Larger brake master cylinder sizes?

Pete Whitstone

Pete Whitstone

True Classic
Hi all,

Is anyone aware of any master cylinders that fit our car but have a larger bore than the stock 19.05mm (3/4") bore? I'm wondering if there are other, somewhat larger cars in Fiat's line-up, like the 131 or something, that might have a larger bore but share the same mounting?

Short of that, what's the feasibility of boring the MC to get a larger piston size?

Thanks,
Pete
 
Hey Pete, I know we threw this stuff around quite a bit in the past....discussing brake upgrades. But I don't recall anyone identifying a good replacement large-bore MC. A couple of considerations were mentioned but they had some rather major obstacles. The use of dual cylinders, adjustable, 'race car' pedal box set up is too involved in my opinion. And there was one idea from a car with a vacuum booster, but also not practical due the size. Certainly would be nice to find a suitable bolt-on replacement. Perhaps some fresh perspective will open new possibilities. I guess the other option would be to modify the mounts, etc to use a MC from another vehicle entirely (maybe VW?)? Perhaps a fairly simple adapter plate could be made to convert the bolt pattern, with a customized (hybrid) push-rod?
 
The majority of Fiats I have seen all had 19mm MC’s. I have seen a couple of other MCs with a mounting hole pattern like but not the same as the X.

What will be the difficult is the bolt patterns.

One could try to find larger guts and then have an X MC drilled out to a larger diameter as a possible approach.

Why do you want a larger diameter MC? (Out of curiosity)
 
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So something to consider: Find internal guts of the size you want and then machine a new MC body to the spec required.

The MC body isn’t a particularly difficult object, its really about getting things where they should be. One could create it so it can be sleeved (as you really don’t want to have to do this a second time) by one of those companies. If it was sleeved you could likely make it out of aluminum and not have any issues as you would really only need to install it and likely never service it.

It doesn’t need to end up looking like a casting, it could be a square billet in large part aside from where the hoses, hard lines and bolts attach.

As this I assume is for the 037, does it need to even be packaged in the pedal space anymore? Woudn’t this open up other possibilities?

If this is for the X, a larger MC will likely require more push than your leg can comfortable deliver, could you instead look to a remoted MC in the frunk which could also allow you to use vacuum booster? It could still be an all mechanical solution not like the Scorpion’s remote booster. There are some well engineered 90°mechanical setups used by people who want to clean up engine bays (shaving the engine compartment) and some retrofits for older American cars. This could allow you to look at a variety of different existing MCs.
 
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I looked into this in my brake upgrade to UNO TURBO front brakes. I did purchase a 20MM reamer and was going to ream the stock cylinder. The internals were from a Nissan, I think maybe a pick up truck, can't remember now. With that in mind I then turned my research to using two Lockheed 6" remote servo brake vacuum boosters ( 1.9 or 3.2 ratio) with Wilwood proportioning valves. The servo's would be placed in the front trunk where the windshield washer fluid container is located. The stock Fiat Master with the valves in each circuit would be for adjusting the brake bias. What turned me off of the 20MM was greater brake pedal force required due to the larger master cylinder.


TonyK.

Grimsby Ontario Canada.
 
The top unit is what is needed, try and get it. I have been on a waiting list since October, they are made in Spain and limited production that sells out fast.

TonyK.

Grimsby Ontario Canada.
 
kmead said:
So something to consider: Find internal guts of the size you want and then machine a new MC body to the spec required.
Click to expand...

Yup, that's what I was thinking.

kmead said:
One could create it so it can be sleeved (as you really don’t want to have to do this a second time) by one of those companies. If it was sleeved you could likely make it out of aluminum and not have any issues as you would really only need to install it and likely never service it.

It doesn’t need to end up looking like a casting, it could be a square billet in large part aside from where the hoses, hard lines and bolts attach.
Click to expand...

Whoa there, Karl. You jumped to the opposite end of the spectrum on me. :D I asked about alternative bore sizes because I DIDN'T want to reinvent the wheel. Using MCs from other cars is certainly on the table, and getting them mounted would be far less fab work than starting from scratch. But first I wanted to explore the path of least resistance.

kmead said:
As this I assume is for the 037, does it need to even be packaged in the pedal space anymore? Woudn’t this open up other possibilities?
Click to expand...

Yes, for the 037. I have not changed the pedal box and don't really plan to. This is a longer term project, right now the car utilizes the stock hubs/brakes, but that will have to change at some point. My plan has always been to finish the car with what is on hand, than look at upgrades later, especially in the brake department. I think I've found the donor vehicle I want to use for this, which has stock 54mm slave cylinders in front and a 7/8" MC bore. No, it's not a Fiat 500, I'm looking at 2002-2004 Acura RSX parts.

Good information on the boosters, I have no doubt the pedal effort will be impossible without one.

Pete
 
kmead said:
a larger MC will likely require more push than your leg can comfortable deliver
Click to expand...
Actually the pedal effort will not increase IF the calipers (slaves) are also increased in size by a proportional amount. Hydrodynamics: equally larger volume on both ends = same effort, same pedal throw. It's when you start changing one end more than the other that the dynamics change. This is why going to a larger MC is worth the effort if you upgrade the calipers' piston size.

I'm with Pete here, not worth reinventing the wheel or going to an elaborate remote/side mounted, boosted, or other creative design. Even boring the original MC to fit another (larger) piston assembly can get rather involved. These are all great ideas and sound like things I would do, but making an adaptor to fit another master cyl onto the original position seems much more doable and more reliable. Plus it would be easier to engineer than building a complete new style system from scratch. Just my view.

