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Thread: N52 Bottom end : Magnesium cracking?!

  1. #1
    Join Date
    Jul 2022
    Granby, QC, Canada
    My Cars
    BMW 328i E92 2007

    N52 Bottom end : Magnesium cracking?!

    Hello all, I searched a bit on the web and found absolutely zero helpful info related to my current issue/question, thus I registered on this forum in an ultimate attempt to get more knowledge! Thanks in advance for your help!

    I basically tore down two N52 engines today (one being N52B30AE and the other one being N52B25).

    TLDR : the magnesium of the bottom end is cracked at several places close to the main bearings, on both engines. Is this normal?

    More info:

    The first engine (N52B30AE) was the one that came with my BMW 328i E92 2007 (built 2006/11). At this time, I was lacking experience in BMWs, so I had the car inspected upon buying. They found nothing, so I went ahead and two weeks later I tracked the car....well only for two laps and a half before I realised my oil was at 165 C (I assume it would be around 300F?) and the engine started knocking in the base 🤦. While removing the engine, I noticed the waterpump had the electrical wires "hot glued" 🤦🤦🤦🤦🤦.

    So I went ahead and bought the very first engine I could get, which was a N52B25. This engine went very well for about 60 000 kms. I tracked it last summer. I have to mention : it got slightly hot, but never as close as the first engine (125C in the oil, which is likely around 235F). The engine ran well for several weeks after the track day, but I was also gentle with the car. I was going at another track day this summer and decided to give the car a shakedown before going : I basically floored it until 7k rpm and shifted the second gear. At this point I noticed the engine started knocking again in the base 🤦🤦🤦.

    So here I stand now : I opened both engines to the last bit, hoping to rebuild one very healthy engine out of the two with all the required new parts.

    But, after inspecting the parts thoroughly, I noticed both bottom ends showed visible cracking into the magnesium shells, up to the aluminium.

    The thermal expansion rate of the aluminium is twice that of the magnesium, so my current hypothesis is that the overheat led to this cracking. BUT. common sense makes me question if this is really the case, or if it's even just something normal to happen to those engines?

    I don't know how to share picture on this forum but here is a link to a google Drive that contains some pictures:

    Thanks a lot for any help that you can share!! Much appreciated!
    Last edited by MecEng4Win; 07-25-2022 at 10:29 PM. Reason: Adding link for pictures

  2. #2
    Join Date
    Feb 2022
    United States
    My Cars
    2007 BMW 760li Quadturbo
    N52's arent made for the track so i think your theory of thermal expansion is correct, i would probably not use those blocks for a rebuild, used n52's can be found for dirt cheap.

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