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  #1  
Old 7th December 2018
dtwaddell42 dtwaddell42 is offline
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Default Master cylinder bore question

Hey guys I'm hoping you can help me understand this a little better.

I've got a '74 front caliper that from everything I can read has a 1/2" bore, please correct me if I'm wrong. I'm wanting to replace my front master cylinder to one that doesn't have switches attached to it. This is going on a custom build so I'm trying to clean up the handlebars. I can find other master cylinders but they are not 1/2" bore, they tend to be 5/8 to 9/16.

Can I successfully run these other master cylinders with my stock caliper even though the bore sizes are different? I guess I'm thinking that the bore size should just mean it takes less pull on the lever to equal the same force at the caliper?

I may be way off on this but I'm having trouble finding a lot of material on doing something like this.

Any help would be appreciated.
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Old 7th December 2018
needspeed needspeed is offline
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The stock master cylinder has a 3/4 bore. I believe a 5/8 would work for you. Going smaller is what would give more braking force with less effort at the lever, though the lever might have more movement.

I know the stock bore is 3/4 but
I've no personal experience with this so wait for others to comment.
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Old 7th December 2018
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9/16 (or even 5/8) bore would work if yours is 1/2. The pull will be just a tad stiffer, but not enough difference to worry about. All else being the same, stepping up in master cylinder bore increases to volume of fluid moved, so it takes less lever movement to push the same amount of fluid as the smaller bore.
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  #4  
Old 7th December 2018
dtwaddell42 dtwaddell42 is offline
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Cool thanks for the input.
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Old 7th December 2018
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I have a 77 with stock 3/4 bore with single caliper. It’s a bear to pull, and lacks any feel. I would recommend you go smaller, I.e. 9/16 vs 5/8 if you want better feel, which would mean more throw.


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Old 8th December 2018
dtwaddell42 dtwaddell42 is offline
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So something like this should work then?
https://www.ebay.com/itm/Genuine-Har...item23b0f8a542
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I would get some better specs, and the HD part number of 12345-67 is bogus.


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The area ratio between master cylinder and pistons is what you want to compare.

Take the area of all the pistons, divide it by the area of the master cylinder.

You're looking for something in the 26-28ish range. It can be kind of sensitive. I currently run 27.0 ratio on my bike, which is a single 00-03 Sportster caliper with a 1/2" bore 04-06 master cylinder. It's nearly identical to the new M8 bikes with offset 32/34 four piston calipers with 1/2" masters.

Stock 02 with single caliper is ~21.3 ratio. A bit stiff and lack of feel. Lots of lever force, not very much feedback.

That goes for typical HD levers. This can be a little different depending on geometry, so you basically have to experiment, or take some measurements to make sure it will work with your setup.
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