Disc Brake Upgrade for a 120

I met a Jaguar XK120 owner years ago and saw his very cool right hand drive car. He had the opportunity to drive that car across Australia… ship to South Africa… drive North the entire coast plus more and he arrived in England. The car was shipped to the US and I also sat in that car … a moment to remember. He was an Engineer and he told me about mounting Mark 9 front disc brakes onto his 120 as a bolt on modification.

1 Like

Presumably, this was a disc-wheel car?

Easy part:

Yes, the 5-lug Mk9 hubs use the same bearings and hence, fit the XK spindles. The Mk9 caliper brackets fit the XK uprights, and the wheel lugs are the same bolt pattern. The only physical difference is that the Mk9 caliper mounting bracket uses larger bolts than the XK drum brake backing plate,so you would need some sort of spacer. You cannot use the Mk9 upright and brakes as an assembly, as the geometry is different.

Hard parts:

  1. Drum brakes are self energizing and disc brakes are not, so the pedal pressure will be higher. This will also affect the front-rear balance and can lead to premature locking of the rear brakes.

  2. Mk9 brake rotors are hard to come by. The easy fix is to use XK150 calipers, which use a thinner rotor. A badly worn Mk9 rotor can be turned to the XK150 spec. The XK150 caliper mounting bracket uses the same size bolts as the XK120 drum brake backing plate.

Mike you have been there and done that, very cool you know the options there. Steel wheels for sure and he worked it out with one of those solutions and adjusted or adapted to the drivability. Here is a person who got to drive his XK120 wide open on occasion during his journey, especially in remote Australia!

Hello Adam, I’ve been down this road with MK9 setup for my steel wheeled XK120 OTS. If you search these archives you will find volumes on this topic,… much of it written by guys like Mike & myself. The rotors for the MK9 are available from one vendor, but pricey. I bought new pair with the intention of fitting the MK9 disc. I also purchased years ago, a MK9 front suspension with that idea. This was all because for years, no conversion was available for steel wheeled cars, however now there is. After weighing the cost, effort in making MK9 work, as Mike points out, I opted for the conversion kit offered by SC Jaguar Parts.
I think it sells for about $2500 and has been a great conversion. My brother, at the same time, converted his car also using a different conversion kit from a different supplier, for his wire wheeled 120. That was a nightmare while mine was a snap.
Our steel wheeled FHC also needs this conversion done, and maybe I’ll think about using the old MK9 set up,… or may take the easy way again. It was darned nice to have everything show up, all fit nicely, and be done without machining spacers, dealing with different bolt sizes, etc. etc. Best of luck. Knight


@ Knight_Martorell

Awesome recap of what is available today.
Makes me wish I a 120 to fix up.


There was a continuing problem with the front brakes on my Wife’s treasured XK-120 FHC. Typical, apparently, of a seldom driven car, the front brake cylinders often leaked resulting in fluid soaked linings and related driving problems.

I solved the problem by installing a Wilwood conversion kit sourced from Moss Motors, on sale, for about 1,500 USD. The kit included hubs, bearings, rotors, calipers, brake pads, brackets and hoses. It was a very easy installation. Actually, less effort than dealing with a cylinder leak.

The pedal does require a bit more pressure but it is not noticeable, and there has not been a premature rear wheel lock up. In general, the car is much more driveable.