Front brake calipers & pistons

[Chip]

So my brakes were sticking and I've decided to clean and replace the seals in the calipers (1996 R1100RT).

 

The question: Are the pistons in the right and left brake calipers the same? I know that the two pistons in each caliper are slightly different sizes, but is the setup the same in the right and left?

 

Here's the scenario:

After removing the brake pads, I noticed something strange. The two (2) pistons in the right caliper (opposite the ABS sensor and speedo cabe) appear "normal." In other words, they are inserted in the caliper such that the open end of the piston faces toward the brake pads. Another way to put it is that, if you imagine a glass, the rim of the glass would actually touch the back of the brake pad and the bottom of the glass is in the calipers. I hope I'm describing this such that it's understandable.

 

However, the pistons in the left caliper have the bottom of the glass/piston facing out such that it touches the back of the brake pads. I haven't pulled them out yet so don't know what the back of the piston looks like.

 

In any case, is this normal? I checked the microfiche but don't even see pictures of both calipers.

 

As far as I know, the calipers have not been cleaned before so the setup appears to be as built in the factory.

 

Is this the way they're supposed to be?

 

Help

[bmwmick]

So my brakes were sticking and I've decided to clean and replace the seals in the calipers (1996 R1100RT).

 

The question: Are the pistons in the right and left brake calipers the same? I know that the two pistons in each caliper are slightly different sizes, but is the setup the same in the right and left?

 

Here's the scenario:

After removing the brake pads, I noticed something strange. The two (2) pistons in the right caliper (opposite the ABS sensor and speedo cabe) appear "normal." In other words, they are inserted in the caliper such that the open end of the piston faces toward the brake pads. Another way to put it is that, if you imagine a glass, the rim of the glass would actually touch the back of the brake pad and the bottom of the glass is in the calipers. I hope I'm describing this such that it's understandable.

 

However, the pistons in the left caliper have the bottom of the glass/piston facing out such that it touches the back of the brake pads. I haven't pulled them out yet so don't know what the back of the piston looks like.

 

In any case, is this normal? I checked the microfiche but don't even see pictures of both calipers.

 

As far as I know, the calipers have not been cleaned before so the setup appears to be as built in the factory.

 

Is this the way they're supposed to be?

 

Help

 

Jay,

I believe all 4 pistons should have the "rim of the glass" pointing the back of their respective pad. That would lead back to your original problem too. Are you saying all 4 pistons on the left side have the flat side facing out?

Mick

[Chip]

Are you saying all 4 pistons on the left side have the flat side facing out?

 

Yes. That is exactly right. The other thing I noticed (which makes me think it's okay) is that the flat side of the pistons on the left side have a single dimple in each one. The pistons on the right side, when removed, do not have the dimples on the flat side...they're smooth.

 

I ran out of time today to get the pistons out of the calipers, but tomorrow, I should have time to do it. It'll be interesting to see what the pistons look like. I'm thinking that if everything is okay, those pistons will be enclosed cylinders...otherwise, that's a lot of brake fluid in the caliper.

 

That said, I haven't had any braking issues. The pads from both calipers are wearing evenly, etc., etc.

[Stan Walker]

Are the two calipers made by the same company? Even if so, it could be a design change and you got some of each.

 

I would put them back together the same way they came apart!!!

 

Stan

[John Dickens]

On my bike, from new, ALL the pistons in both calipers have the closed side facing the pads.

[big-t]

However, the pistons in the left caliper have the bottom of the glass/piston facing out such that it touches the back of the brake pads. I haven't pulled them out yet so don't know what the back of the piston looks like.

 

Assuming those pistons are waterglass shaped,you would have a lot of wasted brake fluid in that caliper.

 

I'm guessing someone put them in backwards,my front fender is off,assuming I can remember I'll check my 96 tonight(caliper has never been apart).

[big-t]

 

Duh...Monday morning,asleep at the wheel

 

The most obious reason for...NOT...having the flat part of the piston against the brake pad is heat transfer.

 

Too much surface area would transfer all the heat generated by the pad into the brake fluid much quicker, boiling the fluid under hard braking,...no brakes

[Ken H.]

To my way of thinking, the key question is, where's the seal groove? It certainly needs to be inward, furthest distance away from the pads. So as not break seal as the pads wear and the pistons move outward.

[bmwmick]

Ken,

You are correct but I believe there are TWO sealing rings on each of the 4 pistons on the R1100RT front brakes. I've just never seen one with the solid portion of the piston pointing towards the brake pad. Odd

 

Mick

[Chip]

To my way of thinking, the key question is, where's the seal groove? It certainly needs to be inward, furthest distance away from the pads. So as not break seal as the pads wear and the pistons move outward.

 

Ken:

I don't recall seeing a seal groove on the pistons I was able to remove (2 so far on the right side...the others are buggers to get out now that I can't use the brake pressure to force them out). In any case, I'll check this evening for such a groove.

[John Dickens]

To my way of thinking, the key question is, where's the seal groove? It certainly needs to be inward, furthest distance away from the pads. So as not break seal as the pads wear and the pistons move outward.

 

Surely the seal groove and seal is in the caliper bore not on the piston?

[big-t]

To my way of thinking, the key question is, where's the seal groove?

 

 

Unless I'm misunderstanding the question...The seal sits in a groove cut into the caliper,the piston surface should be smooth,except for the dirt seal groove at the very outer end.(rim of the waterglass end)

[Chip]

Assuming those pistons are waterglass shaped,you would have a lot of wasted brake fluid in that caliper.

