Re: Truth on M5 caliper rebuilds...(sorry for leng (archive)

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I am reproducing a response from John Aho, from who I got the original information about the bolts being TTY (now modified to non resuable, but not necessarily TTY). Reminds me of Sir Alfred North Whitehead's, "Seek simplicity and distrust it."

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.)
Even with the correct bolts (NEVER substitute a "similar" bolt, even with the same class) the caliper halves do not assemble exactly as they were, and there is a likelihood of leaks. 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."

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. (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.)

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."
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?

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.

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!)
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.) 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.

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

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