Princess 4 Pot Caliper Conversion

The heading contains probably several of the sexiest words in the UK classic car modifiers vocabulary, for modern car modifiers other words might be more suitable - but for me these were just what the doctor ordered.



Question - why are Princess calipers so popular even now?  The following is my own opinion and should not necessarily be considered as gospel, each modifier should do their research and form their own opinions.  The key thing about Princess calipers is that they were designed in the 1970s and bolt onto lots of cars from the same period without a lot of work.  Why do they fit, because through the 1960s, 70s and into the 1980s there were two key players in brakes in the UK and they influenced motor manufacturers right across Europe - the two manufacturers were/are Girling and Lockheed.


At that time the standard method of bolting calipers onto suspension uprights was to have two lugs cast into the inner half of the caliper, bolted through onto a threaded mounting plate/bracket.  Girling and Lockheed both used a format of 31/4" (82.55mm) to the bolt centres.  1960/70s cars general used imperial bolts around 7/16" unf (Marina included), with later 1970/80s cars (including all Fords and Austin Princess) used M12 metric bolts.  This meant that for the Ford Escort rally boys they could nip down the scrap yard, take some Princess calipers off a scrap car and bolt them directly onto their suspension uprights.  They then needed to use spacers to push the caliper back a little (10mm) and space out the Lockheed Princess calipers to accept vented disks and they could bolt on Capri 2.8 vented brake disks and the had a sexy, low cost, heavy braking piece of kit.  Of course that was then and this is now, single piston calipers can be much more powerful than old opposed piston calipers, but the basic engineering principles of squeezing both sides of the disk remains.  Racing  and high performance cars still use 4 (or 6 or 8) opposed piston calipers, but these days they are alloy rather than cast iron, however as an upgrade for the Marina standard Girling Type 14LF two piston calipers, Princess calipers are an obvious improvement and they look sexy through the wheels.  The other reason I chose them was that I was given a pair by someone (from the back of their shed) - so my starting point was dictated to me, if I had been given some Brembos I would have used them.


Princess brake specifications

Piston diameter = 38mm x 4 per caliper

Standard brake disk width = 13mm

Standard brake disk diameter = 270mm

Brake union fittings = M10 x 1.0

Dual line brake actuation - in case of failure of one brake line.


Here's a picture of the Princess calipers donated.  Obviously it allows for a bigger (longer) brake pad than the Marina - which is "a good thing".


Calipers as donated

Austin Princess sourced calipers seem to be a kind of Holy Grail, but AP Lockheed supplied them for other vehicles as well.  Some have double brake pipe fittings and some a single fitting, which to my mind would make a better conversion for a single line car.  I have done a little research on alternate sources and have listed some here.

  • Some older LDV vans/pickups - usually single line and either solid or vented disks

  • Some older Ford Transits - usually single line and either solid or vented disks

  • Various TVRs and other low volume sports cars

Superficially, Rover SD1, Range Rovers/Range Rover Classic & Landrover Defenders brake calipers look the same but they are a bigger (and heavier) Lockheed design with a different mounting bolt width at 3.5" and bigger 41mm brake pistons - they will fit MGBs and TR7/8s though.


Things to do

To get the things fitted and usable, there were a number of challenges to overcome as I was pretty much on my own with this conversion;

  • Find suitable way of bolt calipers to uprights

  • Find suitable brake disks

  • Find suitable way of mounting brake disks to Marina wheel hubs

  • Find caliper spacer kit

  • Arrange method of applying brakes

  • Fitting inside the wheel rim

Plus of course, cleaning, reconditioning & painting the calipers.


Bolt calipers to uprights

As as already been pointed out the Princess bolt hole is 12mm and the Marina bolts 7/16"unf.  Despite a friend saying, " don't worry they nearly fit, just bolt them up!"  I actually value my life so I though through the options;

  1. tap out the Marina mounting brackets to M12

  2. sleeve the caliper bolt holes to 7/16"

  3. get creative

I didn't want to damage the Marina parts as I may want to put her back to original one day - so option 1 was out.  Sleeving the bolt holes felt wrong and where would I find v thin tubing - option 2 was out.  


Marina caliper mounting bracket

Shoulder bolt - before and after

Option 3 saw me source some M12 x 1.25 (fine) threaded bolts cut to the right length and then tap them down to 7/16"unf by hand.  The thread pitch between the two are virtually the same and with a bit of effort a 7/16"unf Die will do the job.


What I have created are what apparently are known as "shoulder bolts", where the blank shoulder is 12mm and the thread is narrower at 7/16", the one on the right is the finished article.


The M12 bolts are available from Namrick or you can now buy them already made up on Ebay - M12 bolts with 7/16th threads - do a search for "Caliper Bolts Escort RS Mexico MK1 MK2 Capri 2.8i" it should take you to a vendor called Motorsport MC.

Brake disks

I then bolted a caliper up to a Marina suspension leg to measure the fit against a Marina disk.  The Marina disk fitted quite well in the caliper between the pads, only requiring one washer to place it centrally.  However the diameter of the disk is too small at 248mm.  The tops of the Princess pads stuck out above the edge of the Marina brake disk.  If you put a Princess brake pad on the Marina disk it fits perfectly, the difference is that the Princess caliper is taller to accommodate the standard Princess disk (270mm).  A quick measure showed that I would need a disk of at least 255mm to capture all the pad, ideally brake disks should protrude beyond the brake pad by at least a millimetre to avoid the pad breaking up.  The second thing was that at 9.5mm the Marina disk was a lot thinner than the standard 13mm Princess disk which would mean further for the pistons to travel when being applied.


