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Thread: E38/750 rear brakes on e31

  1. #1
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    E38/750 rear brakes on e31

    Project notes. Nothing new here, just pics confirming it's no lie.
    Where you see "e38" in these docs, it is understood as specific to the vented e38/750 and not necessarily the solid e38/740. {ed: except that the 740iLP got vents iirc}
    Where you see "e31" in these docs, it is understood to exclude the CSi model.

    E38 328x20 vented.
    E38 length hose extends to the rear, then terminates perpendicular to the caliper.
    StopTech rotor.


    E31 324x12 solid.
    Original E31 hose goes in at an angle.


    Hose comparo
    While you might feel the e31 hose is a bit short, even within the e31 app, the e38 hose seems a bit long.
    On the hunt for the Goldilocks of hoses, preferrably braided SS.


    These e38 dims confirmed.


    Note that the e38 and e31 share the same disc height (and hat thickness) but different disc thickness.
    Therefore we conclude that discs have the same shield clearance, and the vented disc has been grown outboard with respect to solid.


    Disc comparo




    Calipers are stamped with piston and disc diameter.
    Confirmed via direct measurement, the e38 & e31 share the same piston diameter.
    While the E31 had unique ATE left/right ID's, 89 CN1 and 90 CN2 respectively, I couldn't make out such on my E38 examples and therefore can't report.




    Loaded caliper comparo.
    The E38 is an 8mm wider version of the E31.




    {ed: Pads
    Some pad suppliers even sell the E38 and E31 under the same part number, while others will (wisely) split the two since the E31 thickness should be limited to what can be loaded.
    I would not presume that the pad's radius on the disc has changed, and I will check precisely someday.}

    Rebuilding
    Piston popping into a catcher's mit.


    Painting with G2 epoxy.


    New seals, boots, hardware.
    Nice picture but the successful sequence for getting a boot on is to...
    - lubricate the piston and seal
    - wrap the boot around the very inside end of the piston,
    - extend the boot and secure it into the caliper slot,
    - push the piston in past the boot and the seal.


    Apropos, the e38/750 rear brakes behind the BBS RS743, an e38 staggered option.
    {ed: I did not check if these fit under e31 16's and have none to do so.}

    Last edited by Hyper; 07-12-2017 at 11:27 PM. Reason: Pic link repair, re PB affair

  2. #2
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    So correct me if I am wrong, this upgrade to e38 rear caliper lets us use a vented rotor vs solid? Is this the only benefit?

  3. #3
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    Quote Originally Posted by dragon850 View Post
    So correct me if I am wrong, this upgrade to e38 rear caliper lets us use a vented rotor vs solid? Is this the only benefit?
    Vented, plus a slighly big diameter than the solid one ( 4 mm IIRC )
    Last edited by dudu850; 08-08-2016 at 07:21 AM.

  4. #4
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    Quote Originally Posted by dudu850 View Post
    Vented, plus a slighly big diameter than the solid one ( 2 mm IIRC )
    I see, but same clamping force from the calipers right?
    What about rear euro CSI calipers, more clamping force and larger rotors?

  5. #5
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    The benefit is the transfer of the braking distribution to the rear so the fronts don't lock up so early.

  6. #6
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    Quote Originally Posted by dragon850 View Post
    I see, but same clamping force from the calipers right?
    What about rear euro CSI calipers, more clamping force and larger rotors?
    As just added to my previous post, rotor is bigger... Calipers are the same diameter.
    Might worth the effort to check this URL..

    http://forum.roadfly.com/threads/105...calipers/page2
    Last edited by dudu850; 08-07-2016 at 02:16 PM.

  7. #7
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    Quote Originally Posted by dragon850 View Post
    I see, but same clamping force from the calipers right?
    What about rear euro CSI calipers, more clamping force and larger rotors?
    Rear Euro Csi's give you 35Lbs more clamping force making the car stop flat, Pretty much perfect imo, The E38 finned rotors are the same as Csi Euro rear rotors, The E38's are about a third of the cost when I got mines way back, The wider and finned rotors are worth having but will not work with oem rear calipers.

  8. #8
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    {ed: Warning, this post contains info not relevant to the thread title, but later included under my comprehensive thread -> Survey of Rear Calipers.
    *******
    Larger pistons getting checked out.
    This E90 42/20 caliper did bolt up, but not quite correct until an "easy" bracket mod.
    About 10% more area to balance the (calculated) 10% torque boost up front.
    My procurement and rebuild cost, plus nice pads: $100/pair.




