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    Advice on diff. bracket fix / cracked spot welds

    I'm about to embark on the PN# 41118398664 additional diff bracket fix as I have 4 cracked spot welds - so I figured I'd do both jobs (despite not having a problem yet with the diff bracket.)
    Can anyone please spell out, or direct to a picture, what modifications must to be done both to the new and to the original bracket in order to get the new one to fit in place.
    Looking at the new bracket in my hand it's obvious that it cannot simply slip on top into place....and it's not immediately clear what must be modified in order to get it to sit correctly.

    (p.s. is it wise to double up the bracket despite not having a problem yet? - and just do the spot welds.)


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    Not to the bracket itself, but...

    ...I would replace the subframe bushings with something in urethane. I like the design of PowerFlex and usually install the Race version for additional rigidity. By replacing these bushings the subframe doesn't shift under load so the differential doesn't lull on the ear
    ---------------------------------



    1998 Z3 1.9 - few modifications here and there
    1999 Z3 2.8 - individual edition (British Traditional)
    2003 Z4 2.5i


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    Re: Not to the bracket itself, but...

    Zeta,
    Are you saying that as preventative action it is wiser to change to urethane bushings rather than double up the differential bracket?
    or in any case I should double up the bracket?

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    My experience with the 1.9 is that the...

    ...condition of the diff bracket and the spot welds didn't changed for 80K miles after I've installed the urethane bushings.

    Replacing the bushings will solve the problem at its source; doubling up the bracket or any other change back there will address the weakness, but will not cease the diff to pull on the bracket.

    If you'r bracket is cracked/damaged, I would consider it replaced; if you're just experiencing some rust around the spot welds or some signs of fatigue on the bracket, I wouldn't and just replace the bushings.

    The problem back there is that the diff is mounted rigidly to the suspension frame. The assembly is than attached to the unibody in 3 points: 2 in the front of the subframe and 1 on the differential. The mounting is not rigid, but to acoustically insulate the car from the road it uses rubber mounts in each mounting point.

    The two in the front are particularly soft and in part hallow: when you put a lateral load on the subframe (e.g. you make a turn) the inertia on the unibody cause it to shift. As it shifts the 3 mounting point is subject to pulling/pushing. Since this is the weakest point, it can take so much, before the fatigue causes the bracket to rip and the spot weld to fail.

    By replacing the front bushings with urethane (a much harder material than rubber) you don't allow (or significantly reduce) the subframe to shift and the lateral load is now primarily absorbed by the front two mounting points, which are much stronger and because of their design not subject to fatigue.

    Although I have no proof of it, I don't believe that the failure is related to the power of the engine itself (so it's not the torque applied to the diff that causes the ripping), but the more powerful cars have a combination of larger tires, stiffer suspensions or sway bars. All these differences causes allow the car to take faster turn (i.e. the tire don't loose traction as easily) and a faster turn causes more load on the subframe. That's why, in my opinion, the more powerful version are more likely to show signs of fatigue.
    ---------------------------------



    1998 Z3 1.9 - few modifications here and there
    1999 Z3 2.8 - individual edition (British Traditional)
    2003 Z4 2.5i


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    Re: My experience with the 1.9 is that the...

    Thanks for your great insights......
    All I have is 4 small spotweld cracks. Pic attached.
    So I gather that you would go with "fix the welds" and "replace the two rear subframe bushings with Powerflex".
    I still have a couple of questions;
    1. The car is my daily drive and I don't stress the car. How much "harder" will the drive be with the new bushings?
    2. How complex is the job? (in hours for my BMW mechanic to do)
    Photobucket
    http://i262.photobucket.com/albums/ii110/bonboniera_2008/4Fracturedwelds.jpg

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    I wouldn't go straight to the welding solution...

    ...as it looks like you're problem is just starting to show up now. If that was my car, I would start by replacing the bushings and monitor the trunk floor if it gets any worse. Aside form monitoring the spot welds, I would also suggest using a straight edge to check if the trunk floor gets worse.

    As far as the replacement of the bushings goes is quite a long and tedious job: since I don't have the BMW tool to remove the bushings (I use a combination of bearing puller & blowtorch...) I normally drop the entire subframe. This also gives me access to the fuel tank straps: that's another known problem. The padding on the straps easily comes off causing the gas thank to rattle. I normally take a 1" hose and slice it lenghtwise to replace the original padding.

