do it otherwise.
A number of people have been been in touch with me over the last couple of years in regard to the master cylinder brake modification sleeving it to 1/2 inch as detailed in Classic Motorcycle Mechanics.
Here follows the simple version...
The original Lockheed master cylinder as fitted to Norton Commandos from 1972 (and to some Ducatis) was given a 5/8 bore for reasons which only promote cynicism and do not bear repeating here. The end result was the brake did not work very well. The lever felt very wooden, and it was hard to get enough braking from it.
I decided to try and improve the hydraulic advantage by sleeving the master cylinder down. I had experienced good results with a 1/2 inch Grimeca cylinder on a Norton so took that size as a reasonable objective.
Having heard of seals from a 1980s Kawasaki GPZ 500 (EZ500 in some countries) being used for the this purpose I took that as a starting point. I obtained a M/C from one and drew up a piston which had the shape and width of the Kawasaki piston but otherwise the length of the Norton.
Click for larger image
My master cylinder was sleeved in stainless steel. If using SS, and I would recommend it, the piston should be made from something hard but not SS. I recommend Aluminium Bronze 3. (As a friend said, AB3 will still be there when the Vogons invade Earth) Do not use soft alloys. While they may stand up to the wear inside the cylinder they will pickup and smear at the contact point with the lever. There is no reason why the cylinder could not be sleeved in aluminium just as original, and the piston made in steel. But if you think about it those materials were chosen for poor reasons and we can do better. Why put steel in a rust prone environment?
Click for larger printable image of piston design
The piston is a fairly straightforward turning job. The Kawasaki seal kit number is 43020-1098. This may be an Australian part number, it's from a GPZ 500 from early eighties, single disc type. In the US this bike was called an EZ 500.
In the kit you get a piston, seals and spring. Biff the piston and use the seals and spring. If you need a new boot and spring clip they are available from the usual sources of Norton parts.
The original cylinder had a trap valve in the same end of the bore to slow the fluid in the downward direction (ask yourself, why would anyone make the brake slow to act? No, don't. ) Leave this out. It wouldn't fit the smaller bore anyway. With this exception, assembling the brake is exactly as for the standard setup. If anything about your setup is not standard, for example the calliper, or the lever (any change to the lever is significant, mail me if you need to know why), deep thought may be needed.
While I have not struck problems in those I have assembled personally or heard of any problems with the many assembled to these instructions elsewhere there is need to be cautious . There are variations to dimensions in the original casting, for example I am told there is up to 160 thou difference possible in length of the internal bore. When you assemble your brake, before you fill and bleed it, make sure the lever can travel right to the bar.
Mine has now done in excess of 20,000 miles, most in tough city traffic. It gets a lot of hard use. I dissassembled it recently because the boot had ripped (it was not renewed when I did the sleeve job) and I could find no evidence of wear anywhere.
The all up price in Australia is about A$90 for sleeving, $45 piston, $49 seal kit, a total of less than A$200. About $100US. Cheap for a good brake. If you are having problems getting your cylinder sleeved a number of people from around the world have sent their's to the person who did mine in Sydney. One bloke in the UK got his back in 18 days.The outfit which does the sleeving does many brake cylinders every day.
attention Terry Milligan
18 Harp St,
ph 02 9787 3111
He takes the usual credit cards. I don't mind calling him for you to let him know you want to send one.
Pistons can be made by...
Po Box 92,
(Ph 0246 32 7202)
I don't know whether Alan would want to do business worldwide. I have no financial interest in these businesses.
While soaking the seals in brake fluid inspect the piston to see there are no sharp corners that will cut the seals. Put the seals on carefully, the main trap is letting the rear one fall into the groove for the boot. It is hard to get out. I nurse the seals on with a tiny screwdriver without sharp edges.
Put the boot on the piston in the same way it goes on the original. Lubricate the cylinder with fluid and carefully insert the piston, careful with the seals. Before pushing against the spring arrange the boot and its spring clip in the right place, then push the piston down with your thumb and prod the boot and spring clip down into place with a screwdriver. It pays to start the clip by pushing one side down first so it does not get caught in the slots for the screws that hold the M/C to the switchgear.
Holding the piston down in place (don't let it pop out, the less you have to put the seals in and out of the entrance to the cylinder, the better), put the lever in and let it take the place of your thumb, and then holding the lever against the pressure of the spring, slip the bolt into the pivot. Pop a tiny amount of grease on the end of the piston and you are done...
The modification was printed in Classic Bike Mechanics Oct 1998. The article included an interesting (and compelling) before and after comparison test as follows...