    Also ...
    - Test fitted the E90 44/22 caliper, pads shimmed. A 20% increase in area. $120/pr. I'll probably end up here and get back to ya.
    - Fyi, for the E46 42/22 and E39M5 & E46M3 42/20, the hose comes off at an awkward angle. Would require R&D for a banjo takeoff, possibly machining a flat to prevent rotation.
    - Probably someday, a 42/12 on a stock rotor. From E53 X5 standard rear.
    - Probably never, there is a 46/20 in the bin. Interesting, the Z8 shares the e38 rotor and pads.
    - The E38 40/20 rear has been out on the road, combined with the E60 348x30 Brembo fronts, and you can read impressions over there.
    {ed:forgot...}
    - E60 42/20, inbound.
    - E53 X5 42/20, inbound. From my old friend, the 4.6iS.
    Last edited by Hyper; 08-02-2017 at 02:36 PM. Reason: Pic link repair, re PB affair

  9. #9
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    Is it just bolt on of the E38 750 caliper with pads? or do you have to shim anything?

    I just don't have the means to manufacture something myself.

    I see you get the slightly larger vented disc, which is cool, but would it change the brake bias at all? I find the single pot fronts to be great, but can always feel that the fronts do most of the braking. I swear I never have to clean the rear wheels, only the fronts get brake dust.

    Wonder if this would work good with hypers front bbk he's working on.

  10. #10
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    Is the E38 750i rear brake solution still in vogue? I understood my car to have the BBKoB but turns out it only has the front. I ordered new blank rotors and my favorite brake pads to go all around to replace the drilled and slotted rotors and found the rear is still unvented E31 stuff so now I have some heavy vented E38 rotors that would be very costly to return.

    Rock auto has one side of the rear but not the other. Caliper includes the bracket. They have the other side caliper without the bracket from another remanufacturer. Is there another economical source for the bracket?

  11. #11
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    hmm that is very unfortunate at the moment. I was planning on trying this in about a year or so. once I get my 8 back on the road. i'm surprised those brackets aren't available from bmw anymore. I guess that goes along with the tie rod lever arms becoming NLA. They will probably make more in the future or you may find some on ebay. The brackets are the same for both sides so you could buy 2 rear lefts if you wanted to and just eat the cost of the additional caliper or sell it on ebay. Obviously the caliper is different. its the same part number according to RealOEM. 34211163335
    I am surprised anyone selling just the bracket seems to have them outrageously over priced.

  12. #12
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    Exclamation

    I'm just in the process of tackling this upgrade myself (it's been a long time coming though!) I too feel the stock front end has too much bite over the rear, and I'm adding the E39 M5 42mm callipers and vented discs to the rear of my 99 840. Unfortunately the project is once again on pause over the winter and is unlikely to be completed before the end of summer. But I'll let you know how it goes eventually!...

    photo_2023-05-03_09-05-42.jpegIMG_1933.jpg

  13. #13
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    I did the E39-M5 rear calipers upgrade with a Euro semi-floating directional fined rotors a couple years ago.
    I purchase several sets of E39-M5 rear calipers for $300 each on eBay.
    The Euro directional fin 328x20mm rear rotors were $200 each from Schmiedmann
    There are two issues with brake upgrades, torque and thermal Bias,
    Brake torque can be calculated.
    Brake thermal bias needs to be measured (laser or contact measurement).

    -Original Post-
    E39M5, Euro 328x20mm Rotor with 42mm caliper Retrofit:
    The Euro ///M aluminum hat rear rotors are the semi-floating directional fin vented rotors.

    BMW P/N:

    Left Rear Rotor: 34-21-2-228-801
    Right Rear Rotor: 34-21-2-228-802
    Calipers/42mm: NLA, eBay – used
    Caliper Carrier/72mm: 34-21-2-229-705

    Upgraded Performance:


    1. More efficient rear rotor cooling: Directional ///M curved-fin rotors will cool more efficiently than the straight-fin, all-cast iron vented rotor.
    2. Rear brake bias upgrade: advised only for cars with upgraded front rotors. The E39-M5 rear caliper (42/328) with 42mm piston is a direct fit caliper for the E31 with 328x20mm rotor. Approximately 2% increase in rear brake bias. Final brake bias (mathematically) is Front 67%/Rear 33% with Euro CSi 345x32mm rotor with ATE 4 piston calipers
    3. Cooler front brake rotors: an artifact of an increase the rear brake bias will unload and provide cooler front rotors. A win-win



    Retrofit issues:


    1. E39M5 Euro ///M aluminum hat rotors have a 185mm E-Brake drum.
    2. Rotors have a dual rotor retaining screw setup.
    3. Must order Euro Parts outside of US - Euro parts orders NLA in US.



    My initial retrofit preference would be to upgrade the rear brake dust shields from another series BMW with the 185mm E-Brake shoes and install an E46-M3 rear axle hub with the dual rotor retaining screws.

    However, none of the 185mm E-brake dust shields I reviewed and/or purchased could be easily modified to fit the E31 wheel carrier.
    Also, the 185mm E-Brake shoes do not correctly fit the E31 Rear wheel carrier, dust shield or E-Brake shoe expander.
    The 185mm E-brake shoes expander has a different cable-end attachment and doesn’t work with the pin-type E31 brake cable.
    Hence, an OE 185mm E-Brake shoe retrofits seems more complicated than I anticipated.

    So, I chose another way.


