# Can I use a master cylinder 20mm bore instead of a 22mm?



## EDG (Apr 21, 2006)

Hi!
I need to replace the master cylinder on my 16v Scirocco. My car use a 22mm bore and a friend have a 20mm. Looks the same but the difference is in bore. Could be this a problem?


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## GTijoejoe (Oct 6, 2001)

*Re: Can I use a master cylinder 20mm bore instead of a 22mm? (EDG)*


_Quote, originally posted by *EDG* »_Hi!
I need to replace the master cylinder on my 16v Scirocco. My car use a 22mm bore and a friend have a 20mm. Looks the same but the difference is in bore. Could be this a problem?









reducing MC size will increase your brake gain (higher pressure less force).
You should be fine.


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## gunnr0991 (Aug 1, 2006)

*Re: Can I use a master cylinder 20mm bore instead of a 22mm? (GTijoejoe)*

A smaller dia. MC will result in less braking, as you will be moving less fluid to the calipers and/or wheel cylinders, and it will take more travel to get the necessary amount of fluid to the calipers/WCs. 
Yes, there will be less force required, but you won't get the volume you need, it's just like your hydraulic floor jack, doesn't take a lot of force, but takes more strokes.
Force = Pressure X Area
Same pressure on pedal X less area = less overall force
But going from 22mm to 20mm isn't too big of a deal, as long as you can get the required stroke, the rest of your system is on good shape and you are using good pads.


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## spitpilot (Feb 14, 2000)

*Re: Can I use a master cylinder 20mm bore instead of a 22mm? (gunnr0991)*

By reducing the cylinder bore you will be taking out some of the margin for brake fade...if you get into a situation where you induce significant fade..trailer towing and/or mountain driving..fast street or trackin..you might not have the reserve "stroke" on the pedal that you need to accomodate fade since the smaller diameter requires more pedal stroke to start with just to get normal brakin action...I'd change back to larger cylinder as soon as you can get/afford one.


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## EDG (Apr 21, 2006)

*Re: Can I use a master cylinder 20mm bore instead of a 22mm? (spitpilot)*

Thanks for the information, I forgot to mention something I,m using braided lines. Appreciate the help.
Happy new year from Puerto Rico!


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## GTijoejoe (Oct 6, 2001)

*Re: Can I use a master cylinder 20mm bore instead of a 22mm? (gunnr0991)*


_Quote, originally posted by *gunnr0991* »_A smaller dia. MC will result in less braking, as you will be moving less fluid to the calipers and/or wheel cylinders, and it will take more travel to get the necessary amount of fluid to the calipers/WCs. 
Yes, there will be less force required, but you won't get the volume you need

As I stated smaller MC increases brake gain, increased brake gain increases braking force. You will be moving the same amount of fluid (since your system is still the same), it just requires more stroke to do it. We are discussing less than 10% difference in bore sizes, increasing stroke would be roughly 20% more and increased pressure would be roughly the same difference.
Your description for force and pressure is pertaining to the caliper's pistons, NOT MC sizing, less force for less area is drivin by pressure, the pressure made by your MC. 
Simply stated, System Pressure = input force/MC bore area.
I think your overall understanding is good, perhaps just confused?
Fluid volume when sizing a MC is decided based on caliper piston area, disk to pad gap, pedal stroke/feeling, and MC stroke run out. Since the difference is so small between 22 and 20, he should have no problems to make the system work.


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## spitpilot (Feb 14, 2000)

*Re: Can I use a master cylinder 20mm bore instead of a 22mm? (GTijoejoe)*

It will work..but VW put a bigger MC diameter in there for a reason! Remember the area of the MC piston goes down with the square of the radius (diameter/2) so what looks like a little diameter change will affect volume of MC output more. Just a heads up on sacrificing design margin..whenever you do that you want to be sure you understand just what you're givin up! When I comes to brakes..I always go conservative...if you skimp on engine you'll just get less HP or MPG or less smooth runnin...brakes not working in severe conditions..just when you might really..really need all the brakin you can get...that can cause major damage..to others as well as yourself...modify braking system with care...if you're gonna modify it at all! http://****************.com/smile/emthup.gif


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## GTijoejoe (Oct 6, 2001)