 

I'm guessing someone put them in backwards,my front fender is off,assuming I can remember I'll check my 96 tonight(caliper has never been apart).

 

So...what did you find?

 

I found that the pistons in both calipers are in the correct way. The pistons in the right caliper have the rims facing the brake pads and the pistons in the left caliper have the *head* of the piston touching the brake pad. The pistons are distinctly different across the calipers. I have no idea why this is the case.

[bmwmick]

To my way of thinking, the key question is, where's the seal groove? It certainly needs to be inward, furthest distance away from the pads. So as not break seal as the pads wear and the pistons move outward.

 

Surely the seal groove and seal is in the caliper bore not on the piston?

 

Correct! Each piston is 'sealed' with two seals plus the dust seal. Would have been a better way for me to state that.

I'm almost tempted to compare my left and right calipers now.

 

Mick

[big-t]

So...what did you find?

 

 

A quick trip to the garage revealed my pistons are installed the way I expected,with the rims on all eight pistons against the backs of the brake pads.My calipers have never been apart.....so.

 

Me thinks someone put yours together wrong

[Joe Frickin' Friday]

To my way of thinking, the key question is, where's the seal groove?

 

 

Unless I'm misunderstanding the question...The seal sits in a groove cut into the caliper,the piston surface should be smooth,except for the dirt seal groove at the very outer end.(rim of the waterglass end)

 

...which makes me wonder about the dirt seals on Chip's apparently inverted pistons. How are they staying put?

 

I've replaced a number of brake pads on my bike and cars; never seen a piston with its flat face in contact with the brake pad before.

[Ken H.]

Yeah, personal opinion, I still think the one side is in backward, but I suppose it will work, still stop the bike, that way.

 

I agree with one of the other posters, the reason it is done that way ("rim" edge contacts the pads) is better heat isolation. The generated heat is partially dissipated into the exposed air space of the hollow piston, not (so) directly into the brake fluid as would be the case with 'bottom' side out.

[Chip]

Me thinks someone put yours together wrong

 

This is turning out to be very informative. I was at BMW Motorcycles of North Houston this morning and spoke with the Parts Manager. He explained that the pistons in my left caliper are, in fact, different than the pistons in the right caliper and that is why there is a difference in the part of the piston that faces the brake pad.

 

What he did not know is why the pistons in the calipers were different (btw, both calipers are Brembo). I recall someone else indicating that a manufacturing change may have occurred. Finding out if that is the reason is my next mission

[Chip]

Big-T:

Quick question: Do you have Brembo calipers and ABS on your '96 R1100RT?

 

I spoke to the folks at Brembo N.A. this morning. The short story is that the left caliper differences *may* be due to the ABS system. They were not sure why the difference as it is BMW that prescribes the specifications for the calipers and Brembo does not design or manufacture the ABS system.

[big-t]

 

 

If its any help,you can look up parts on line by using your serial number.

 

 

http://www.maxbmwmotorcycles.com/fiche/fiche.asp

[Chip]

If its any help,you can look up parts on line by using your serial number.

 

 

http://www.maxbmwmotorcycles.com/fiche/fiche.asp

 

I've done that. Only one pic of the caliper and the pics of the pistons are no help.

 

Do you have Brembo brakes and ABS on your '96 R1100RT?

[big-t]

Do you have Brembo brakes and ABS on your '96 R1100RT?

 

Yes

[GSman]

on my 96 RT/ABS with Brembos, the lip faced out on all of them...

[Chip]

on my 96 RT/ABS with Brembos, the lip faced out on all of them...

 

I wonder...after market pistons?

Years ago (in a lifetime far away), I put after market *engine* pistons in my Husky

 

However, as I said earlier, the calipers have never been rebuilt, so the calipers and piston installation are as they came from the factory.

 

This evening, I'm going to see if I can take a couple of good pics of the pistons for viewing pleasure.

[bmwmick]

Jay,

Have you removed any of those 'solid' pistons yet? I'd be interested in what the back side looked like. If they are flat on BOTH sides, I'd guess it was just a production line change.

 

Mick

[Chip]

Have you removed any of those 'solid' pistons yet? I'd be interested in what the back side looked like. If they are flat on BOTH sides, I'd guess it was just a production line change.

 

Mick,

I did not remove the pistons from the caliper. The parts guy at the BMW dealership suggested I not do that (those caliper rebuild kits are expensive).

 

However, he did state that he has seen calipers with the "solid" pistons. Yep, most likely a production line change. I've searched the internet a bit in an attempt to read about something like that but haven't found anything yet.

 

That said, I am a persistent son of a gun and if information like that is out there, I'm bound to find it.

 

Unfortunately, when I got home this evening, I was so excited to replace the brake pads and test drive the bike, I forgot to take pictures...didn't remember until I had finished bleeding the brakes and had the fender on.

 

Again, there is no doubt that the "solid" pistons were different.

[big-t]

However, he did state that he has seen calipers with the "solid" pistons. Yep, most likely a production line change.

 

 

OR.....There was a new guy on the production line that morning.. I wonder how many he assembled like that,

 

 

Since they have been in use for 10 years without a problem why worry now.Should they be installed backward the only real problem I can see is they might overheat the brake fluid should you get into some aggressive braking situations,like a track day or a long decent off a mountain.

 

Of course it would be nice to know what really happened.