Nissin Master Cylinder Question- 3/8 (9.5mm) piston size?

Back ground info- Bought a 2001 KX 125 with a known front brake issue- Guy said it probably just needed a rebuild kit and it would be good to go-   He had some aftermarket levers and had drilled a hole in the flange that contacts the piston to allow for a bolt to go through- the stem was so long that it was hitting the throttle casing.  Anyways--  Tossed the old stuff-  got new levers and ordered a kit.


And the trouble starts- I got online and ordered an All Balls rebuild kit and it wasn't the right size-- Got online and ordered yet another kit, this one for a 2001 - 2005 KX 125  from RJATV that came with a new piston, seals, clip, spring and washer- again too big to slide into the master cylinder-   The Master cylinder is a Nissin and rather than the standard 11 that is stamped near the banjo bolt,   mine reads 3/8



I measured the pistons of the rebuild kits and they were 11 mm.....the one that came with the bike measured 9.5mm


I am completely at a loss on what kit to order as this master cylinder is clearly not the original nor is it the standard nissin 11mm that most KX/RM/CR/YZ 125-250 use. I have googled until my finger tips bled and have found nothing-   I have scoured ebay in search of master cylinder pictures in hopes I would spot one that has 3/8 stamped on it-   No luck.   It has been difficult to say the least as I can't even go to Bikebandit or Rocky mountain ATV and order OEM because it will be the 11mm piston -


The easiest thing was to just find a good used 11mm master cylinder and be done with it ( I am just not sure if I will have to buy a new brake line and caliper)  but the curiousity is killing me-    Does anyone have a suggestion of what bike used a 3/8 nissin master cylinder?     Any help would be great!




no idea what used it.

but it has more advantage over the caliper

should have a longer throw and more power,

unless it pulls to the bar......

There should be some sort identifying markings on the master cylinder indicating a part number right ? Is there a one or two digit number anywhere?

Edited by Swes

The simple answer would be to but the OEM master for the KX125.

The only numbers on the master cylinder   show    "304"   &     "3/8"  near the banjo bolt   and on the back side near the site glass   "6 N 1"


I will run a VIN check to ensure the bike is in fact a 2001 as requested by the folks from RJATV


I did run across a picture on ebay of a 2000 KX 125 Master cylinder-   This one is exactly like mine-


The only issue- I found other pictures of other 2000  KX 125 Master cylinders and they showed the typical "11" near the banjo bolt which indicates it is the 11mm piston.  The lack of consistency has me worried and doesn't tell me that in fact this is a 2000 KX 125 Master cylinder on my 2001 bike.

I know it is common to switch out parts for better working ones-  like a 2005 Honda CR 125 brake may have been more responsive so people would switch them out.


I am not the original owner so it is unknown what the master cylinder came off of.

The simple answer of spending $236 for an OEM master cylinder compared to buying a $22 rebuild kit does not make much sense. I was just hoping to see if anyone else has had a similar issue or were familiar with the 3/8 nissin master cylinder.


The guys from RJATV are only worried about the year of the bike to match up the master cylinder---  The only issue with that solution is that it only works if it was the stock master cylinder for that bike.  


It could be that they used a 2000 stock part and used it on a 2001. It could be a master cylinder from a KTM or other model (which the guys had a few KTMs in his garage when I bought the bike) ...  


Bottom line-  I need a nissin master cylinder rebuild kit that includes a 3/8   (9.5mm) piston.



I appreciate all of your help guys- 

Okay- For those that are interested.....It turns out that the bike is a 1997 and not a 2001. I am still getting over the fact that I didn't run the VIN to validate the year. The bike ran like a top and was well taken care of other than the front brake not working so I didn't think too much.... that and  the guy was oblivious to the fact of what he had.   Even now he swears up and down it is a 2001.   The Frame doesn't lie unfortunately.   So, now that I am looking at rebuild kits for the proper year, it all matches up.  It is in fact a 3/8 bore and the piston is shorter and is cusped to fit a ball bolt that goes through the flange of the lever. 


I am trying to cancel an order for a 2001-2005 master cylinder but if it ends up being shipped- do you think it can be fitted with the brake line and caliper from a 1997?


I will let you know when I get the part in. should be interesting.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now