Re: Follow up message (LONG) (archive)

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I did a quick read of that thread you pointed to in the other message, and have a couple of problems with the advice given.
As to who says these bolts are not reusable, that would be both BMW and ATE. Back a long time ago, these bolts were sold at the dealer, and on the rare occasion that warranted it, the caliper halves could be split. (The o-rings
are still available, but the bolts are NLA today.)

Some of those caliper bolts are still available, like the E12 solid rotor, caliper bolts, which are the same ones as used as the vented rear E9 rotors, calipers. The O-rings are actually NLA. But can be sourced through, I believe, Volvo or Saab, which includes them in their brake rebuilding kits for multi-piston calipers. Also, BMP has now started offering O-rings to rebuild these calipers. You should check your ETK/HTK before making assumptions, or even your BMW dealer.

Even with the correct bolts (NEVER substitute a "similar" bolt, even with the same class)

I never suggested to substitute a �similar� bolt, and would never suggest that. Only replace those bolts if you have the correct strength, overall length, thread length and pitch, and shoulder length and diameter.

the caliper halves do not assemble exactly as they were, and there is a likelihood of leaks.

This is completely untrue. If you have ever taken apart one of these calipers, you will notice that at the point of mating (where the caliper half meets the spacer, or the other caliper half, on the O-ring side) the port is machined to the spec of the inside diameter of the O-ring. The counterbore is also machined in for the outside diameter of the O-ring, and is also less than the thickness of the O-ring.

This enlarged opening on the O-ring side allows for a small reservoir to pool. It also allows for the spacer, or the other half of the caliper, that does not include the seal and that does not have this �enlarged� hole, to align with the port of this reservoir � in other words, the machined parts allow for inaccuracy of the alignment. Also since the counter-bore is not as deep as the O-ring is thick, when you tighten the bolt down, it compresses the rubber seal. You would have noticed this because it squeezes out all of the excess brake fluid or assembly fluid when reassembling the calipers.

That's why they always did discourage stripping the fixed calipers apart, and why ATE now says in their current year's schiebenbremsen literature WRT fixed caliper bolts: "..are no spare parts. Caliper housings are not to be disassembled."

In response to ATE�s literature, one of the common beliefs is that ATE has not stopped making individual caliper parts, but stopped selling them, in order to increase sales of the rebuilt caliper units. Go talk to technicians who have been in this business for over 15 years, and they will remember a time when ATE will sell every single part to every single caliper (bolts, seals, o-rings, pistons, etc). ATE has found that it is better business to sell whole, rebuilt caliper units than individual parts. Go talk to part houses that now only sell these calipers as whole, remanufactured units to BMW owners. This is a case of economics, pure and simple.

Consider it like a cylinder head gasket -- the reason those can't be reused is really an alignment issue. The cyl head gasket can never contact the exact same surfaces impressed upon it originally, and that's why they would likely fail.

A head gasket bonds to the surfaces to which it is mated to. You always must clean and prep the surfaces of engine components, which removes any leftover gasket material. Leftover gasket material will cause the new gasket to incorrectly seal and leak. The old gasket most likely is damaged when removed � due to the fact that is has become attached to the metallic surfaces which it has been designed to do.

The other thing you will notice is that if you are doing a valve adjustment, you can reuse the valve cover gasket numerous times, as long as it is not damaged and you prep the surfaces. This is not suggested as most of the time the valve cover is damaged, and is a technique that you would use only in an emergency. The caliper on the other hand does not use a gasket, but uses a rubber. The rubber does not attach to the metal, and the metal does not attach to itself, therefore the fact that you�re comparing it to a paper, cork, or head gasket is unfounded.

(I know a cast iron BMW caliper is not as soft as a gasket, and has rubber rings to seal, and I know it can be done, I'm just suggesting why there is a risk.)

It is not cast iron. Cast iron is so brittle that it would crack if you dropped it! Brittle materials are not used in calipers or brakes! This is a high temp, heavy duty, corrosion resistant steel. Not to mention the fact that using cast iron machining would be infinitely more difficult than steel to get a polished surface such as the bores of the calipers.

The four-pot calipers in question are the same basic part, with the same seal rebuild kits, as fit many other older BMW.
2002 tii used them with solid rotors, and no spacer between the halves. E12 & E24 >81 used 'em with vented rotors, and fitted a spacer to accommodate. E28 M5 & E24 M6 had even thicker rotors & needed thicker spacers. These three versions above all used different hardware kits for the pads, and different length bolts as the widths required. But, they are the exact same castings & work the same (same size pistons, etc.)
BMW had stated in their older service manuals regarding splitting the halves: "Only strip fixed caliper in an emergency." "Always renew expansion bolts after every stripping process."

I�ve covered this and the reason why BMW recommends renewing bolts in cycled fatigue situations. See above. Expansion also does not imply TTY.

BMW provides a tightening sequence & the torque spec has only a "minus" tolerance, as these stretch(y) bolts have a very critical ELASTIC range. These are expansion bolts to accommodate the growth of the cast iron at high temps. Are they TTY? Well, once you torque 'em, you have to yield to the idea they can't be reused! Do you know the bolts won't get plastic during the breakaway torque to remove?

