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Thread: M30 Top End Rebuild - Worth it? 400k Miles?

  1. #1
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    M30 Top End Rebuild - Worth it? 400k Miles?

    Alright, I know the word is the M30 bottom end is bullet proof, but I am looking for some reasonable advice from the experience on this forum on just how bullet proof.

    I have an e34 535i with a M30B35 with ~250k on the ticker.

    I would like to get another 100 - 200k out of it.

    She runs great right now. Engine is original. Maintenance has been kept up to date.

    I have the resources right now to rebuild (source a rebuilt) head and swap it in.

    Is it reasonable to expect to approach 400k+ on the bottom end of this engine if the top end rebuild is done right?

    Is that too optimistic?

    Bottom line is, I’m not interested in doing a full rebuild of the bottom end. It’s top end or nothing.

    Fire away…





  2. #2
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    Nothing.... If it isn't burning oil from valve stem seals and the valves can still be adjusted to spec on the cam-side measurement, I wouldn't "let the magic out" by taking the head off. I've seen a couple healthy undisturbed m30b35s with well over 600k that lived pretty spirited lives and never received more than occasional valve adjustments and fluid changes.

    I have a personal m20 with around 500k in my e28 and it doesn't burn a drop of anything. Valves still adjust alright although there's some serious varnish on everything in the engine. Not disturbing it.
    1990 Alpine 535i/5 - BC Coilovers, LSD, M5 Swaybars, 540 Brakes, Slide Bearings, A/C Mods, Sport Seats - 270k and counting
    1986 Alpine 528e - 500k+ miles daily driver
    2002 330i/5

  3. #3
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    I’m chiming in to save you from yourself, the classic just because you can doesn’t mean you should. My 735i had a head gasket go and because I had the money I had the head fully gone through “to make it bulletproof”. Engine had 220k on it. And after the head was built and able to hold all the pressure it was designed for, the rings decided they couldn’t. Blow by became too much and the oil pan gasket got pushed out and now it won’t run without hemorrhaging oil.

    Engines have two “compression sealing sections” the valves in the head (head gasket of course) and the piston rings. Both age together and when one is restored to new, it will definitely exacerbate the other.

    Bottom line, like danespann said above, if there’s no current need, don’t touch it. If you do though go ahead and do rings!
    Last edited by 6erWEBB; 09-28-2022 at 10:06 AM.

  4. #4
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    Thank you for the dialogue, gentlemen. I too am concerned about the rings and blow by. This is exactly the kinda feedback I was looking for.

    I am happy to leave the engine alone. But the reason I am thinking about the head rebuild is I am considering getting rid of my daily driver that I don’t like much and making the e34 my DD. I am concerned about doing that with a car that has over 250k and was hoping maybe a top end rebuild would give at least a reasonable amount of security for some years to come.


    I would welcome continued feedback. Is the general consensus what has been stated above already?

    Thanks Again!

  5. #5
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    I'd look for wear on the camshaft lobes and eccentric adjusters. I had the misfortune of using a reground cam from Ireland Engineering; it was soft (and crap). Maintained m30's are reliable.

  6. #6
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    Quote Originally Posted by royAdler View Post
    But the reason I am thinking about the head rebuild is I am considering getting rid of my daily driver that I don’t like much and making the e34 my DD. I am concerned about doing that with a car that has over 250k and was hoping maybe a top end rebuild would give at least a reasonable amount of security for some years to come.
    If your concern is keeping the car reliable as your daily, just make sure all the things surrounding the engine are in top shape to keep the engine protected. Fresh oem cooling system will keep the head cool and there shouldn't be any real reason to remove it so long as it is kept from overheating. Oil with good zinc content and frequent valve adjustments will keep cam wear to a minimum.

    - - - Updated - - -

    Quote Originally Posted by 6erWEBB View Post
    My 735i had a head gasket go and because I had the money I had the head fully gone through “to make it bulletproof”. Engine had 220k on it. And after the head was built and able to hold all the pressure it was designed for, the rings decided they couldn’t. Blow by became too much and the oil pan gasket got pushed out and now it won’t run without hemorrhaging oil.
    I don't see any reason bottom end would be of concern if an engine hasn't been overheated badly. Sounds here like the engine in question may have been overheated far enough to make the cylinder walls out of round or it ingested coolant for long enough to glaze the cylinder walls and gum up the rings with burnt coolant residue.
    1990 Alpine 535i/5 - BC Coilovers, LSD, M5 Swaybars, 540 Brakes, Slide Bearings, A/C Mods, Sport Seats - 270k and counting
    1986 Alpine 528e - 500k+ miles daily driver
    2002 330i/5

  7. #7
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    That could be, when I tear it down I’ll have to report back. I’ve heard it happening to others though, however I’m a body man not a mechanic.

  8. #8
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    Seems to be the general consensus, but I agree, if the motor is running fine, make sure it is up to date on preventative maintenance, give her a tune up, and drive it like you stole it.

    I acquired my 'well seasoned' '90 535 with almost 275k and DD'd it for a good 12 years. Have taken it on multiple 1000+ mile road trips around the Southwest and West coast. Used it as my ski vehicle, camping rig, canyon carver, you name it.

    She has technically left me "stranded" a couple times, but they were arguable my fault for differed maintenance. Alternator died at one point, but still got my co-worker and I all the way to work. I tore an engine mount during some spirited driving. Didn't realize what happened for several miles, and finally had it towed the remaining mile for fear of overheating/further damage (only tow under my ownership). And the radiator inlet blew off a few years ago about a quarter mile from home.

    M30B35 is a fantastic motor with even a mild attempt to maintain it.

    With that said, I have torn into just about EVERY other system in this vehicle over my ownership, so don't waste time and money on unnecessary work.

  9. #9
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    Quote Originally Posted by Tre0Tre34 View Post
    I acquired my 'well seasoned' '90 535 with almost 275k and DD'd it for a good 12 years.
    Wow! So how many miles are on that old gal now?




    Sounds like the general consensus is “if it ain’t broke, don’t fix it.” I can get behind that mentality.

    As far as just top end versus top and bottom end rebuild, sounds like some differing opinions. That is what I have read elsewhere too and I’m not surprised. It’s probably a bit of a crapshoot at those high miles.

    Thanks again everyone for the input!





  10. #10
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    Quote Originally Posted by royAdler View Post
    Wow! So how many miles are on that old gal now?




    Sounds like the general consensus is “if it ain’t broke, don’t fix it.” I can get behind that mentality.

    As far as just top end versus top and bottom end rebuild, sounds like some differing opinions. That is what I have read elsewhere too and I’m not surprised. It’s probably a bit of a crapshoot at those high miles.

    Thanks again everyone for the input!
    Little over 340k miles currently. Still a pleasure to drive.

    Enjoy yours, whatever you do with it. They are a labor of love, but plenty worth it.

  11. #11
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    As a long time M30 guy I feel compelled to offer my thoughts on this topic.
    I agree that if it ain't broke don't fix it. As for the bottom end wearing at the same rate as the top I disagree. Oiling problems from loose banjo bolts, infrequent or incorrectly done valve adjustments are common and accelerate wear to the top end. A leak down test will determine how the rings are sealing, or not.
    While the machine work is straight forward finding a shop that knows how to properly assemble this head could be a challenge.
    I've had many high mile M30s, some over 300k that while a bit tired still ran well enough I wouldn't bother to open them up.
    The only bottom end failure I've seen was one that was run without oil and spun it's mains so if anything fix those oil leaks.

    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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