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As the title states, I'm in the process of trying to spec out a clutch master cylinder. Math-wise I'm all set, but I was curious what to do if I can't find the slave cylinder piston diameter.

Someone suggested that this doesn't really need to be sized and just to order a 3/4" MC....id rather try to figure out the force needed and how long the throw would be, but am I overthinking this?

For what its worth, I'm using a 2011 Focus 5 speed MT with the stock slave cylinder.

Thanks,
Casey

According to Dorman, the m/c bore size is .75"

One thing I have heard, although not seen in person, is to watch the restrictions in the lines. If you are using adapters and braided hose, the holes can be small enough to cause delayed clutch engagement. I used -3 AN stuff and all is well on mine.

Cheers.

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Build log: viewtopic.php?t=9291

I'd measure a focus pedal, from pivot to pad center and from pivot to pushrod to get the pedal ratio.
If the ratio is greater than the pedal you will be using ( and they usually are), I'd go with slightly larger bore master, like 7/8".
Only if the bore is so great that the clutch is difficult to modulate due to very little pedal travel between engaged and released, would I consider fitting a smaller bore.

_________________
Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS viewtopic.php?f=5&t=13225&p=134742
Simple Spring select viewtopic.php?f=5&t=11815
LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12

Thanks guys. I'm fine with matching the original mc size, but where did you find that info? May be helpful in the future.

I would have measured the stock pedal, but do not have a donor car.

Typically I use the Napa parts program I use at work to look up specs. Napa is usually very good about listing bore-sizes etc. In this case, no luck so I searched their part number (often you can get the manufacturers info which will have that) and ended up on Dorman Products website which listed it.

In my car I just stuck the OE sized M/C on the Wilwood pedal (which I assume is much lower ratio) and all is well. Most OE clutches are so light that a little more effort is nice IMO.

Cheers.

_________________
Build log: viewtopic.php?t=9291

My Mazda B2000 had a hydraulic clutch. I swapped a Chevy 2.5-liter four and a T5 into it. The Chevy bellhousing had a hydraulic cylinder on it.

I measured the Mazda and Chevy master and clutch cylinders and the clutch forks and figured that a 1" master cylinder would work. I had to fabricate bracketry, a pushrod, and a metric clevis to put a Tilton (hey, they were on sale at the time!) master cylinder on it.

Unfortunately, the clutch doesn't *quite* disengage when the pedal is fully depressed. I bought a 1-1/8" cylinder, but other things have meant that project is stalled for a while.

My guess is that the pedal ratio under the dash was different. But I didn't have an S-10 to compare it to, even if I'd thought about it...

Using the Rock Auto web site, I have about a 90% success rate. They have a large choice of suppliers and it is rare that at least one does not provide whatever spec you are looking for.

Bill

I found a pic of the focus pedal. It is a falling ratio and appears to be about 7:1 before falling but it is a poor photo. If I’m correct, .750” may be adequate with a 7:1 normal pedal and would still have reduced modulation over the original design. The master should last longer too, due to less side loading of the piston in the bore.

_________________
Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS viewtopic.php?f=5&t=13225&p=134742
Simple Spring select viewtopic.php?f=5&t=11815
LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12

My old Chevy truck had a noticeable falling ratio; it was very easy to handle in traffic; easier than the quite linear clutch in my Geo.

If you have room for the pedal travel, it's nice.