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Thread: M30B35 Pistons?

  1. #1
    Join Date
    May 2019
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    Sandy, Utah
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    1991 BMW 735il

    M30B35 Pistons?

    Hey all,
    New to the forum so I apologize if I'm missing something. I have a 1991 735il with the M30B35. The engine had a real bad knock, so I tore the engine down. All the bearings and crankshaft look good but a couple of the pistons have some good scarring on them, I will try to attach a picture to it. I'm just curious, is there any way to still be able to use these ones, or does anyone know where I can get some for a decent price? Everywhere I look they're a couple hundred each, at least. Any help would be greatly appreciated!file.jpg
    Last edited by Caydan0522; 05-17-2019 at 12:53 PM. Reason: Photos

  2. #2
    Join Date
    Sep 2006
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    NW suburban Chicago
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    hiss by my window
    Yeah, those are junk alright.
    Have you diagnosed the knock? Are the wrist pins bound?
    What do the bearings looks like?
    Tops of pistons? crushed ring lands?
    what do the bores look like?
    I have a set of pistons in FAR better condition than this I'll offer you on the cheap.
    Last edited by ross1; 05-17-2019 at 08:12 PM.

    If you can leave two black stripes from the exit of one corner to the braking zone of the next, you have enough horsepower. - Mark Donohue

  3. #3
    Join Date
    May 2019
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    Sandy, Utah
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    1991 BMW 735il
    All the bearings look good as well as the ring lands, cylinder walls still have crosshatching but little bits of scuff marks that can be honed out. The head got checked out by a machine shop and is in perfect condition. The Tops of the pistons look alright but have lots of carbon on them. The crankshaft is in great condition too. The guy at my local machine shop said it looks like someone either used the wrong oil, or it got overheated at some point causing piston damage which caused the piston to slap against the cylinder wall, hence the knock. I am definitely interested if you have a set. Since I already have this thing torn down I'm just going to do a complete overhaul on it.

  4. #4
    Join Date
    Jan 2008
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    VA
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    1987 325ic
    Quote Originally Posted by Caydan0522 View Post
    Hey all,
    New to the forum so I apologize if I'm missing something. I have a 1991 735il with the M30B35. The engine had a real bad knock, so I tore the engine down. All the bearings and crankshaft look good but a couple of the pistons have some good scarring on them, I will try to attach a picture to it. I'm just curious, is there any way to still be able to use these ones, or does anyone know where I can get some for a decent price? Everywhere I look they're a couple hundred each, at least. Any help would be greatly appreciated!file.jpg
    I am going to be tearing down and parting out the exact same motor from a 91 735il. Let me know is Ross's offer falls through or if you need anything else.


    Sent from my iPhone using Tapatalk

  5. #5
    Join Date
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    hiss by my window
    Quote Originally Posted by Caydan0522 View Post
    All the bearings look good as well as the ring lands, cylinder walls still have crosshatching but little bits of scuff marks that can be honed out. The head got checked out by a machine shop and is in perfect condition. The Tops of the pistons look alright but have lots of carbon on them. The crankshaft is in great condition too. The guy at my local machine shop said it looks like someone either used the wrong oil, or it got overheated at some point causing piston damage which caused the piston to slap against the cylinder wall, hence the knock. I am definitely interested if you have a set. Since I already have this thing torn down I'm just going to do a complete overhaul on it.
    Given all that and if the wrist pins aren't binding I'd tend to think overheating, pistons grew and scuffed. Glad the bores will clean up. I think poor oiling would have shown up in the bearings too.
    Got your PM, took some pics of mine for you to see.

    If you can leave two black stripes from the exit of one corner to the braking zone of the next, you have enough horsepower. - Mark Donohue

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