Rear Caliper piston sizes
 

Hello fellow gassers!

Been having an awkward problem and i think it's time to direct it to this forum, the one-stop place for all gasgas related info! If you can't fix this... nobody can! :(

Rebuilt my bike completely replacing anything that needed replacement, and one of the things done was to rebuild the rear caliper as the piston had melted the seal around it and would not go fully out, leaving me without a rear brake on a dangerous race. (crashed big time too)

Anyway, As i was putting things together a problem came up that was unpredictable and really hard to fix.

I ordered the seals for the caliper and i received two seals with two different part numbers (not a rebuild kit) and they are both thin (both seem like fitting in the outer groove only) and they are different diameter???

After this delaer stopped responding to my emails i tried another and asked him for the thick seal and i received it, but this too is the second size, the one that won't fit due to diameter (seal is bigger than the piston)

My bike is a 2013 (EC250) and even though i read about a difference between the calipers of pre-2009 to post 2009, there is nothing online explaining what the difference is.

Coincidentially a friend brought a bike over to do some similar rebuild work and we have the exact same piston.

I own a second bike (YZ125) and the second size of seal i receive is the one that fits both the rear piston of the YZ (slightly wider piston) and the front pistons of both bikes.

The only piston i see with a smaller diameter is the rear GasGas piston and i thought it was funny that a 250 had a smaller piston than a 125, but then it sopped being funny when i tried finding seals for it!

Any idea what's going on here? does my bike have the 2009 caliper? (There's some story behind this bike... let's consider it a second hand bike so we save some talk, but it's a bike built from an official dealer that i found different model parts on it. (all the bad stuff) but at least the frame and engine have been verified to be 2013ers)
So the chance of foul play exists and big time. Not rushing that conclusion, just letting you know that i didn't receive this "in the box".

Somebody know if there's a difference in piston sizes between the -2009 to +2009?

I might end up using a thick seal for the front, but that's not my favorite way of making a break reliable to race! ( i don't race no mo', but i do ride it like i stole it!)
edit: no i can't, they are different diameter dummy!

Your knowledge is valuable and highly appreciated so i can get back to doing that!

P.S. and my gift of appreciation for your help :

The piston in the image is titanium (but that doesn't affect the size :P)
Titanium does not transfer heat as fast as steel and it is 50% lighter for your unsprung weight concerns. It's a super upgrade (only for the rear) and priced real close to the original part.
find them at www.titanclassics.com (you will find it by size, not model/bike as GGs are not listed!)


It's a KTM world ;-)

Thanks again!

More it's a Honda world surely? It's a Nissin caliper isn't it? Not an expert on the. . I'm going to say 'newer ones' cause I'm in a 2007 timewarp :).

I'd research CR250 piston sizes and measure yours.

As an aside the sliding pins wear and replacement can be bought from, heck I think I just chose an RM with Nissin and ordere might have had to trim it. The pad pins also get grooves but again easy replacement aftermarket for Nissin from something else.

Check the pad listing, I thought they went to a Yamaha caliper later. I am pretty sure they changed to a smaller caliper around 2007/8/9.
Piston size has no correlation to engine size, rather the master cylinder it is mated too.

Where can i find that info my friend? Thanks for the response!

Well, ok, but i thought a 250 and 300 would implement wider pistons for better heat absorption by dispersing it to a bigger piston.
So, did both pump and caliper change in 2010 as they are mated together?

True, if we're talking brake pistons! (Nissins!) but i meant that it is hard to locate your piston (or other performance parts) for a GG in a KTM world.
It was about the website of where to get the Titanium piston.
Which btw, i forgot to mention that titanium remains corrosion free and it's an overall superb upgrade for that dirty unsprung crucial element on our dirtbikes!

Thanks, i'll do that, but still... if someone knows for sure if the piston has changed in 2010, then i have something more against this seller that sold me a bike made of older model parts!
My 2013 came with a 2011 kickstarter among other problems i'd rather not mention and it was purchased to race an extreme race.
It had a faulty pump, a faulty estarter (fixed with your help here) missing spacers in the bearings, missing washers in the swingarm, a purple rear disk (not coated, but from extreme breaking) and i hold back from writing what else was used/old on it.

But thanks for solving half of my problem so far by telling me where to find the darn seal!

I am lucky (or smart) hanging on to a YZ 125 1998 and there are a lot of things that match both bikes as the GG seems more YZ based than a CR. (don't get confused by red :P)

I wish the same was true for the piston. Because your experience is right, this one smells like a Honda :D

added some pics of what i did to those calipers and getting stuck in silly stuff like that!

If this caliper turns out to be of a different bike, then i will consider getting the right one, not for the extra size, but for a bike being as it is supposed to be from factory! Especially now that the good old GGs belong to the past!

If i sell it i can't add notes about finding this seal from this bike and that from that bike for the new owner!

I wish i hadn't cerakoated it first before discovering this problem coz if i replace it now, the cost skyrockets to new paints etc. and as the bike won't be getting sold i might keep it that way.
But there must be a place to discover what changed in the 2009+ upgrade!!! for crying out loud :D

Solution provided by Powerhouse Automotive Ltd (Powerhouse UK,from their ebay store (Powerhouse-UK) as i ordered a set:
Mind you, i didn't even ask and i received this professional response translated in my language too:

"Hi there.
I need to check that you are happy with your order.
Different models of the EC250 use different size pistons - the kit you have ordered is to repair a rear brake caliper with a 25.5mm diameter piston, whereas earlier models have 27mm.

Please confirm which of the two measurements you have and I will dispatch the correct kit for you."

I asked why and they too don't seem to know why.
So there you have it gassers!
There are two pistons for our bikes and nobody knows why!

The difference between the pre-2010 model caliper can be discovered by looking at the brake line.
Tip by <Enduropuro.it>
It does not relate to pistons though.


Pre-2010 EC brake line goes below the caliper
2010+ break line rises to the caliper.

I own a 2013 with the brake line going up, yet i have the smaller piston.
Foul play? It must be too good (like finding a perfect caliper from another bike and swapping it and the question would be "why?")

In pictures you see the two sets on the same piston.
i am adding one more post with a couple more and the brake lines in case someone in the future comes across (bangs his head at) the same problem if they try to order the official seal set and not an after market one.
Note the difference in width too.
The parts on the diagram are not correct! (both wrong on width AND size!) (one is 25.5 and the other 27mm and only one is the size for its groove)

On my way to contribute some money to this forum as well, as i've used its services long before this post to find my way around this weird (but best) bikes!

And to thank Powerhouse-UK and their BrakeMasters rebuild kits: It is the only rebuild kit i have ever received that each part comes on separate bags, not shaking around the box
and more importantly, with a superior seal quality that is also apparent to the eye (pics)
You can research seal quality online and see how much fake rubber exists out there!
Everything else is top notch too,better than other rebuild kits and don't miss tonotice the golden bleeder bonus!


Cheers Old-skewl GG team!
-Another Gmaniac!

Title can change to *Solved* as gassers are not about knowing the right answer, but about riding to it and this thread has done it's job!
:cool::cool: