Part 4: Brakes – Drums to Discs and Everything Related

Updated: Feb 9, 2017

Previously on My Fiat 600D
Part 1: The Car – Before I Bought It
Part 2: The First Six Months
Part 3: Restoring the Undercarriage

In this episode, I’ll be converting the Fiat 600D’s front drum brakes to Fiat 850 front disc brakes.

While I was driving my little city car around in the city, doing no more than around 35 mph, I realized that the stock, 7-1/4″ drum brakes were not up to my standards.  Even with everything stripped out of my light car, making it even lighter, the 4-wheel drums were not quick to slow it down.  In fact, the poor braking performance was noted in the Road & Track road test article from Nov, 1961, “Unhappily, the brakes were not up the standard set by the rest of the car; the required pedal pressure is much higher than one would think necessary for so light a car and when the brakes are applied hard, they set up a shuddering that is very disconcerting.  The car will definitely stop, but the weird pulsing that feeds back through the brake pedal is something to think about while you wait for things to come to a halt.”  I couldn’t agree more.

Fiat 600D front suspension

Fiat 600D front suspension

Primarily because I like to tinker, secondly because dual circuit brake systems are required on most tracks, and thirdly because of safety concerns, I decided to upgrade the brake system. There are three disc brake swaps that I know of: Girling, ATE, and Fiat 850.  I don’t know where to source reasonably priced Girling disc brakes, so those were out.  The ATE brake kits are almost a grand, but the nice thing about them is that you can use 12″ wheels.  In the end, I already had the front discs from a Fiat 850 Spider and 13″ wheels, so I went that direction in the beginning.

Discs first.

The 850 set wasn’t in too bad shape, as you can see below.  The rebuild wasn’t too bad. I sent the uprights to Dave at Apple Motors to rebuild the kingpins. They were stuck beyond my ability. When I got them back I cleaned and covered them and the rest of the parts with POR-15 and replaced all the rubber.

Fiat 850 disc brakes ready for rebuild and transaxle ready for scrap metal.

Fiat 850 disc brakes ready for rebuild and transaxle ready for scrap metal.

The rotors are Brembo, P/N 25073. Specifications: Outside Diameter (mm): 227, Nominal Thickness (mm): 10.8, Minimum Thickness (mm): 9.

Fiat 850 disc brakes ready for install

Fiat 850 disc brakes ready for install

Fiat 600D with Fiat 850 Front Disc Brakes and Spax Shocks

Fiat 600D with Fiat 850 Front Disc Brakes and Spax Shocks

The other brake conversion I may do at some later point is install the rear disc brakes from a Fiat 124. That is probably overkill at this point and quite a bit heavier, but I do have them already if I decide to go forward with it. The Brembo rotors are the same as the Fiat 850 fronts. In addition to the calipers themselves, you’ll need a mounting plate to attach them to the 600’s rear swing arm. You’ll also want to use a brake proportioning valve. Here is the document I used, though I take no responsibility for it’s accuracy or effectiveness.

Fiat 600D rear disc brake conversion diagram 1Fiat 600D rear disc brake conversion diagram 2

The Fiat 124 rear calipers before rebuild and Brembo rotors PN 25073.

The Fiat 124 rear calipers before rebuild and Brembo rotors PN 25073.

Fiat 850 front caliper and Fiat 124 rear calipers after rebuild and ready to install

Fiat 850 front caliper (left) and Fiat 124 rear calipers after rebuild and ready to install

A word of caution – ‘They’ may tell you that the front disc conversion is a bolt-on procedure, and technically ‘they’ are correct. The Fiat 850 front discs bolt right up (after drilling out the lower mounts to the larger 600’s diameter). HOWEVER, the stock 600 brake master cylinder is single-circuit and may not move enough brake fluid volume for the 850 brake piston to work effectively. Moving a little front wheel cylinder requires less fluid than moving a disc brake piston. /Warning

Master Cylinder second.

The common dual-circuit master cylinder upgrade is from a Zastava 750.  It has the necessary dual circuit, is built for disc brakes, and, being licensed by Fiat, it fits in the same general area.  You can purchase one from Chris Obert at Fiat Plus here.  However, there are some pitfalls that I fell into on the way.

First, the Zastava MC is much longer and has different mounting holes than the 600 MC. Also, thanks to Body Shop Guy, the 600 MC has more Dodge Ram Red paint over-spray on it than the Zastava does.

Fiat 600D vs Zastava 750 Brake Master Cylinders
For test fit purposes I mounted it using the rear hole so as to keep it in the same position relative to the brake plunger as the original as shown below-left.
Fiat 600D Brake Master CylinderFiat 600D Zastava 750 Brake Master Cylinder
You can see below that the pitman arm strikes the front of the MC, which isn’t good if you ever want to make any turns.

Fiat 600D Zastava 750 Brake Master Cylinder

You can solve the striking issue by using the pitman and idler arms from a Fiat 850.  However, the problem remains where the sector shaft blocks the MC inlet.  Frustrating!

Fiat 600D Zastava 750 Brake Master CylinderFiat 600D Zastava 750 Brake Master Cylinder
Also, the Fiat 850 center link is much too wide and the higher steering arms really throw off the outer link geometry.

