Welcome to Netrider ... Connecting Riders!

Interested in talking motorbikes with a terrific community of riders?
Signup (it's quick and free) to join the discussions and access the full suite of tools and information that Netrider has to offer.

How to determine Bore of Master cylinder?

Discussion in 'Technical and Troubleshooting Torque' started by Mr White, Jul 27, 2012.

  1. Hi all, as you can tell im new to the motorcycle world in a way.

    I've been learning basic mechanics with fairly good success but my latest issue is with my newly acquired '83 suzuki gsx 250, which is missing the front brake master cylinder on the handle bar, along with the brake lever.

    I have rung around to suzuki dealers and wreckers in Melbourne and I've now learned that it IS necessary to operate the front caliper, and that I cant just throw any old master cylinder on, because It has to have the right 'bore'.

    I've also learned that a new or used one will cost anywhere from $150 to $300!!! So Im assuming the 'bore' in this case is the width of the tube sending fluid to the caliper or something?

    Has anyone had experience with this or know if I can use, say, a Bandit's part to install on my bike?

  2. The bore is the internal diameter of the cylinder.
    It's important because too small will not move enough volume of brake fluid to fully actuate the brakes, too large will not give enough pressure to properly brake the bike. You need the correct bore to work with the lever design and caliper design.
    Surely any competent motorcycle wrecker could assist you with the correct master cylinder for your bike? If not then the local dealer could provide you with a part number for it.
    • Like Like x 2
  3. Correct, bore = diameter of the master cylinder and piston. Bigger the bore the larger the VOLUME of fluid moved per stroke.

    Pressure is determined by the lever and its leverage ratio.

    At a very basic level if I had a single disc front end with single piston caliper I would need a relatively small bore master cylinder.
    • Like Like x 1
  4. There's only about 3 common sizes for master cylinders on bikes and they are not that different from each other. A wreckers should be able to supply you with a master cylinder for your bike. The good news is that Suzuki have a lot of parts they share between bikes, particularly in the 1980s.

    The other thing is of course ebay. If you go to international ebay sites, you can get new master cylinders pretty cheap.
    • Like Like x 1
  5. This.

    It's a very long time since I looked at the front end of a GSX 250 but the simplest way to tell what might fit is to have a look and see how many pistons the fluid has to push.

    Take a look at the caliper(s). Do they have two pistons (one on each side of the disc) or one (with a sort of sliding mechanism to allow the piston pushing on one side of the disc to pull the caliper body over to push on the other side of the disc)? Suzis of the era generally had two piston calipers but you never know.

    Secondly, is it twin or single disc? Memory tells me that the 250s were single disc but, again, I may be wrong.

    OK, so if we assume a single disc with a twin piston caliper, that gives us two pistons to push. Further, because Suzuki were quite well behaved about parts interchangeability at the time, it is highly likely that the MC from any other single disc Suzuki from ~1977 to 1985 will fit and work satisfactorily.

    And yes, definitely try overseas Ebay. US and UK are both good bets.
    • Like Like x 1
  6. Bang in what you need at www.findapart.com.au, if you find one, given it's age, I would also order a M/C rebuild kit, about $30 on eBay.
    • Like Like x 1
  7. pw n pat are on the money

    the bore size you need is in relation to the total bore size of the calipers, this is extremely important.
    once you work out what size master goes with what size calipers you have - buy a NEW ONE
  8. Thanks for the education guys. I looked around for something that might suit.. but in the end I put in an application on findapart.com.au like MV said. Many people got back to me the next day straight away!

    I think I got an okay deal, compared to all the other alternatives I came across. For a used GSX 400 master cylinder which should fit my bike $70. I also got some parts for my kawasaki included and got a discount :)

    Pretty happy and looking forward to attaching a bloody brake to my suzuki so I can actually ride it safely..
  9. I'd be trying to get hold of a seal kit for that Mcyl. It's gotta be a good 25 years old and has probably stood dry on the wrecker's shelf for a while.
  10. Findapart is good, has saved me many times!

    +1. Good piece of mind.
  11. Hi, its me again.

    Okay so I have recieved the master cylinder today, its in pretty good condition but of course it is old and the sight glass is all cloudy and crackled.

    For one thing I know it wasnt sitting on the shelf dry like you mentioned PatB, firstly because he was telling someone to remove it from his bike while he was on the phone to me, secondly because my express post bag arrived with all the bubble wrap inside saturated with brake fluid! The tax invoice is useless because all the writing and ink has dissolved.

    Anyway, how important is it to reseal the parts and maybe use a rebuild kit? If I attach it and refill it with new fluid etc (and I suppose bleed the brake which I will have to do some reading about), and the brake functions safely, wouldnt it be okay to leave it like that? I mean, atleast for the time being.

    Also, it all seems fairly straight forward and I can refer to guides to make sure i install it correctly, but there is a small slit underneath where I think the brake sensor/wire connects to somehow work the rear brake light. As far as I can see, there is the wire with a small plate with screw holes that I think attaches to the underside of the reservoir thing, but I dont see how that functions. Can anyone shine any light on that?

    Cheers folks!
  12. Maybe Im not as good at Googling as I thought, but im finding it hard to find clear information to help me install the M/C and about how the electronic brake switch thing works.

    But from a link a helpful gent on this forum provided the other day, I've come across this schematic which kinda explains it to me:


    So that wire with the contact plate on the end is supposed to also connect with a switch, which is inserted in the underside of the master cylinder to be triggered by the brake lever? Is that right?

    Sorry to anyone reading this, it seems like im talking to myself, but this is how I learn hehe -_-
  13. Okay I should correct my terminology: The whole thing is the switch assembly? With what I was calling the contact/plate actually being the cover, and inside the little thing I was calling a switch is the 'contact' .

    So in that case, im missing the contact? New question, can I get one down at dick smiths?
  14. No u cant
  15. cant get a contact at dick smiths?