Disc Brake Conversion - The Pioneering Work

In 1998 a project was initiated by the Z Magnette Register to research and implement a satisfactory front disc brake conversion for our cars. Individual cars had been converted, such as Mike Laflin's, referred to below, but the objective was to understand the process and describe the method so that there was a viable option available to members. The pioneering member whose car was to be the "guinea pig" was  former Committee Chairman David Johnson. What follows is his account of the work and the close involvement of Alan Dixon, whose engineering company had made Mike Laflin's original caliper support bracket. A bespoke bracket is the only component in the conversion that has to be manufactured. Everything else is standard MGA or MGB.

 The technique developed by Mike, David and Alan has also been summarised into a separate workshop note that can be printed out for easy use.

Front Wheel Disc Brakes for a Z Magnette

 A small number of Z Magnettes have appeared, over the years, wearing disc brakes. One I remember with four discs. Many Z Magnette readers will recall Mike Laflin‘s article in the July Safety Fast of 1997 about his Shorrock supercharged ZBV. He described a method of fitting MGA discs and a special mounting plate to achieve more satisfactory brakes than the standard drums. As a result of that work, Mike suggested that other members might be interested in fitting a similar conversion, so Paul Batho canvassed opinion in Safety Fast of October 98.


The original Laflin recipe contained MGA disc brake hubs, discs, MGA callipers and specially manufactured back plates on which to mount the callipers. Mike also fitted the slightly different disc wheels found on the disc-braked MGA. Subsequently, it was thought that by using MGB callipers, it would, perhaps, be easier to get the necessary ingredients for the conversion. This enticing delicacy was mooted in Safety Fast and a number of members, of which I was one, placed orders for a set of the said plates.The engineenng company who made Mike‘s original plates was presented with a stub axle and an old MGB calliper and then set about roughing out a new backplate.

Last Summer, I became involved in the trial of this system because Mike had, as in every year, become busy with agricultural activities which took up most of his waking hours. So it was that, one day, I received a weighty parcel containing the trial back plate. By this time I had equipped myself with the necessary hubs, new MGA discs and an old MGB calliper. Well, the back plates fltted and so did the calliper, and the disc was almost central in the calliper, but I was not happy with the position of the latter in relation to the Zs suspension. I spoke to Alan Dixon, the owner of the engineering company in Thetford. Ah!,‘ he said, ‘I will come and see you,“ at which point I wondered why he was willing to travel all the way to East Kent to pursue the completion of such a small order. This concem was soon answered. The following Sunday afternoon, Alan & Shirley Dixon arrived, in a splendid MGC, sporting an M.G.C.C. badge. “A pleasant drive,“ they said. Together we spent several hours, in hot sun, sorting out the fine adjustments needed to create suitable back plates.

The revised plates were duly delivered within a couple of weeks and the complete mixture fltted to my Magnette. This time the callipers fitted in a very similar position to those of the MGB. Such was Alan Dixon‘s skill that, on my ZA, no shims were required. lt was also possible to use a standard set of Magnette front brake hoses, which fit the “B“ calliper, although it may be that they could be shorter by around 1/2 - 3/4 of an inch. A major bonus is that the disc braking system, using MGB callipers, fits under standard Z Magnette wheels.*) There followed a few local trial runs, then a trip to Thetford and around Norfolk to test the new braking system. In all, more than 500 miles have been covered, much of it in heavy or fast moving traffic, but some on the winding lanes of Norfolk. The first difference found was that the steering and running of the car felt more pleasant, even before the brakes were applied firmly. I had not thought about this possibility, but I guess it has something to do with the fact that the revolving masses are better balanced than when using drums.

So far, I am happy with the new set-up and the braking is comfortable and sustained. I have used standard Mintex pads so as not to complicate the trials and no servo is fitted. Personally, I tend to prefer the system without a servo because I believe it transmits more feel when driving an old and relatively heavy sporting saloon. No doubt the fitting of a servo would reduce the foot pressure required and be more to some folks liking. What has not been researched is the possible effect on the performance and wear at the back brakes, although there does not appear to be a noticeable difference. For those who have ordered backplates from Alan Dixon and who are familiar with the removal of drums, shoes, backplates, Meccano etc, this is not a particularly arduous task, if carried out slowly and carefully. You will need, apart from the items already mentioned, four 7/16" UNF bolts, some 1/2"  longer than those existing at the stub axle, and four 3/8" UNF bolts/nuts/washers to fasten the discs to the hubs, all high tensile. In addition, a new set of MGB pads, springs, pins and tabs are required. lt might also be a good time to replace the front wheel bearings, but this will depend on the state of those existing.

So there we have it. The original batch of backplates is twenty pairs, and most of those are taken, however, you may still be able to get a pair off the shelf by ringing Alan on 01 842 752336. Depending how much you have to pay for some of the items, it should be possible to do the whole job for under £300 (exduding new bearings).

David Johnson ( Z & Farina Magnette Register)

(first published in Safety Fast January 2000)

Some comments on MGA/Magnette wheels from Colin Pratt:
With the introduction of disc brakes on the front wheels of the Magnette, should the wheels be changed to the more dished shape like those used for the MGA when this model changed from drum brakes to discs? It's probably not absolutely necessary but presumably MG thought more clearance was needed for the disc brakes when the 1600 MGA changed from the 1500. As I think you know, many people with MGAs have a mixture of 1500 and 1600 wheels on their cars, possibly without even realising or noticing this fact, and I haven't heard of any problems. However, it could be that brakes could get a bit warmer with the older wheels without being noticeable except in extreme circumstances.  

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