Fitting Front Disc Brakes - Workshop Note

This workshop note is intended as a supplement to David Johnson’s Safety Fast! article explaining the origins of his project and his pioneering work in trialling the newly manufactured calliper mounting plates. We are indebted to David, Mike Laflin and Alan Dixon whose early work has made it possible to offer Magnette owners a tried and tested way of benefitting from disc brake installation. Feb 2014: Thanks are also due to Guy Maathuis for his further contribution to this article, including the excellent pictures..

How is the Modification done?

The basis of this conversion is to use a specially fabricated mounting plate to fit the MGB calliper to the Magnette stub axle. An MGA disc (rotor) is carried on MGA 1600 hubs, which, fortunately are a straight swap for the Magnette’s. The hydraulic connection to the calliper requires no modification from standard Magnette. The mounting plates are the only item that requires special manufacture. All other parts are readily available from MG parts suppliers, except possibly the hubs, which may need hunting down. My own conversion used a set of the original plates manufactured, like David’s, as a small batch for the Register. However, I have noticed that conversion kits are now advertised for MGA 1500s. It may be that the same mounting plates will work on a Magnette but it looks as though the angle is wrong. If any Magnette owner has successfully used parts intended for the MGA, the Register would welcome hearing about it. The MGA plates mount the calliper at an angle slightly above horizontal, whereas the Magnette plate, as produced so far, is horizontal.

You will need to source MGA hubs. If you were thinking about converting to wire wheels, new after-market hubs are available because wires are popular on MGAs and they are often converted from steel wheels. If you want to retain the original steel wheels, then you need to source second hand hubs because new items are not made. With older hubs there is a risk that bearings may be a bit loose, so try to check this before parting with your money.

Why use MGB Callipers rather than MGA?

The conversion uses MGB callipers, which are a better solution than the original MGA part. They are a later and more developed design and, of course, the parts are very readily obtainable from specialists, whereas the MGA calliper is no longer manufactured. It will be useful to have a copy of the Haynes MGB Manual, which covers the operations involved in refurbishing and refitting the callipers. However, if you are using second hand callipers, it must be emphasised that if the calliper pistons or cylinder walls show any sign of wear, then the calliper should not be used, but part-exchanged for a replacement.

Required Parts

 NB: the part numbers given below are taken from the Moss Europe MGA & MGB parts catalogues dated March 2009. These numbers may differ from original numbers appearing in the MG factory parts lists because over the years, new parts supersede old ones. The numbers may also change if these parts lists are updated. The brake disc is also known as the “rotor” in the USA.

 1 pair             Calliper Mounting Plates                   Available now from Magnette parts specialists

1 pair               MGA 1600 front hubs                        Obtain second-hand

1 pair               MGB callipers                                     27H4650 & 27H4651

1 set                 Brake pads                                          GBP202

1 pair               Brake hoses (see below for high-spec alternative)

1 pair               MGA brake discs                                  BTB108

8no                  Bolts – adaptor plate to stub axle      1 1/2" long 7/16 ths 20TPI UNF High tensile

4no                  Calliper fixing bolts                               ATB4074

2no                  Calliper bolt lock-tab                            BTC114

8no                  Brake disc fixing bolts + nuts             BTB145 & FNZ506


1 set  Hub bearings + oil seals (unless you know your existing bearings are nearly new. Transferring old bearings is a false economy: 2no GHB176,  2no GHB177, 2no ACF4004

1 pair Goodridge stainless steel braided brake hoses. These are superior to the standard parts because they allow less expansion of the rubber pipe and therefore give the brakes a firmer, more positive feel.

1 small can of calliper paint (Halfords and most accessory shops)

1 pair of MGB backplates


NB: where an operation is marked “(WM)” it means that it is fully covered in the Magnette workshop manual and does not need to be repeated here.

