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Thread: Check for stripped headbolts before head removal?

  1. #1
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    Check for stripped headbolts before head removal?

    99 m52TUb28: Unknown history, but I have it now. Leaking coolant out rear of head, lower "dry" compression on cylinder 6 only.

    Reading up on the stripped head bolt problems, can you check if they bolts are stripped before you remove the head? If not and you put on the new gasket and some of the bolts are stripped, you would have to remove the head, do the timeserts option and buy another new gasket and bolts that were torque already?? Is this right?

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    The head bolts them selves don't strip what happens is when you removed the original head bolts they tend to pull the threads from the engine block with them. Which would require you to install timeserts or helicoils. If your headgasket is leaking there is no hope in a can perment fix you will have to remove the bolts and see how lucky your going to be you will need a headgasket kit and new head bolts since the head bolts are torque to yield meaning upon final torqueing you are strenching the bolt you do not want to re use already torqued head bolts. Installing timeserts is not all that difficult helicoils require you drilling and tapping perfectly vertical to the block. Hope this answers your question and or confusion. Btw your going to want to bring the head to a machine shop to be trued up and checked for cracks. I just picked mine up and it was .003 off cost me $135 to mill and crack check it. Well worth the $$$ so I won't have to do twice.
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    Your 99 model may have a cast iron block instead of the aluminium that came on later. If it is cast iron,,you don't have to worry about stripped head bolt threads. To tell if it's cast iron--just hold a strong magnet to the block and see if there is attraction
    Last edited by Poolman; 10-17-2013 at 06:13 PM.

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    The M52TU has an aluminum block, with cast iron cylinder liners.

    Why do you want to know if they're stripped before opening the engine? You won't know until then.


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    Try re-torquing the bolts before you pull the head off. That should give you an idea if the threads are ok.

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    If you don't check now, you won't know until reassembly and then it's too late. I'd loosen each headbolt exactly 1/2 turn, then retighten exactly 1/2 turn. If bolts tighten, threads should be OK.

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    Thanks for the info. I will try the half a turn and check method before I remove the head. I really don't want to waste a new gasket seeing if the threads are OK.

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    OK--What years of the E39 ,,used a cast iron block? I thought it was after the 2000 models.

  9. #9
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    Quote Originally Posted by Poolman View Post
    OK--What years of the E39 ,,used a cast iron block? I thought it was after the 2000 models.
    If memory serves before 9/98 have cast iron. If you have the double vanos its a aluminum block also knowin as m52tub28
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  10. #10
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    Quote Originally Posted by m735is View Post
    Thanks for the info. I will try the half a turn and check method before I remove the head. I really don't want to waste a new gasket seeing if the threads are OK.

    Loosen and re-torque. Simply turning a half turn won't be very accurate.

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    Re-torque to what torque setting? The factory doesn't provide a torque setting. Loosening half a turn and tightening half a turn duplicates the factory tightening procedure and returns bolt and threads in block to the same stress level that existed before you started. Typically only some of the threads get stripped. If the threads are starting to pull, the resistance to re-tighten will be noticeably different.
    Last edited by pshovest; 10-18-2013 at 07:56 AM. Reason: typo

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    It shouldn't hurt anything to loosen them, but what order? Loosening order? Loosen one then re-torque? Then move to the next, correct?

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    I was mistaken.......the factory procedure is xxNM, 90degrees, 90degrees. So only loosen 90 degrees. I would do one at a time, and in the reverse tightening sequence.

