Need help on front calipers - X1/9 vs 128 vs 850.

[myredracer]

I bought a pair of each of these calipers a number of years ago. The 850 ones are for my Abarth clone project. But I'm not sure why I have a pair of 128 and also steel ones for an early X (both new). I bought a set of rear calipers for a Scorpion for my 1300/124 restoration and bought a pair of new calipers for the front but don't know which ones I intended to use. What is the difference between the X and 128 calipers and which ones would be better to go with the Scorpion rears? Does one have a larger dia. piston? The 1300/124 is heavier in the rear which is why I originally thought about the Scorpion calipers after someone on another forum recommended them (no idea which forum).

I also see that the X and 128 calipers are wider which I assume is because the pads are wider?

 

[kmead]

The Scorpion rears do have a larger diameter piston and move the brake bias rearward, which on an 850/600 based car is not a bad thing given their propensity to lock up the fronts. It is a significant change given that you are moving from wheel cylinders to disks in the first place. Some careful testing may be in order to look at a pressure limiting valve.

I assume the 850 calipers are the late ones, not the early setup (pre 68 as I recall?). There are many differences between the 850 and X calipers, the biggest difference is obviously the width which does allow a much thicker brake pad. The 128 and X both use the same diameter piston, as does the 124, 124 calipers supplanted the early X calipers which simplifies the design and eases servicing.

I am not that versed in making such extensive changes to the brake system on an 850/600 but given the small diameter of the master cylinder, I would think you would want to change/upgrade that part of the system.

 

[myredracer]

Yes, the 850 calipers are the late style. The master cylinder is a dual one from a Zastava 750 and is a commonly used upgrade in 600s. Not sure how the piston diameter compares to an 850 MC. I also have a MC from a Multipla which has a larger dia. compared to a stock 600 one is another upgrade that was used back in the day. The Zastava MC is not a simple bolt-in swap however and requires a new mounting stud location and re-positioning a bit so it won't hit the pitman arm.

Not sure about front/rear brake bias and it I'll need a proportioning valve. Maybe I was planning to use the X or 128 front calipers to give a little more braking force to the rear brakes without having to use a proportioning valve?

 

[Dr.Jeff]

Unfortunately I'm not that well versed on the various Fiat calipers and their differences. But generally speaking, if you increase the piston sizes on the calipers (either/or larger fronts, swapping the rears from drum to disc, or larger rears), then a larger master cylinder piston is needed to maintain the throw (travel) of the brake pedal and the 'feel' of the braking effort. The links below are a series of articles that offer better explanation. An adjustable brake bias valve in the rear circuit would help to find the balance that suits your application and personal preferences best (example photos below).

Articles:
http://www.stoptech.com/technical-s...ias-and-performance-why-brake-balance-matters
http://www.stoptech.com/technical-support/technical-white-papers/brake-system-and-upgrade-selection
http://www.stoptech.com/technical-support/technical-white-papers/rear-brake-upgrades
http://www.stoptech.com/technical-support/technical-white-papers/proportioning-valves

And another simple article: https://grassrootsmotorsports.com/articles/grm-vault-grassroots-guide-braking-systems/

Typical adjustable proportioning valves:

 

[NM850]

Not sure about front/rear brake bias and it I'll need a proportioning valve. Maybe I was planning to use the X or 128 front calipers to give a little more braking force to the rear brakes without having to use a proportioning valve?

Don’t worry about “having” to use a proportioning valve. They are easy to plumb and very useful in setting up the balance for different tracks and/or conditions. I’d actually change the front calipers, mostly to save unsprung weight and get more modern dual or quad pistons. That may not fit your replica theme though.