Prepare the hubs with new bearings (if used), pack them with grease and fit a new rubber seal (WM). Do not be tempted to over-fill with grease or it will expand when hot and risks contaminating the brakes. Attach the brake discs to the hub. The disc fits behind the hub flange and is fitted with four studs on each side + lock washers & nuts. The head of the stud is on the outside of the hub and sits in a recess so that it does not obstruct the fitting of the wheel. Check the wheel studs for damage or undue wear to the threads and replace if necessary (Part No.1G4359)

If you are not using brand new callipers, do whatever reconditioning is needed before you start the main work. Fit new pad retaining clips and clevis pins. Care: brake dust should not be inhaled so do not use compressed air to clean old callipers. Use a spray can of brake cleaner as per instructions supplied with it. Callipers can be painted with special paint sold for the purpose. A small quantity of copper grease on the back of the pad where it touches the piston will help avoid any squeal. For normal road use standard pads should be fine but there is a range of pads available if the brakes are to be subject to heavy or competition use etc. e.g. EBC “Greenstuff” pads that advertise 20% better stopping power and reduced brake dust because they are Kevlar-based. However, they are significantly more expensive than standard pads!

Jack the car up and support it on axle stands, preferably under the chassis leg so that there is free movement of the suspension & steering. A trolley jack can be used later under the spring pan if you want to compress the spring during the conversion.

Bleed the brake fluid from the front of the system using the bleed nipples on the wheel cylinders. On each side, remove the flexible hose from the back of the wheel cylinder (WM). Take precautions to ensure that no dirt etc can get into the brake pipes. Remove the brake drum and brake shoes. Remove the brake back plate from the stub axle.

Clean and refinish the stub axle and suspension parts as required.


These require modification to fit flat on the adaptor plate. Follow the sequence of photos showing the work completed by Guy Maathuis (Feb 2014)








Mount the adaptor plates on the stub axle flange as above using the new extra long 7/16ths bolts with lockwashers and nuts. (Please note: ALL NUTS AND BOLTS MUST BE HIGH TENSILE). The plate should be mounted so that the calliper fixing holes are behind the wheel (i.e. at the rear of the wheel-arch) and the machined recess at the centre is facing outwards. The hole at the centre of the adaptor plate will sit snugly over the raised ring at the base of the stub. The plates are handed and the mounting holes are irregularly spaced so each plate will only fit the side for which it was made.

Mount the hub on the stub axle, complete with brake disc attached. Adjust the bearings (WM), fit a new clevis pin to the castellated nut and fit the grease cup. Check for free and smooth rotation of the hub. Check visually by spinning the hub to ensure that the edge of the brake disc shows no run-out and is not distorted.

Make sure that the calliper pistons are full retracted. Mount the calliper to the adaptor plate. The plate has threaded holes to accept the standard calliper fixing bolts. Fit them with a new lock-tab under the bolt heads. Torque to 40-45 lbft or 54-61Nm. (Only bend the lock-tab ends over a flat on the bolt-head when you are satisfied that the disc and calliper are correctly positioned in relation to each other). Fit the pads, with clips and retaining pins. Part the ends of the pins, so that the pin is held in position. The disc should fit in the middle of the pads and rotate freely without fouling. If the disc is not central in the calliper it may be necessary to use shims to bring it into position.

Reconnect the flexible brake hose (WM). Check that it has enough length to allow full steering movement from lock to lock with the suspension fully extended without the hose being stretched. (With the car supported under the chassis leg and allow the road spring to expand fully. Then do the “lock to lock” test). Check also that it is not too long and will not chafe against any other component during normal use. Check again when the wheels are back on and under normal load.


Re-fill & bleed the brakes (WM). A “one-man” bleeding kit is useful for this. Check that the pads are not binding after the brakes have been applied or rapid wear will result.

Re-fit the wheels. Take note of Colin Pratt’s post-script to David Johnson’s article. When the MGA was first fitted with disc brakes, the factory decided to fit wheels that were modified to a more dished shape so as to ensure clearance of the calliper. In practice we believe that standard Magnette wheels will not cause a problem, but do check carefully for any risk of fouling during rotation. Unless you are proposing to use the car in extreme circumstances, we do not believe, either, that the calliper will be adversely affected by over-heating.

With an assistant, carry out a basic test for correct brake function with the wheel still jacked up. Spin the wheel by hand and make sure it comes to a stop when the brake is applied. Then drive the car slowly, preferably in an off-road location, and apply the brakes gently to make sure that brake operation is smooth, balanced and not pulling to one side.

