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Discussion Starter · #1 ·
Hello,

I have been searching the threads and can not find one that mentioned the MC piston size for the aforementioned bike. The bike is a dual disc four total piston set up. Any help or (links to a MC) would be appreciated.

Thanks
Kris K
 

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If you have the MC, look on the side as it is cast into the MC body.

14 = 14mm
1/2 = 1/2"
5/8 = 5/8"

Are the most like numbers you will see.

What size pistons do you have in the caliper(s)? I am guessing you will want to match MC to get best ratio?

Jerry
 

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Just went through this myself. The dual disc uses the 5/8 size MC. BTW, just bought the MC linked above and it is a beaut. Recommend not fooling with the 35 year old rebuild. Just spend the money and start fresh. Besides the kit is about the price of the kit.
 

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Capt,

Its not a sure thing to use a 5/8" MC without first checking to see what piston sizes you have in the calipers. The first generation dual piston sliding calipers came in different piston sizes for different applications. You should check in case the PO(s) changed a caliper(s) before you bought the bike.

Too big a MC piston will work, but will not generate the same pressure to squeeze the disc(s); too small of a MC piston and you then do not move enough fluid to lock up the brakes.

It takes only a few seconds to check. THis is good reading: https://www.vintagebrake.com/mastercylinder.htm
 

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Discussion Starter · #6 ·
Capt,

Its not a sure thing to use a 5/8" MC without first checking to see what piston sizes you have in the calipers. The first generation dual piston sliding calipers came in different piston sizes for different applications. You should check in case the PO(s) changed a caliper(s) before you bought the bike.

Too big a MC piston will work, but will not generate the same pressure to squeeze the disc(s); too small of a MC piston and you then do not move enough fluid to lock up the brakes.

It takes only a few seconds to check. THis is good reading: https://www.vintagebrake.com/mastercylinder.htm
That's interesting spacetiger, I got a new cheap MC on ebay with a 17mm piston and it sucks, the thing is, is that it barely moves the caliper pistons for some reason, when it is fully depressed there is braking action but really not enough, i don't know if I just set it up wrong or what.
 

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Kris,

It sounds like you do not have the system bled well enough; there is still some air in the system if your caliper pistons are barely moving. The 17mm piston is much bigger that stock, so the stroke (lever pull) should be more than enough to move the caliper pistons. BUT, you will not generate the same pressure because the ratio (hydraulic ratio) has changed to be a smaller number (caliper piston area/14mm area > caliper piston area/17mm area).
 

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Is there an MC where the reservoir angle is adjustable? Since I changed the bars when I went naked on my GL700 interstate my reservoir is sloping and therefore not as full as it could be! Brakes still work well but always nice to have reserve.
 
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