Front Master cilinder Kit, piston doesn't fit my BMW K75 1986 ......F%&$

[foliver]

Hi.
i cannot bealieve this one.
I've bought a new magura front master cilynder kit 13mm, it's uposed to be 1978-1996

But it doesn't fit.

It measures 12,92, and the one into the bike measures 12,52........

I should bought 12.....but i bought it prior to the dismantle, so....

Here i am.

What can i do.

Or, i leave the old one as it is, the rubbers are not destroyed, but is a peety, and the cilinder needs to be polish a bit to take of corrosion.

Or, i take the seals of the new 13mm one, and i use them into the original piston, after polishing ...( hoping they don't break).

Or, reduce the diameter of the new 13mm to allow to fit ( a little danger cause i can damage the seals, but is doable)

Or buy another one, i don't want......but...

[frankenduck]

I'd just use the old one if the seals look good. I wouldn't bother polishing it aside from rubbing it with some 0000 grade steel wool. Also clean the master cylinder bore with some 0000 steel wool.

And inspect the master cylinder bore for pitting. If it is pitted then the piston seals won't matter and you need to replace the master cylinder.

[Laitch]

Hi.
i cannot bealieve this one.
I've bought a new magura front master cilynder kit 13mm, it's uposed to be 1978-1996
But it doesn't fit.It measures 12,92, and the one into the bike measures 12,52........
I should bought 12.....but i bought it prior to the dismantle, so....

They have different measurements because the 12.52mm reflects wear. The K75 takes a 13mm front master cylinder, not a 12mm. Polish the bore with a scotchbrite pad or very fine steel wool if it isn't pitted, as frankenduck indicated. If it isn't pitted and you are able to polish is slightly, that piston should fit into with the seal after being lubricated with a small amount of brake fluid. Be sure the opening rim of the cylinder is smoothed and free of the crystallized fluid crust that often accumulates on a neglected cylinder.

[frankenduck]

What piston did you order?    

32 72 2 302 356?

[foliver]

Hello Frankenduck and Laitch

I have ordered and they send me:
Front Brake Master Cylinder Kit 13mm - BMW Airhead & K Bike Models; 32 72 2 302 356 / EnDuraLast ( euromotoelectrics)


Ok, here is what happened in the last 2 hours:

The Master cilinder bore was ok, and was polished with 0000 steel wool, the internal diameter of the bore is 12,68 mm ( ?)
All the master cylinder was cleaned in petrol, and polished with steel wool, and painted some days ago (exterior).
I was following Frankenduck idea of using the old piston, but it needed to be polished a bit, and it has some pitings on the larger circunference , the one with 12,52mm.
So i instaled a Drill in a vise to do it, and it was good.

After that and seeing that the drill way worked fine, i decided to reduce the diameter of the new piston.
The challenge was not to touch the seals, and not to everheat the piston cause i didn't know how the rubber was going to react with heat...
I started cutting a 220 sandpaper strip of 14mm that i folded at the half, so 7mm double face.
It was fine but slow, so i went to 150 grain, and it was better, and it worked.
After i achieve the desired diameter i sand a bit with 220, and steel wool to make it shine.
The rubbers went ok.
So i tested and it fitted nice.
here some pictures.
I asked my son for help me with the drill trigger, he didn't want to leave fortnite, but he liked the job, and he did all the measurements with the precision gauge. It was nice.

[Laitch]

It shines like the Cruz del Sur, Fabian! Looking forward to the report about its function.

[foliver]

It shines like the Cruz del Sur, Fabian! Looking forward to the report about its function.

jajaja if it doesn't works, i'll write from the hospital ! 

I need to do calipers, forks, new tire and handlebar, it will take some time to know how they works, but i'll let you know!

[Laitch]

You'll get an idea of how well they work when you bleed the brakes. Only call from the hospital if you like to dine there.

[jad01]

They have different measurements because the 12.52mm reflects wear. The K75 takes a 13mm front master cylinder, not a 12mm. Polish the bore with a scotchbrite pad or very fine steel wool if it isn't pitted, as frankenduck indicated. If it isn't pitted and you are able to polish is slightly, that piston should fit into with the seal after being lubricated with a small amount of brake fluid. Be sure the opening rim of the cylinder is smoothed and free of the crystallized fluid crust that often accumulates on a neglected cylinder.

This^

I just rebuilt the master cylinder in my '86 K75C and the new piston/seals fit fine; no problems and is working great.   

[Lennee]

I'm curious what brake symptoms led to you rebuilding the master cylinder? Not trying to hijack this thread with my new account, but I got my 1990 K75 working again last week and the front brake is being goofy. It stops the bike fine, but then requires me to manually push the lever back into place. Is this similar to your original problem? 

[foliver]

Hi Lenee. Welcome.
No, my situation was different than yours. My bike was parked 6 years and losts all the fluid.
I decided to renew the hole system just to have a fresh restart.
I don’t know what can be the origin of the symptoms of your bike…..others with more experience can chime in

[frankenduck]

I'm curious what brake symptoms led to you rebuilding the master cylinder? Not trying to hijack this thread with my new account, but I got my 1990 K75 working again last week and the front brake is being goofy. It stops the bike fine, but then requires me to manually push the lever back into place. Is this similar to your original problem? 

If I were you I would pull the piston out and clean both it and the master cylinder bore.  Also make sure that the lever is not binding where it pivots.

Similar to this: (but there's a circlip that holds the piston in)
http://www.kbikeparts.com/classickbikes.com/ckb.tech/0.ckb.tech.files/master.leak/master.leak.htm



(I'm about 60 miles north of Olympia on 101.)

[Lennee]

If I were you I would pull the piston out and clean both it and the master cylinder bore.  Also make sure that the lever is not binding where it pivots.

Similar to this: (but there's a circlip that holds the piston in)
http://www.kbikeparts.com/classickbikes.com/ckb.tech/0.ckb.tech.files/master.leak/master.leak.htm



(I'm about 60 miles north of Olympia on 101.)

Great, excellent link, I'll have to send you an email about parts once I take things apart and see what I need. I reside in Hoodsport many weekends so it sounds like we may be within spitting distance.

Getting the bike running again has been fun. First time I thought I had it fixed the fuel line to the rails burst mid-ride and almost caught my bike on fire...whew... Now I just need to fix my front brake and lube my spline and I should be good for a while!