Disc brake conversion

Please would someone advise on what parts would be needed to convert a standard drum-braked 140 to 150 front disc brakes? Hub, upright, calipers, discs?

Don’t waste your time and money.
XK140 Drum Brakes being state-of-the-art design in 1955/6 and specifically matched to the XK140, if properly maintained and in good condition are extremely good brakes in all circumstances short of high level track use.
XK150 Disc brakes are first generation design, and don’t actually work any better than XK140 drums, albeit the disc-brake concept did allow for later development into much better performing brakes if properly designed as a package. Keep in mind, if you modify front brakes in isolation, you will risk impacting on brake-balance, so you MUST similarly upgrade rear-brakes to ensure a compatible balanced system. If you drive gently such that brake-balance doesn’t matter, why go down the brake modification path in the first place anyway.


Unless you are doing serious canyon carving, I agree with the other poster: build the drums correctly, vent the backing plates, and they work fine.

Much better to upgrade the ignition to electronic.

1 Like

No doubt sound advice above, but should that not dissuade you, you will need XK150 hubs, rotors and the adapter bracket that mounts the caliper to the regular XK120/140/150 upright plus some method to reduce the fluid pressure to the rear brakes, aka a proportioning valve. Without a servo, pedal pressure will go up significantly.

You can keep the uprights. Wilwood calipers require adapters IIRC, but Coopercraft calipers mount directly on the std 150 adapters. If you also convert the rear to disk brakes several sizes of wheel cylinders are available to accomodate better balance. Changing the rears might involve a bit of work to accommodate the hand brake cable. Expect to add a servo as used on the 150. Discs should be readily available and will bolt directly on to the 150 hubs. Also remember that the disc arrangement used 3/16 brake lines vs the existing 1/4 lines. Many people have been satisfied with the conversion. If your drums have been cut too often it may be easier to find new 150 discs than drums. I switched over my everyday Mk7 to discs and drove it all over in all conditions including snow. I never looked back.

First question …why d o you want to change?
I made the change as my car had a number of items missing from the brakes including all the drums. I used a kit from Guy Broad here in the UK. it is essential to use a servo if you do make the change as the pedal pressure is ridiculous without one. its a big modification for very little (if any) gain. I was able to leave drums on the rear as i had the original cylinders refurbished and bought new drums (expensive!). The servo is only applied to the front. the brake balance is fine. In hindsight i should have sourced original parts for the front and had drums all round.

I have several XKs and can compare a 140 OTS with disc brakes to a 140 DHC with drums (the one owner (Jack Stamp in Wichita, Kansas) blue car in PP´s book on the Original XKs.
The difference is dramatic!. Ok, the drums may not be optimized in terms of brake shoe material etc, but this doesn´t matter as I never use the brakes heavily. I seem to keep a good distance to cars in front of me, and downshift a lot.
But I would not hand the drum braked car to anyone else, whereas the disc braked car is just like a modern car to drive. It also has the T-5 gearbox.

1 Like

To Paul: what electronic ignition do you prefer? I have Pertronix on a few cars, but it stopped working on a 420G. Needed a new ignitor.
I have the EDIS megajolt in one E-type and another similar system in another E. They are very good indeed. I have several cars on stock dizzys with points, and they also,work well, though a little less crisp

did you fit the T5 yourself or with the help of a kit (if satisfactory, of what provenance)? a friend of mine is on the look… and I’m tempted to bias him towards the T5 (esp. if I end up working on the car later…)

If the work isn’t a put-off, EDIS, no question.

I had a Mallory Unilite in Tweety: the difference in performance, between the stock Lucas and it was dramatic.

Then, I installed @Ray_Livingston ‘s prototype EDIS… and, the difference twixt it and the Mallory was even MORE dramatic!

Mileage increased, hot/cold startability and drivability was markedly better, more stable idle, and I was able to dial out all the pinging, through a laptop.

1 Like

I’ve also been told that mk9 fronts (and xk150 rear end) make for an all disk setup on a 140 : any take on that ?

I know many will disagree, but I simply don’t see the point in making an XK drive like a modern car. For me, the whole reason for owning an XK is for the experience of what the car was in its own era. I don’t want to change it into something else even if that might be better.
Just my take on classic car ownership.


I couldn’t agree more, Eric. For me, that’s what owning a classic sportscar is all about, i.e. the experience as a whole.

1 Like

If you use MK9 parts you are limited to disc wheels as well. Also rotors are scarce and expensive. IIRC I was not able to match the Mk9 caliper to the wire wheel 150 hub and rotor. Mk9 rotors and 150 rotors do not interchange.

1 Like

In this case, I completely agree: changing ignition just makes it less fiddley and more reliable.

Changing the brakes wholesale will fundamentally change the car’s nature.

1 Like

I like the experience of pulling off a successful upshift or downshift with the Moss box without making a hash of it, one gains immediate bragging rights! It is after all part of the XK experience.


1 Like

I am keeping the Moss box with overdrive, haven’t made a definitive decision on the brakes yet but will lay in a front disc set, but maybe start off with the drums I already have. I will, however, fit electronic ignition and an alternator. Even my ‘65 Cobra has a Pertronix III in place of the dual points in the Autolite dizzy. It also has a Dynator to give reliable charging and the tachometer drive. Neither item will affect the drive, but will improve reliability.

NOBODY was ever able to relieve me of my offer of an all-expense paid, at Morton’s Chop House, steak dinner, for a completely clean 1-2-3-4, then back down, 4-3-2-1, drive of Tweety.


thank you, food for thoughts: I suspect after market or 72 spoke wheels would not make much of a difference with the mk9 caliper, either … there’s more than what meets the eyes when it comes to make a good braking system, esp. if one would rather remain more or less “period correct” (e.g. skip carbon rotor alloy Wilwood kits)

1 Like

having used the “123” setup on Lotus Twincam (but not on Jaguar 6 cylinders) I find it offers quite a bit of tuning possibilites and all neatly enclosed within the distributor, so a good compromise if one is not quite ready to go all out with the computer as a passenger.