*Re: Can I use a master cylinder 20mm bore instead of a 22mm? (spitpilot)*


_Quote, originally posted by *spitpilot* »_It will work..but VW put a bigger MC diameter in there for a reason! Remember the area of the MC piston goes down with the square of the radius (diameter/2) so what looks like a little diameter change will affect volume of MC output more. Just a heads up on sacrificing design margin..whenever you do that you want to be sure you understand just what you're givin up! When I comes to brakes..I always go conservative...if you skimp on engine you'll just get less HP or MPG or less smooth runnin...brakes not working in severe conditions..just when you might really..really need all the brakin you can get...that can cause major damage..to others as well as yourself...modify braking system with care...if you're gonna modify it at all! http://****************.com/smile/emthup.gif 

I totally agree with you. If you don't know what you're doing than don't touch it. But I design OEM brakes for a living, I know how to size brake systems, there is NOT always a right or wrong answer, when you change a system characteristic one way, you will be sacrificing something else in another, we put smaller MC's on our OEM/prototype vehicles all the time to determine different system characteristics. True performance braking systems are some of the easiest systems to size by design, because the braking habits are always the same. 
The percent differences I gave were rough estimation of numbers I ran, so indeed you are right with volume, the effect is more, thats why the difference was almost double http://****************.com/smile/emthup.gif 
Can I tell you exactly why VW determined a specific size MC. Of course not, because there is no way to know the determined performance/feeling GOAL by the manufacturer unless you speak with the designers, things are not that simple. Although with my experience I can spectulate many things. There is more to a brake system than MC's and calipers, boosters and brake pedals are always taken for granted.
One of the largest misconceptions of a brake system is the effect of a MC, novice's believe that smaller= less braking and in fact its the opposite. Determining brake gain is how you determine a systems effectiveness. 
I admit its always challenging on forums to determine who's advice to take, since no body always understands someone elses background or experiences. http://****************.com/smile/emthup.gif 
HAPPY HOLIDAYS http://****************.com/smile/emgift.gif










_Modified by GTijoejoe at 1:05 PM 12-29-2008_


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## spitpilot (Feb 14, 2000)

*Re: Can I use a master cylinder 20mm bore instead of a 22mm? (GTijoejoe)*


_Quote »_
One of the largest misconceptions of a brake system is the effect of a MC, novice's believe that smaller= less braking and in fact its the opposite. Determining brake gain is how you determine a systems effectiveness. 
I admit its always challenging on forums to determine who's advice to take, since no body always understands someone elses background or experiences. http://****************.com/smile/emthup.gif 
HAPPY HOLIDAYS http://****************.com/smile/emgift.gif









_Modified by GTijoejoe at 1:05 PM 12-29-2008_

Yup...you read about how fewer and fewer US kids are doing well in basic math and science and the evidence is here every day...simple physics seems to be well off many folks' radar here. In addition the concept of studying somehting B4 you attempt it is also lost on many...its deemed "not manly" or something to read a shop manual B4 you pick up a wrench and start tearin into your car...I read far too many Vortex posts that say... "Well I decided to change my ......now the engine won't start/idle etc...please help!"....Duh..they call it a timing belt for a reason..it sets timing..but only if you pay attention to timing marks during replacement!


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## gunnr0991 (Aug 1, 2006)

*Re: Can I use a master cylinder 20mm bore instead of a 22mm? (spitpilot)*

So, I'll admit to some backward thinking, a smaller MC will result in more system pressure... input force will remain the same, output force (at the end effectors) will increase.
However, while the lever effect of the brake pedal and the booster's effect are often neglected in the thinking of brake systems, a smaller MC results in more pressure input as the pressure generated by the MC is determined by the Brake pedal force / area of master cylinder
A MC diameter reduction of 2mm results in a decrease of effective area of approximately 17% A 20mm MC has an area of 314 sq.mm, and a 22mm MC has an area of 380 sq.mm
By taking away area you increase the pressure, easy way to think of it...
Have your girlfriend step on your foot with flats, see how it feels, then ask her to try it with heels.
yes, force stays the same, area decreases, therefore pressure goes up.
Yet, it would still take 17% more pedal travel to move the same amount of fluid, but once the pads are in contact you are applying more output force, as the piston area is the same, but the pressure is more, and braking force would increase. As long as the pedal gives you the necessary amount of travel at least.


_Modified by gunnr0991 at 11:33 AM 12-31-2008_


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## kickice24 (Apr 15, 2002)

*Re: Can I use a master cylinder 20mm bore instead of a 22mm? (gunnr0991)*

"It was my understanding that there was to be no math."