Fiat 600D with Fiat 850 Steering Arms and Center Link

So, it was back to the drawing board…

My solution:  Starting with the m/c mounted using the rear-most hole, I moved it back toward the rear of the car 23mm and drilled two new holes. The Zastava MC is just a bit narrower than the original, so I spaced it away from the chassis rail 3mm to line it up to the plunger more closely. (It’s not perfectly lined up, but spaced away any further caused clearance issues with the brake pedal). I used two 90mm length bolts, fender washers, and nyloc nuts to secure the m/c to the chassis rail.

Fiat 600D Zastava 750 Brake Master Cylinder Fiat 600D Zastava 750 Brake Master Cylinder

The picture above is looking at the Zastava MC from below.  In it you can clearly see the space issues you will run up against.  Toward the front of the car, you are limited by the steering arm and toward the back of the car you are limited by the brake plunger. On the top, you run into the bottom of the chassis.  On the driver side you mount it against the frame rail and on the passenger side you are limited by the brake pedal.  It’s a very tight fit!

Again, moving the m/c toward the rear causes clearance issues with the brake plunger, so it necessary to remove a similar amount of material from the plunger. The plunger also was originally installed with the curve upward. With the m/c so much closer, I found it fit much better installed with the curve downward.

Fiat 600D Brake Plunger

If left as-is, the sector shaft clears the inlet, but the 600D pitman arm will still strike the front of the m/c. You can use the 850 arms to solve that problem, as I mentioned above, but that would really change the steering link geometry. I didn’t know how that would affect the car’s performance, so instead I put spacers behind the steering box to move it forward about 6mm. The combination of moving the m/c back and the steering box forward allows the use of the original 600D pitman and idlers arms.

Fiat 600D Zastava 750 Brake Master CylinderFiat 600D Zastava 750 Brake Master Cylinder

Now that I solved the MC placement mystery, that just left the matter of the 45 year old, very crusty brake lines to deal with.  They didn’t all stretch to meet the new MC and the fitting type didn’t match anyways…  Typical.  At this point, since there were really no brake line kits to be had for this custom installation, I decided to convert the whole thing to AN/JIC fitting, which are very reliable and parts can be found at just about any automotive store.  More recently, though, there are now brake line kits you can purchase for this swap here.  This would have made this swap much easier.  All well.

I made the following couple of diagrams to help with items I would need for the AN conversion.

Fiat 600D AN Brake Conversion 1Fiat 600D AN Brake Conversion 2
The center and rear lines are done.  Here are some before and after pictures.

Fiat 600D Rear Brake Lines Old Fiat 600D Rear Brake Lines New

Fiat 600D Rear Brake Lines Old Fiat 600D Rear Brake Lines New

I’m still working on the front lines…  Coming soon.

Gerry from NZ asked if I had to change any of the track rod ends/ arms etc, or is it a matter of changing the steering geometry. The answer is yes. Changing the steering geometry is one thing I didn’t want to do. When I moved the steering box and idler arm forward, the linkages pushed the front of the kingpins and, therefore, the wheels, outward. There wasn’t enough adjustment available in the original tie rod sleeves to bring the wheels back into alignment. Thankfully, the fix was simple in that I used the same steering/idler arms, center link, and outer tie rods ends. However, I used short inner tie rod ends and the 223mm tie rod sleeves from a Fiat 124 Spider.

If you have any questions or comments, contact me using the contact page or email me at myfiat600d@gmail.com.  Cheers.

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17 thoughts on “Part 4: Brakes – Drums to Discs and Everything Related

  1. Pingback: Part 5: 767cc Engine and Transaxle | myfiat600d

    • Thank you for the comment and idea. Carlo Abarth did this with his cars that used the Girling disc brakes and you could buy them in the US from Al Cosentino’s FAZA. Taking that route does not, however, deal with the safety provided by a dual circuit MC. Additionally, dual circuit MCs are generally required by racing associations – even historic ones (in the US anyways). Regards! Aaron

  2. Hi Aaron, my friend has just put the 850 kingpin and discs on his 600, and wondered if you had to change any of the track rod ends/ arms etc, or is it a matter of changing the steering geometry.
    Thanks in advance.Gerry
    my email if you choose is
    midgegerry@xtra.co.nz

  3. Great site. Finally someone described the issue with two circuit pump. Where i can buy one?
    Additionally I assume your car after changing the brakes requires 13 inch wheels right?
    Is there any way to change brake drums into 12 inch brake discs? I would like to not get rid of my original brakes….

  4. Hi
    I’m in the process of restoring a Fiat 900t – I have a set of 850 Spider front discs assemblies to upgrade to in due course.

    I’m also in need of new set of front & rear shocks and notice that you have gone for Spax – normal or adjustable?

    Any chance that you could let me know the part numbers so that I can try and track them down in the UK?

      • While at a car show swapmeet I found a set of shocks the same dimensions of my original fiat setup only the different that the lower end had eye instead of the stud,so I cut off the eye and welded a bolt to the end to the same dimensions and size of the top end ,installed them in the car with no problems retune the steering box tighten the front end and now the car rides like a Ferrari ,all at fraction of cost and with great results,$20.00 for the shocks $2.00 for hardware,at times you have to improvise,its called R&D

  5. Will a 12″ wheel fit over the 124 disc brake conversion in the rear? ( I see that it will not fit front 850 brakes and there is a 12″ brake kit for that)

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