    1. Re-line a set of 180mm E-brake shoes for an 185mm drum arc. ($35 a shoe x 4 = $140)
    2. Widen the Euro E39-M5 rear rotor 185mm dust shield grove to accommodate the E31 180mm dust shield. (Free at local tire shop I frequent)
    3. Fabricate a drill guide with drill bushings to accurately drill the retaining screw hole (6.7mm), tap (6x1mm) hole and bore the retaining screw 12mm shoulder.
    4. Drill and tap the E31 rear hub (on the car) to match the rotor dual retaining screw setup. (tools on hand, purchased 6.7mm and 12mm cobalt drill bits. $35.)
    Attached Thumbnails

    https://www.bimmerforums.com/forum/s...+rear+calipers
    Last edited by m6bigdog; 12-19-2023 at 12:12 PM.

  14. #14
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    https://www.turnermotorsport.com/p-2...ines-rear-e38/

    Is this what I want for rear brake lines to fit my 750i calipers? Or can I use a different banjo bolt and use the ECS branded lines that appear to be E31 specific that are already in service on the car?

  15. #15
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    Nevermind I found some banjo bolts locally that fit the fine thread on the 750 caliper.

  16. #16
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    Quote Originally Posted by LappingLuke View Post
    Nevermind I found some banjo bolts locally that fit the fine thread on the 750 caliper.
    That sounds like your setup is a better match for the OE E31 brake line layout.
    Please reply/post the Banjo Bolt setup you use on these E38-750 calipers to include the brake lines.
    I anticipate that will work with the E39-M5 caliper.
    Did you resurface the caliper at the Banjo Bolt.
    Enquiring minds want to know...

  17. #17
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  18. #18
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    Damn it I just deleted a perfectly good post trying to correct Automistakes thanks to this forums screwed up mobile interface

  19. #19
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    Now I'm on desktop and its easier to type...

    What I said was out of the photos above, the first photo is the completed connection made to the E38 caliper. I did not resurface anything on it and it is sealed enough to not drip as is but I guess we will see if it holds up to pedal pressure. For what it's worth, the surface wasn't terrible but not "machined smooth" like the caliper I removed. Originally I assumed this banjo bolt connection was the original configuration for E31 because there is a "wall" feature that stops the flat connector from rotating when the hose is pushed around and theres a machined smooth area for the copper washer but now I'm not as sure as I cannot find lines like this even for E31, only the straight in threaded kind that are apparently correct for E38.

  20. #20
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    Context 1: I'm suddenly dropping in on this thread after being away from the planet.
    • At first glance, the current posts look chaotic. Mistakes, misinformation.

    Context 2: I'm the designer of LappingLuke's BBKoB solution at the front and also the rear caliper solution.

    • LL's 850 is ex-Alex840 and I've spent many hours on it, at its home base in Portola Valley California. Brakes, suspension, body panels, paint.
    • As far as brakes, my UPB46 car and LL's are identical BBKoB up front. Rears differ.
    • At the rear, LL's are e31 12mm solid disc with (iirc) e53 42/12 calipers whereas my rear rotors are e38 20mm vented disc with (iirc) e90 44/22 calipers.
    • For casual road use, e31 solids are perfectly acceptable, even preferred. The significant change is the rear piston size, to balance the BBKoB up front.
    • Rear brake lines were migrated to an off-the-shelf solution, necessarily different from original e31 stock, because non-stock calipers.

    Luke, perhaps you should get with the original engineer for info about your project, whatever that is.
    PM or email me at 8pivotalMoments@gmail.com.
    Fyi, I will not be hanging around on this thread, chatting.

  21. #21
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    Thanks for helping to clear the air. I have spent a lot of time reading your history of threads on this project through the years and I apparently made some assumption about what had been done to the car in the past, connecting dots that ought not to have been connected. Later today I will verify what was on the car, since you seem at least a little uncertain of what is on it. If I need further clarification I will reach out via email. Thanks for making yourself available in this way!

  22. #22
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    Quote Originally Posted by LappingLuke View Post
    ...connecting dots that ought not to have been connected...
    Nailed it exactly!
    You (mis)applied a general discussion of a high-level topic to your low-level specific situation.

    *** And iirc...
    Your rear brake solution was installed by me in 2016 Oct.

    Caliper: e53 42/12
    Seals: Centric 14334024.
    Hardware: Centric 11734013.
    Nipple, M10x1.0: 34116768018.
    Cap: 34111163435

    Rotors: Centric 12534032
    Pads: Jurid, pn 571918J/JAS

    Your SS rear lines are from an 2010 Audi A3 w/272 (rotor dia?).
    This Hyper engineer selected these based on (268mm) length and (banjo) end-style.
    ECS pn ECS41072KT. >> Shown as NLA but included in their full kit for that app.
    The lines are likely available from other sources or could be custom fab'd.
    Banjo bolt, M10x1.5, pn 34326779818
    Last edited by Hyper; 01-10-2024 at 10:08 AM. Reason: And iirc...

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