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    Quote Originally Posted by m735is View Post
    It shouldn't hurt anything to loosen them, but what order? Loosening order? Loosen one then re-torque? Then move to the next, correct?
    Loosening them 1/2 turn and torqueing them 1/2 turn is not going too put you back at the required clamping force due to the fact that the bolts are stretch in the final stages of torqueing them to achieve to required clamping force needed to contain all the element in your engine your headgasket is already compromised requiring replacement. The bolts currently have a chemical bond because of the two different metals invloved with each other "steel, aluminium" so when you try to break this bond the weaker metal gives."aluminum" But to awnser your questions start in the center and in a circluar pattern work out doing one at a time but this a can of worms suggestion and will proabley force you to deal with the problem of just removing the head install a new gasket and possibly timeserts and reassembling. Is there a reason why you don't want to fix this the proper way? Example. experince, money, time, etc. There is plenty of diy on doing this entire procedure on the net it is not that difficult even with the installation of timeserts.
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    The goal is to test the integrity of the block threads. Loosening 1/2, tightening 1/2, returns headbolt to the EXACT same stretched condition it had before loosening. Hard to see how this results in less clamping force then before loosening.
    How would you improve the test?
    What is the "proper way".......timesert 14 threads, most of which are OK?



    Quote Originally Posted by bartelbe View Post
    .....Loosening them 1/2 turn and torqueing them 1/2 turn is not going too put you back at the required clamping force...........
    ............Is there a reason why you don't want to fix this the proper way..........

  16. #16
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    Just so we're clear, I will be removing the head. I just want to see if I have to order the timeserts kit before I start the complete teardown. Plus I would like to know if the aluminum is going to strip BEFORE I put on a new gasket and get to a point of final torquing new head bolts only to find out I have to remove it and buy another gasket and bolts later.

  17. #17
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    I agree with pshovest. Just loosen each one and re-tighten to same spot.

    During the re-tightening, you could use a click style torque wrench and set the wrench to the factory intitial bolt torque setting. When you re-tighten each bolt, make sure you get the click. This will instill some confidence that the threads can at least withstand this force. I would think if you had some bad threads, that you may not reach this torque as the threads would start to strip.

    As an alternate and best thread test, you could go buy I extra new head bolt. Remove just one existing head bolt and install this new bolt per the factory torque procedure. When you do the final additional tightening (stretch), you could use a beam style or digital torque wrench to verify what torque is required for the final stretch. Then you could loosen each of the existing bolts the 1/2 turn, and re-tighten to this final (higher) torque number. This way you could be assured that the threads are all sound.

    I would look for any trace of aluminum on the bolt threads as I removed each head bolt. Any trace of aluminum and I would be leary of that tapped hole.

    Anyone know if BMW used special thread lube during initial factory assy? Are stripped holes very common?

    Good luck.

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    Wow....


    The highest stress on a thread is during the tightening process. Thus there is no way to tell if the threads will pull out of the block without actually trying. Loosening half a turn and then tightening half a turn will give you an idea. As will 90 and 90. Better would be to fully loosen and then do the proper torque procedure. But even that is no guarantee the threads will hold the next time.


    /.randy

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    Quote Originally Posted by philly98540 View Post
    During the re-tightening, you could use a click style torque wrench and set the wrench to the factory intitial bolt torque setting. When you re-tighten each bolt, make sure you get the click. This will instill some confidence that the threads can at least withstand this force. I would think if you had some bad threads, that you may not reach this torque as the threads would start to strip.

    As an alternate and best thread test, you could go buy I extra new head bolt. Remove just one existing head bolt and install this new bolt per the factory torque procedure. When you do the final additional tightening (stretch), you could use a beam style or digital torque wrench to verify what torque is required for the final stretch. Then you could loosen each of the existing bolts the 1/2 turn, and re-tighten to this final (higher) torque number. This way you could be assured that the threads are all sound.
    That won't duplicate the torque-to-yield spec.

    The initial torque spec is to eliminate any gap between the head, gasket and block. The turn-angle spec both compresses the gasket and creates spring tension in the bolt. Installing a new bolt with an already-compressed gasket will result in somewhat more stretch in the bolt.

    That said, BMW recommended exactly this procedure to replace flawed head bolts on the M20 engines. The dealers were instructed to replace one bolt at a time, using the same spec as with a new head gasket. That suggests that the bolts have a wide enough elastic range to make the difference in new and compressed fire ring height negligible.