 Road Test

Carry out a road test on a quiet lane or a traffic-free area. Only test the car in real traffic conditions when you are absolutely sure that all is well. New brake pads take a few miles to bed in, so do not expect optimum stopping power immediately. Avoid heavy breaking until bedding in is complete. When applied, the brakes should feel smooth and even, with no snatch or harshness. When you are very confident that all is well, I suggest an emergency stop test, just to see how the new set up responds in extreme conditions. It is also a good idea to adjust the rear brakes up to optimum position before the test or the rear brakes may bite momentarily after the front, which is a bit disconcerting for the driver!

After testing, re-check the fluid level and top up if necessary.


This subject is adequately covered in manuals for cars with disc brakes as standard. As already stated above, no periodic adjustment is needed. However, both the pads and the discs will wear with use. Inspect the discs and pads from time to time. The MGB manual suggests a 6000 mile (10,000 km) or 6 month interval for this.  Some light scoring of the disc is inevitable because dust & grit will get trapped between pad and disc. However, any sign of heavy scoring should be investigated immediately. Pad wear can be judged by visual inspection through the side of the calliper between the clevis pins. If the pad is allowed to wear as far as the metal backing plate, the disc will be damaged, so replacement is advisable when there is, say, 2mm or less of pad material left. Check also for uneven wear of the pads. If one side wears more than the other, it could indicate that the piston on the less worn side is not moving freely and may be seizing. This can happen if the car is not used for prolonged periods, especially if the hydraulic fluid is contaminated with water, which it can absorb from the air. The water causes rust inside the calliper. Brake fluid should be flushed and completely renewed for this reason every few years unless you have used silicone fluid to avoid the problem.

If there is discolouration or cracking of the disc, this suggests over-heating, which may occur if the pads are binding. This would be accompanied by loss of power. You can also check whether the car will roll freely down a slope with the hand-brake off. Rectify the fault and replace the discs immediately. Always replace discs in pairs. Never singly, or imbalance may occur.

Discs will become thinner with normal wear and need replacing routinely from time to time. The MOT test will check that the thickness is adequate and safe. Discs will also acquire a film of rust if the car is not used for a while. This is also normal but if rust persists after normal road use, you need to investigate because it means the pad is not bearing evenly on the disc or the calliper piston may be seized. This would normally be accompanied by poor braking or imbalance.

Disc brakes are self adjusting, so there is no mechanism for manual adjustment. As pad wear occurs, it is simply taken up by the “at rest” position of the pad and the fluid level in the master cylinder falls slightly. The natural reaction is to top the reservoir back up. However, if you do you will need to remember to withdraw some of the fluid before fitting new pads. The fitting of new pads can provoke a fluid overflow as the piston is retracted and fluid is forced back via the pipe into the master cylinder reservoir. Cars with disc brakes normally have bigger fluid reservoirs to accommodate the wider variation in fluid level, so with standard Magnette reservoir capacity you need to keep a closer eye on the level to make sure it does not fall too far whilst not over-filling it. When the MGA 1600 was given disc brakes, a new master cylinder reservoir top extension was fitted to accommodate more fluid. Unfortunately, although the master cylinders are similar, there is no scope to extend the reservoir upwards on the Magnette because of the battery box.

It is possible to fit a low brake fluid warning system. You may be able to get new old stock of period parts from Powertrack or a modern version from TE Engineering. (see "Suppliers")

Problems with Brakes that Stick On

Some owners have reported problems after fitting the conversion. After applying the brakes, they do not release properly. One of the suggested causes is the valve in the master cylinder that is designed to slow the flow of fluid back into the cylinder. The reason for this is to allow the driver to pump the brakes to some degree to build the pressure. It avoids total release between pedal strokes. It may be that if this valve is not working freely it holds the disc pads in place for too long. With smaller tolerances between pads and discs than between shoes and drums, it may be that the conversion is merely revealing a fault that existed previously but was less obvious. I would renew the valve components to check whether this helps. Check, also, that the pistons are moving freely in the calliper. Some owners have resorted to removing the valve, but this will change the feel at the pedal.

You have no rights to post comments