Measuring the Master Cylinder O-Ring. For Science!

[The_Patient]

So I'm going about replacing the usual culprit seal that is at the brake booster at the end of the spring and I need to also replace the master cylinder o-ring as well as it is shot pretty bad. Measuring the bent up o-ring wont help so I took some measurements with my dial gauge at the master cylinder itself, where the o-ring sits into.

I'm wondering if someone can confirm what I found when measuring; for accuracy. Also, I can't seem to source a seal for this online, (Anybody?) but I will check in at Napa tomorrow (Where I got the other brake booster seal.) and see if they have something there.

Here is what I found when measuring the o-ring seat at the MC:

Outer Diameter: 48mm
Inner Diameter: 42mm

Attempt at measuring O-ring (Square style o-ring on my car, not round.):
3mm thick
3mm width

So my measurements seem to add up correctly. This sound about right? The MC was taken off my '84 533i.

And before any complaints are made about "Search!" I did search all of mye28.com using Googles advanced search features and found plenty but nothing about accurate measurements...so...There!

[Mike W.]

That sounds right, the 3MM thickness I'm almost positive of, and the other dimensions sound correct too. At least for the round O ring which is the only one I've seen. I suspect a 1 5/8 X 1 7/8 could be used too, it's very close, but on something like brakes why take a chance.

[The_Patient]

Didn't get a chance to get to Napa will have to wait until Monday. Mcmaster-Carr of course has a nice selection of o-rings and seals. They DO NOT however stock a square profile o-ring in the size I measured. But they do stock round profile o-rings in the exact size I need. Sold in a pack of 25 for about $10...

Has anybody sourced a proper square profile o-ring?? I take it just about everybody replaces that o-ring with a round profile without any leaking issues?

Here are a few photos of my MC I'm working with. As far as cleaning the corrosion off...Brass brush work? Or will it scratch? 2000 grit sandpaper?

[Mike W.]

Didn't get a chance to get to Napa will have to wait until Monday. Mcmaster-Carr of course has a nice selection of o-rings and seals. They DO NOT however stock a square profile o-ring in the size I measured. But they do stock round profile o-rings in the exact size I need. Sold in a pack of 25 for about $10...

Has anybody sourced a proper square profile o-ring?? I take it just about everybody replaces that o-ring with a round profile without any leaking issues?

Perhaps it shows my limited knowledge, but the ones I've seen, all 2 of them, have both been round O rings.

[The_Patient]

Square profile on mine for sure. I don't have any service history on the car so I guess its possible its been worked on and changed. Anyway I know another member sells an o-ring set with round profile so it must work out fine. I need to work on cleaning this thing up, I guess I'll go ahead and just sand the corrosion off with 2000 grit or so sand paper and see how it goes.

[The_Patient]

Also, I read on some other thread or in a guide (Can't remember which) that there is a small mesh cup like screen that sits in where the brake fluid enters the master cylinder. Is this the case on all MC's from the e28? Am I mistaken about that? Mine did not have a screen. Maybe it popped out when I pulled the rubber grommets off under the reservoir, I don't know

[Mike W.]

Also, I read on some other thread or in a guide (Can't remember which) that there is a small mesh cup like screen that sits in where the brake fluid enters the master cylinder. Is this the case on all MC's from the e28? Am I mistaken about that? Mine did not have a screen. Maybe it popped out when I pulled the rubber grommets off under the reservoir, I don't know

Oops. I know you said M/C, but for some reason I was thinking booster. Maybe because that's the O ring that's usually a problem, but that's not what you said.