    My estimation is that loosening and re-tightening a quarter turn should be completely safe, and is likely (probably very likely) to reveal thread failure. Using a new set of head bolts isn't likely to increase the failure detection chance enough to be worth the cost and extra work.

  20. #20
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    Quote Originally Posted by djb2 View Post
    That won't duplicate the torque-to-yield spec.

    The initial torque spec is to eliminate any gap between the head, gasket and block. The turn-angle spec both compresses the gasket and creates spring tension in the bolt. Installing a new bolt with an already-compressed gasket will result in somewhat more stretch in the bolt.

    That said, BMW recommended exactly this procedure to replace flawed head bolts on the M20 engines. The dealers were instructed to replace one bolt at a time, using the same spec as with a new head gasket. That suggests that the bolts have a wide enough elastic range to make the difference in new and compressed fire ring height negligible.

    My estimation is that loosening and re-tightening a quarter turn should be completely safe, and is likely (probably very likely) to reveal thread failure. Using a new set of head bolts isn't likely to increase the failure detection chance enough to be worth the cost and extra work.
    Just do one new bolt to get the torque value. Not too much expense.

  21. #21
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    I finally found time to do this when it wasn't -20 in my garage. I just picked a number of 30 ft lbs on my torque wrench. I didn't even loosen the bolts first. I just went in tightening sequence. 10 of the bolts clicked at 30. 4 of them just spun in the bores. So I would assume I have 4 bolts that don't hold. Looks like I'm timeserting this block.

  22. #22
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    Quote Originally Posted by m735is View Post
    I finally found time to do this when it wasn't -20 in my garage. I just picked a number of 30 ft lbs on my torque wrench. I didn't even loosen the bolts first. I just went in tightening sequence. 10 of the bolts clicked at 30. 4 of them just spun in the bores. So I would assume I have 4 bolts that don't hold. Looks like I'm timeserting this block.
    Initial torque is 22 ft lbs, then 90 degrees, then 90 degrees. I would bet this equals a finished torque of about 70 to 80 ft lbs. Your "test" of 30 ft lbs didn't really put that much stress on the fasteners, and still they failed, so you know that timeserting is the way to go.

    I think you will find that the first holes to strip are those around the middle of the engine- cylinders 3 and 4. The engine probably at one time had a hard overheat. The threads would probably be OK if it never overheated. It seems that an overheat is worse in the middle of the engine. The heat concentrates there.

    I have the full 6 hole timesert jig, and I've done a bunch of these. Make sure you do all 14 holes the first time, and don't try to cheat.

    Also, I like to drill the holes by hand. I think it results in a hole that is smaller, and more "on spec". The aluminum is very soft, and I think using an electric drill at high speed makes the bit "jump around" more than it should, and removes too much material. The blocks I did by hand all worked perfect. The one I electric drilled couldn't hold its timeserts. Drilling by hand is much easier than you would think.

    Good luck.

  23. #23
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    Well, I'm not sure about the middle cylinders generating more heat and causing an overheating issue but I've come across the last two cylinders getting hotter than the rest.



  24. #24
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    While head bolts can be re-torqued, I wouldn't. They're stretched into the plastic range from the factory. Re-torquing them is playing with fire imo. Using new head bolts on an old headgasket should be fine though - they're cheap if you really want to test the block (although I still wouldn't do this, if you get coolant in the threads, you'll overtorque the head bolts.) - I would inspect the block after pulling the head.
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    The M52 is known for running hotter (or leaner, the argument continues) on the last two cylinders. But the M52TU and M54 have a totally different coolant flow pattern. The colder coolant is fed into the engine from the back, heading towards the front. Cyl 6 sees the coldest coolant, opposite of the older M52.

    It seems the headbolt failures start around cyl 3 and 4. This may be as simple as head warpage putting the most stress on the center bolts.
    Last edited by rf900rkw; 02-10-2014 at 03:17 PM.


    /.randy

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