    The increase in cabin noise with bushings is marginal, if any.

    You should ask around some independent BMW mechanics in your area.

    If you do decide to also do the welding then I would suggest you contact Randy Forbes on the Z3 section bimmerfurms and ask for his kit : maybe he can also recommend a shop near you
    ---------------------------------



    1998 Z3 1.9 - few modifications here and there
    1999 Z3 2.8 - individual edition (British Traditional)
    2003 Z4 2.5i


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    Two more questions.......

    Firstly, thanks a million. Yes, it's just at the start - but in the last 6 months one more cracked weld joined the club.

    Q's:
    1. Why would you not reweld the 4 spot welds and get it out of the way? What is the benefit of leaving it and monitoring any deterioration?

    2. Is this the kit I need? Do you have a link for a US EBAY supplier? http://cgi.ebay.co.uk/PFR5-311-POWERFLEX-REAR-BEAM-MOUNT-BUSHES-BMW-Z3-96-02-/120562544510?cmd=ViewItem&pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item1c12166f7e

  8. #8
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    That's the correct part number

    check here:
    http://www.harrisonmotorsports.com/shop/index.php?main_page=product_info&manufacturers_id=2&products_id=786

    or here:

    bimmerworld.com

    However, I read that there was a fire at the factory and that many sellers are now back ordered. Anyone else have better info?

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    I normally use Harrison when I buy PowerFlex...

    ...I would highly recommend you call them and make sure you tell them to be careful not to send you the bushings for the E30, but to triple check they send you the ones for the Z3. They look the same if it wasn't that the bushings for the E30 are about 1/2" longer. I normally go with the"race" version that are the stiffest: they give you superior rigidity without increasing road noise...

    Since you're in there I would consider a couple of other things:

    1) replace the padding on the two straps holding the fuel tank

    2) think about adding the rear toe/camber adjustments, so you can do 4 wheel alignment. I highly suggest his, particularly if you have lowered your car or planing to. It will allow you to dial out some of the camber/toe that comes naturally with lowering springs and causes excessive tire wears. You can either go with the Ireland Engineering option, or this one: http://www.e30tech.com/forum/showthread.php?t=80410. Same thing just cheaper...

    3) If you drop the subframe, tell the mechanic to asses the condition of the 2 u-joints on the driveshaft. It's much easier to remove it and rebuilt it now as opposed to at a separate time. If you notice the u-joints (particularly the one closer to diff which is exposed to road dirt) are starting to seize I would suggest removing the driveshaft and have it rebuilt at your local transmission/driveline shop.

    As for the welding, use your judgment. Replacing the bushings will stop the spreading of the damage, but if you fill the structure has been compromised, welding it could be the best solution. I you really want the ultimate solution and/or you think parts of the structure have to be replaced, I would look into the kit from Randy Forbes...

    In my experience I've noticed that with the PowerFlex when the car is on the jack I can remove the bolt holding the diff ear without any problem. With the stock bushings I need to support the diff with the jack and if I remove the jack the diff drop by 2-3"... This is just another data point on how soft the front bushings are...

    You will also likely notice the car has a more precise handling...
    ---------------------------------



    1998 Z3 1.9 - few modifications here and there
    1999 Z3 2.8 - individual edition (British Traditional)
    2003 Z4 2.5i


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    I learnt a lot here

    BTW - Why do you refer to them as "front bushings" when they are in the rear?

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    Oh yea... I should has said something like...

    ..."frontmost rear bushings"... ehhehehe... It's just that once you have the subframe in front of you you have 2 bushings in the front [of the subframe] and 1 in the rear [of the subframe]. The rear bushing is actually on the differential ear, but since the diff is bolted to the subframe, from a dinamics perspectives they behave as a single unit...
    ---------------------------------



    1998 Z3 1.9 - few modifications here and there
    1999 Z3 2.8 - individual edition (British Traditional)
    2003 Z4 2.5i


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    Redoing the spot welds - More Qs

    1.After drilling out the cracked welds is it critical what type of weld is used to fix (tig or other)?
    2. I assume it is wise to add more welds in the area of the damage. Is this correct?

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