Pete, I'm very interested to hear what you come up with for this. So you are thinking on using the Acura MC and calipers? Please share more details on what you've found so far. Thanks.
 
Honestly I think you will find that modifying this to work where an X MC lives will be pretty difficult thing to do.

465A8F4C-D638-4B21-83CD-25D89FD9D342.jpeg


Agreed that what I suggested is no little task either
 
If you remotely mount the fluid reservoir by using the adapter nipples (like on the stock X master cyl), then it would be no problem to mount another MC in place of the Fiat part. Just adapt the mounting holes and push rod accordingly.

I'm talking about these guys:
22659.jpg


They fit the same hole as a reservoir (so long as the diameter is the same).
 
Sorry I am not speaking of the small matter of the fluid reservoir. I was trying to highlight the mounting and the location of the lines coming off it.

More the attachment to the pedal box and the actuation of the MC itself which is optimized for an intermediary of a booster.

But as I say at work, anything is possible if you want to make it happen.
 
kmead said:
I was trying to highlight the mounting and the location of the lines coming off it.
Click to expand...
Ahhh, I misunderstood. I'd have to look at both of them (stock Fiat and whatever replacement is being considered) side by side to evaluate that part of it. But I certainly see your point.
 
kmead said:
The RSX
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Ya, I see how the Acura MC has an extended piston rod part that protrudes pretty far out. There are a lot of other MC's from various cars that don't have that. I recall some of the VW units from Mk1-2 VW's for example:
vw-rabbit-with-power-brakes-master-cylinder-2277.jpg

These are available in 20, 21, 22, and even 24 (I think) bore sizes. But even then it depends on what brake system is on the car (eg. Bendix vs ATE); other versions of MC on those same models also have an extended piece like the Acura. So finding the right match will be a search.
 
kmead said:
how this Scorpion owner upgraded his brakes
Click to expand...
Interesting note he made about previously using "2nd series Toyota MR2 Turbo" brakes on all 4 corners. No detail was given. Wonder how that was done and if it could be a decent option for us?

His latest upgrade is the same one we discussed at length previously - new Fiat 500 brakes. He used the rotors, but opted for aftermarket Wilwood calipers. We determined the rotors and calipers were a fairly direct bolt-on for the X. However the rears are not ideal. The 500 is front weight biased so the rear calipers are actually smaller than the X's. But I still like the idea of using front calipers on all 4 corners, regardless if it is the stock X calipers of any upgrade. Then add a proportioning valve to balance things. The obvious drawback is a lack of parking brakes (which we also discussed). To me that's not an issue, I'd just add a line lock. But I get why some may not like that.

To the subject of this current thread, he opted for aftermarket master cylinders (dual) and pedal box. To me that is way too involved to fabricate for a street car. A true track car yes, but I think there are much simpler approaches...as we have been saying (earlier posts).
 
The 500 brakes are less than ideal in several ways. The 54mm fronts are way more brake than is needed and the 34mm rear calipers make the front to rear bias even worse than the existing X parts (relative to the 54mm front caliper’s pistons).

I can lock up the front tires pretty easily depending on the road surface with 48mm OE front calipers, I personally have no interest in increasing the front braking bias on my cars.
 
Dr.Jeff said:
Interesting note he made about previously using "2nd series Toyota MR2 Turbo" brakes on all 4 corners. No detail was given. Wonder how that was done and if it could be a decent option for us?

His latest upgrade is the same one we discussed at length previously - new Fiat 500 brakes. He used the rotors, but opted for aftermarket Wilwood calipers. We determined the rotors and calipers were a fairly direct bolt-on for the X. However the rears are not ideal. The 500 is front weight biased so the rear calipers are actually smaller than the X's. But I still like the idea of using front calipers on all 4 corners, regardless if it is the stock X calipers of any upgrade. Then add a proportioning valve to balance things. The obvious drawback is a lack of parking brakes (which we also discussed). To me that's not an issue, I'd just add a line lock. But I get why some may not like that.

To the subject of this current thread, he opted for aftermarket master cylinders (dual) and pedal box. To me that is way too involved to fabricate for a street car. A true track car yes, but I think there are much simpler approaches...as we have been saying (earlier posts).
Click to expand...


I had a few moments to look into this as I have all of the Fiat 500 Abarth brake components sitting on the barn floor. The pad holders use a 12MMx1MM bolts, the X uses 10 MM x 1MM bolts. The centres are the same, but sit to close to the centre line of the hub. When bolting on the rotor the pad holder is not centred to the rotor so the and 1MM or so would need to be removed from the mating surfaces of the pad holder. So boring out the bolt hole on the knuckle from 10MM to 12 MM is not an issue and to revert back installing bushings or sleeves is simple as well. The difficulty lies in the diameter of the Abarth rotor, it is too big in diameter. I measured the rotor and it is 283MM, I suspect taking about 10 to 13 MM would correct the problem. Chucking it in my 4 jaw could take care of this. Fiat 500 Abarth rotors are obtainable on this side of the pond, unlike the Uno Turbo rotors currently on my car. The piston size is 54MM which is on the large size so a proportioning valve would be required to dial back the front brake effectiveness.
IMG_0756.JPG

Stock Fiat X1/9 Knuck shown with Abarth Pad Holder installed.


TonyK.

Grimsby Ontario Canada
IMG_0756.JPG
 
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