As for removal torque, remember that you have to overcome the friction between the bolt head and surface and between the threads. It has nothing to do with the tension in the bolt. Why do you snap off the head of rusted bolts? Because you torque the bolt and deform it plastically, albeit in the opposite direction to tightening the bolt. That�s the only reason you�d snap heads off of rusted bolts.

Bottom line is they are very special bolts, and I'd defer to BMW/ATE on this. Later BMW service manuals (like my Euro E24 English version film) show mostly the same fixed caliper procedures, but have deleted all reference to splitting the halves. I'm certain this is for safety reasons, as even with proper fasteners there was too much risk for them to endorse it.

What is the point talking about splitting calipers in the manuals when the only thing you can buy from ATE is either a complete caliper, or a repair kit made to repair only one piston, not completely rebuild? Also, BMW doesn�t use any bolt-together multi-piston calipers anymore. That is the reason why modern prints of service manuals don�t cover this anymore.

It does not matter that some people have done it with no harm, that's no basis on which to give potentially unsafe advice. (Even If you did do it yourself, don't tell anyone else they should!)

This is not unsafe advice, because the advice I give is completely and utterly truthful. Read the post again, then read all of the knowledge to back up the post. I only suggest people to do this, if they have higher level DIYer experience. This is because playing with brake parts is something that you don�t want to do if you don�t have a clue. Also, I never said anyone SHOULD do it, I only said HOW. I only SUGGEST how and why to do things, I don�t tell them that they SHOULD do them. If you want proof, read the disclaimer on my tech pages. It clearly states that if you do not feel comfortable doing the work yourself, take it to an experienced mechanic. If this is suggesting that they should do everything on their car, then I�m sorry, not everyone should do everything on their car.

Many folks have had problems with this even when the bolts were available & installed as prescribed (ask the old timers like Michel, or anyone with 2002 knowledge.)

Well, I am very good friends with a technician factory trained on E9 coupes. He has no problems taking apart calipers, and the only reason why he doesn�t do it himself anymore is because it because as much in labor to rebuild a caliper as it does to purchase a reman caliper. So talking to the old timers, the consensus is that there shouldn�t be a problem if you know what you�re doing.

This was a known problem when the old 2002 were in their heyday.
I'd reckon if these four-pot fixed calipers were still common, you'd hear more horror stories today.

I thought the E9 and the 2002 were produced around the same time?!?! And these multi-piston calipers are exceedingly abundant, just think of all of the E28 M5s, M6s, E23s, E12s, E9s, E3s, 2002 series, E34s (in europe), E32s (in europe), E24s, E21s still running around. And newer cars also use multi-piston calipers (like the rear of an IS300, or Mercedes, etc.), and if you would like to why Brembo and most modern multi-piston caliper manufacturers are solid instead of bolted together, is because you get less flex out a single piece of steel than multiple pieces bolted together. This in turn gives you better brake feel.

The best thing for this type of caliper is to get ones in good condition, that have never been rebuilt.

Personally, I would never use used brake parts that have never been rebuilt in my own vehicle, because the chances of failure are much higher than those of properly rebuilt units. In fact, what you said is potentially unsafe advice.

That way you have less risk of someone have ruined the seals between the halves with solvent or grease (old thread that would not die: grease or oil has no place inside calipers. Assembly lube is either a paste or fluid that is totally miscible & compatible with DOT 3/4.

The seals will not be ruined if they are contacted by grease or solvent, as long as cleaned off with fresh brake fluid before reinstallation. In fact I suggest that upon removal from calipers when disassembling, that you actually place the seals in fresh brake fluid, and let them sit. Letting them sit in fresh brake fluid lets them expand back to original size because they are no longer under pressure, and also prevents contamination by foreign substances. I would also wonder how you would actually get grease or some sort of solvent on these seals without getting it in the brake system, which would probably end up causing massive pedal softness and also could destroying every seal in your braking seal from inside the master cylinder to the inner piston seal. At which point you must rebuild the caliper, and rebuild your entire brake and clutch system.

You also forget in your list that brake fluid is an acceptable assembly lube. Because for the rest of caliper�s life the seals and metal are contacted by brake fluid.

Take good used calipers & rebuild without splitting. If you would need to split them for repair, they are likely too far gone to save without reboring
& lining & new pistons.

I�m sorry, but some of us here, find that finding these calipers is extremely difficult. Also some calipers like the wonderful Tii calipers or vented rear E9 calipers are extremely rare, unless purchased new at exhorbinant prices. This means that being able to rebuild the caliper is your only option. Rebuilding the caliper is something that I�ve done to calipers that have been seized for 5 years, from which brake fluid comes out as a paste, not a liquid! And I have done so successfully! And once again, I do not suggest that you do this unless you feel comfortable doing brake repairs, and high level DIYer jobs.

Now that I have fully proved this point, I believe it is clear to see where the basis for my statements come from. I hope this has cleared up any misconceptions.

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