I fancied the idea of vented disks and with these measurements in mind set about identifying suitable options - the caliper spacer kits produce a gap for a 20mm - 22mm disk so that was my target.  The last key measurement is the centre hole diameter of the disk as it has to fit over the Marina wheel hub which is 66.7mm.   So where to find such elusive information?  Who makes the most popular aftermarket brakes at the moment - Brembo?  I knew that they also make excellent brake disks and I found that they have a brilliant search engine which allows you to search by one of more parameters - BREMBO.  This allowed me to compile the table below for comparison.



Disk Diameter


Hub Dia


Inner dia  brake  surface


Centre hole


Total Rotor width








Disk Required 255+ 127ish 146ish 66.7 or less 45ish 20-22mm 18-22mm
Marina 248.5 126.7 145.7 66.7 45.7 9.5 8.6
Focus 1.6 16V 98 0n 258 146 159.8 63.6 43.5 22 20
Mazda MX5 1.8 (94 0n) Mk1 255 135 148 55.1 45 20 18
Mazda MX5 1.6 (98 0n) Mk2 255 135 148 55.1 45 20 18
MAZDA/323//1.8 (BA1) (94-98) 255 135 146 55.05 45 22 20


Clearly the Ford Focus or the Mazda MX5 were closest to my needs, the overall width of the disk being the deciding factor for the MX5 disk as it is very similar to the Marina and would avoid spacing the caliper about much.  I sourced a pair via EBay (about 30 against 75 from my local motor factor).  The centre hole was opened out and four new tapped bolt holes made to bolt them to the Marina hubs, this cost me around 30 with my local engineering shop.  The next job was to buy some caliper spacers, a quick Google listed several versions and finally EBay (yet again) came up with the best 'Buy it Now' deal.


I then had the calipers soda blasted and I fitted a new piston and seal kit from BiggRed .  Tea and home made scone not included - thanks Rosemary.

Here's the result ready for assembling and a drop of paint.  I added a pair of copper brake pipes running back to an imperial  T union, with the standard Marina brake hose.  The brake pipes have 10mm metric unions at the caliper end and 3/8" UNF imperial unions at the flexi pipe end.  Time to bolt it all together, fit the pads and clips and offer up a wheel to go over it and that's when I finally ran into trouble.  


  1. The calipers extend out beyond the edge of the wheel hub by 11mm - solution - a spacer of 15mm

  2. The standard 38mm wheel studs won't leave enough thread for a wheel to be bolted on safely without ideally 15mm extensions - total 53mm

  3. The maximum available wheel studs at 47mm in the Marina wheel bolt 3/8"unf thread - shared with Spitfire but different from Mini

  4. The calipers just kiss the inside of the 13" alloy wheels that I am using, even after removing all the original casting flashing from the caliper bodies

Short term solutions

  1. Take caliper spacers out and fit suitable solid brake disks.  Another trawl through the Brembo online catalogue produced 13mm thickness and 256mm diameter solid disks from an Audi 100 1.8 litre 1982-1990.  Once again EBay came to my rescue!  I got two pairs and had them machined to fit the Marina hub (so if anyone wants a pair I'm your man).  Here's the Audi disk fitted, but I had yet to strip out the caliper spacer. 

  1. The caliper is now flush with the hub face, so I have re-installed my 5mm spacers and 5mm longer wheel studs, to give a little clearance.

  2. The 13" alloy wheels will still caress the caliper, so a move to 14" wheels gave me the excuse to shop for some Revolution alloys.  I had been lusting after these wheels for years, I couldn't find any at the right price when I was originally looking for wheels some years back, so then I ended up buying the Performance Minilites.  Now some Revolutions came up at just the right time - 6" x 14" with the 3 3/4" PCD.  They can often be found on Triumph TR7s, GT6s, Vitesses and such, but mine came off a Lotus Elan, so I expect that they are older than the Coupe.

New and Old

It took me 6 months to think of what in fact was an obvious solution to the spacer problem, the only thing is that I don't like it a lot, as it's not very elegant.  I bought a set of real Mini wheel bolt extensions and spacers.  These are 1 1/4" (32mm) wide each so are more than adequate for my needs.  Overall they increase the track by 2 1/2" which is a useful increase.  This was only ever an interim solution whilst I put my thinking cap on, and I needed to do some fairly quick thinking as the Mini spacers gave me horrible wheel wobble from about 40mph.

OK I needed at least a 11-12mm thick spacer for the wheels to clear the caliper, which also meant that I needed to extend the wheel studs as well.  To get the required 53-ish mm length, a switch to Ford 7/16ths studs will do the trick, I have had to buy more Revolution wheel nuts to match, this is OK long term as I plan to fit a Dolomite Sprint rear axle and that is already on 7/16ths studs.  I designed and had made up some 15.5mm thick spacers as shown below, I had the 7/16th wheel studs fitted to a spare set of hubs and stripped down the existing hubs to fit it all together.  The Marina hubs have two threaded holes in them for special spacers for 10cwt vans, I used these to secure the spacers to the hubs with some 1/4" UNF countersunk bolts and then had the complete units dynamically balanced.

I fitted slightly longer cap head bolts to secure the disks to the hubs and then used some K Nuts and spring washer to fix them from behind. The reason for all this is the thickness of the metal in the middle of the MX5 disk - it is about 6.5mm against the Marinas 11mm and I think that there aren't enough threads in 6.5mm to be safe in the long run, so belt and braces and some Loctite thread lock were the name of the day.

Pretty ain't she?

The next problem to raise its head is the length of travel in the brake pedal.  The brakes bite , but only at the bottom of the travel.  A fast brake application is met with a whooshing noise as the volume of brake fluid is pumped through the master cylinder.  I have a solution for a different master cylinder - see the page marked Master Cylinder for more on this.