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## g2turbo7 (Nov 5, 2004)

*Re: Can I use a master cylinder 20mm bore instead of a 22mm? (EDG)*

One thing that was not mentioned is that the brake pedal will also become soft and spongy. Modulating the brakes becomes guesswork as the pedal feel is drastically reduced. It feels like air is in the system.
You may hear arguments as to how fluid is not compressible (which is true), and how it directly applies to a hydraulic system. The practical reality does not correspond. By downsizing the MC you will likely discover my comments to be true. Personal experience speaking here.
Good luck in whatever you do. Just don't let Mr. Darwin prove his theory yet again.


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## VWn00b (Mar 4, 2005)

*Re: Can I use a master cylinder 20mm bore instead of a 22mm? (g2turbo7)*

Ok, somewhat of a related topic for Joe:
Going from a mk3 stock disc/drum setup, to a Wilwood 4 piston/disc setup, what would be the best thing to do in order to keep somewhat close to the stock pedal travel while also increasing the amount of fluid needed by the bigger brakes up front? I know there is the 23mm and the 25mm MC, but I never really understood what would happen to my pedal travel, on paper?
Side note, I haven't put the brakes on yet, but I was waiting to do so until I got another MC just incase I needed it.


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## g2turbo7 (Nov 5, 2004)

*Re: Can I use a master cylinder 20mm bore instead of a 22mm? (VWn00b)*

I would guess they have various size pistons to choose from to match the respective MC. It may not be "one size fits all". Wilwood would obviously know.
The bigger MC would move more fluid faster and reduce pedal travel, while increasing pedal effort (may be good or bad depending on viewpoint). As a previous poster mathmattically spelled out, the slight difference in diameter makes a substantial surface/volume change. Given a choice, from personal experience, I would always choose a bigger/oversized MC. This will drastically effect/increase pedal feel or modulation-- absolutely critical for driving at 10/10th's.
One should also factor in the vaccum booster and it's effects-- another topic not mentioned yet. If you have a race car with low engine vaccum due to high overlap/high lift cams, this could increase pedal effort due to reduced vaccum assist and could compound any hydraulic components mismatching issues (for better or worse).
In the practical world it may not be an issue for a street car. Years ago i did use an aggressice Schrick cam on my low compression GTI engine, and didn't notice anything.
Okay, the espresso is wearing off. I'll stop


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## GTijoejoe (Oct 6, 2001)

*Re: Can I use a master cylinder 20mm bore instead of a 22mm? (VWn00b)*


_Quote, originally posted by *VWn00b* »_Ok, somewhat of a related topic for Joe:
Going from a mk3 stock disc/drum setup, to a Wilwood 4 piston/disc setup, what would be the best thing to do in order to keep somewhat close to the stock pedal travel while also increasing the amount of fluid needed by the bigger brakes up front? I know there is the 23mm and the 25mm MC, but I never really understood what would happen to my pedal travel, on paper?
Side note, I haven't put the brakes on yet, but I was waiting to do so until I got another MC just incase I needed it.

Basically your MC to caliper piston area will need to be very similar, displaced fluid volume is going to drive your stroke spec. Dealing with wilwood with their products, they are the best to ask such questions since they have the specs on their products. http://****************.com/smile/emthup.gif


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## 83mk2scirocco (Nov 13, 2008)

*Re: Can I use a master cylinder 20mm bore instead of a 22mm? (EDG)*

Im going from drum to disc on my 83.The spring on the drums retract at like 100 plus pounds.To over come the retracted pressure. The master cylinder there is a residual check valve thate hols pressure to the rear drums.So if I swap to disc with out changing any thing else there could be aproximitly 25 lbs residing to the rear brakes.In the long run the rear brakes would wear out twice as fast as the front.
The 20mm im guessing is like mine (rear drum) 8V.The 22 rear disc,16v.I would stick with the 22 mm.


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## g2turbo7 (Nov 5, 2004)

*Re: Can I use a master cylinder 20mm bore instead of a 22mm? (83mk2scirocco)*

I did the same rear disc conversion about 15 years ago to my Scirocco. Being so long ago I don't remember, but also don't recall taking the residual valves out. 
Naturally, with just a rear disc upgrade it will cause a heavy rear bias. The car will become very unstable and dangerous.
If the front brakes are not upgraded, I strongly recommend NOT to "upgrade" to rear discs, unless using an adjustable proportioning valve (Wilwood makes one).
I know people that race SCCA IT class with rear drums on their Rabbit and do just fine-- often outbraking other cars, and outcornering just about everyone, too.


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## 83mk2scirocco (Nov 13, 2008)

*Re: Can I use a master cylinder 20mm bore instead of a 22mm? (83mk2scirocco)*

Im going to get all of it.....Im still trying to figure where all perportioning stuff is.Its bolted under neath the mast?


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