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Thread: M52 engine removal by knock sensor bolts

  1. #1
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    M52 engine removal by knock sensor bolts

    Has anyone removed a M52B28 engine by the knock sensor bolts? I did it last week, but it was an aluminum block and the head was off. The transmission was attached.

    Now I am putting a cast iron M52B28 back in the car complete with transmission. The 2 knock sensor bolts are in the perfect spot for the hoist leveler and engine angle. The question is, 'are the bolts strong enough to support over 600 pounds of engine and transmission?

    The bolts are M8s and go 18mm into the block. The block is cast iron so I would assume the threads in the block would pull before the bolts would break.

    Anyone care to comment?

  2. #2
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    Why not use the lift pointe..?

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  3. #3
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    Quote Originally Posted by cjames10 View Post
    Has anyone removed a M52B28 engine by the knock sensor bolts? I did it last week, but it was an aluminum block and the head was off. The transmission was attached.

    Now I am putting a cast iron M52B28 back in the car complete with transmission. The 2 knock sensor bolts are in the perfect spot for the hoist leveler and engine angle. The question is, 'are the bolts strong enough to support over 600 pounds of engine and transmission?

    The bolts are M8s and go 18mm into the block. The block is cast iron so I would assume the threads in the block would pull before the bolts would break.

    Anyone care to comment?
    Two things to consider when calculating bolt stress.

    1) normal stress - the stress in a bolt when the load is in the direction of the bolt axis, causing an extension or lengthening of the bolt. This is calculated by stress = force/cross section area (S = F/A)

    2) bending stress - the stress in a bolt due to the load acting at some angle to the bolt axis, causing the bolt to bend. The worst case would be the load acting perpendicularly to the bolt axis. This is calculated by Sb = Mc/I, where M is the moment caused by the load, c is the bolt radius, and I is the area moment of inertia. The moment, M, of the load will be the applied load acting at the end of the bolt times the unsupported length of the bolt sticking out of the engine block. A load perpendicular to the bolt axis is the worst case because at that angle the moment is maximized.

    For an 8 mm bolt we have the following data:

    minor diameter: 6.466 mm = .254"
    c = d/2 = .127"
    I = pi*d^4/64 = .000204 in^4
    A = pi*d^2/4 = .05067 in^2
    F per bolt = 600 lb/2 = 300 lb

    For case 1, the normal stress if the load is aligned with the bolt axis is then Sn = F/A = 300 lb/.05067 = 5.9 ksi

    For case 2, worst case situation for the load acting perpendicularly to the bolt axis and with a moment arm of let's say 1", Sb = Mc/I = 300*1*.127/.00204 = 186.5 ksi


    For any other angle of load, you will have to decompose the load vector into axial and perpendicular directions and superpose the results of calculations for both Sn and Sb.

    For example, if the load angle is 60 degrees, then the axial load on the bolt will be 300*cos(60) = 300*.5 = 150 lb. So the normal stress will be Sn = 150/.05067 = 3 ksi

    The perpendicular load will be 300*sin(60) = 300*.866 = 260 lb, giving a bending stress of Sb = 260*1*.127/.00204 = 162 ksi

    The maximum bolt tensile stress will then be the sum of the normal stress and the bending stress, Sm = Sn+Sb = 3+162 = 165 ksi.


    Similarly, for a load angle of 30 degrees, the normal stress will be Sn = 300*cos(30)/.05067 = 5.1 ksi, and the bending stress will be 93.2 ksi, giving a combined maximum tensile stress of 98.3 ksi.


    At this point you will have to compare the calculated bolt stress to bolt strength. For case 2 above, where the bolt stress is 186 ksi, even a metric grade 12.9 with a published strength of 177 ksi is not strong enough. You could get by for the 60 degree load angle but with very little safety factor. The load angle of 30 degrees could be done with grade 8.8 bolts (116 ksi strength) and marginal safety factor.

    Bottom line: I wouldn't recommend it, but if you do, be sure you know what your bolt strength is.
    Last edited by Stephen Max; 12-18-2018 at 10:49 AM.

  4. #4
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    Going beyond 177ksi (1220MPa) only means you're in the plastic (i.e. loss of ductility) range of the bolt. 186ksi on a 12.9 bolt would stretch the bolt beyond it's elastic limit. Bottom line: you could use the bolt, but you wouldn't reuse it for anything meaningful. The proof stress of 177ksi is where you'd see the bolt start to stretch (aka necking) before fracture.

    I've pulled apart many bolts (on purpose) throughout college and during my 13 years as a professional structural engineer I've seen a lot of misuse of bolts. Many bent and stretched, very few actually broken. The vast majority of breaks have been due to over tightening, not bending.
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    Hmmm-I just love this forum-I get a course in physics at no charge!

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    Quote Originally Posted by s8ilver View Post
    Going beyond 177ksi (1220MPa) only means you're in the plastic (i.e. loss of ductility) range of the bolt. 186ksi on a 12.9 bolt would stretch the bolt beyond it's elastic limit. Bottom line: you could use the bolt, but you wouldn't reuse it for anything meaningful. The proof stress of 177ksi is where you'd see the bolt start to stretch (aka necking) before fracture.

    I've pulled apart many bolts (on purpose) throughout college and during my 13 years as a professional structural engineer I've seen a lot of misuse of bolts. Many bent and stretched, very few actually broken. The vast majority of breaks have been due to over tightening, not bending.
    You make an excellent point, but 177 ksi is the ultimate strength for grade 12.9, the yield strength is 160 ksi and the proof strength is 140 ksi. So if you did stress it to 186 ksi i think you probably would actually break it, assuming there is no reduction in load due to bending.

  7. #7
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    Given those numbers, and the fact that the bolt will not be perpindicular... seems most any bolt would break in that situation

    Also, kinda sad putting a iron M52 into a Z3 but it makes sense...

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    From a quick glance at the math, it appears there is an assumption of static loads equally distributed. Not true. With the transmission cantilevered off of the back, the loads will be more like 200/400, or even 100/500. And this is before the bouncing and swinging dynamics factor in.

    Use the lift eye cast into the block at the back. The front will be a lot more forgiving; a M8 should suffice.


    /.randy

  9. #9
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    Quote Originally Posted by rf900rkw View Post
    From a quick glance at the math, it appears there is an assumption of static loads equally distributed. Not true. With the transmission cantilevered off of the back, the loads will be more like 200/400, or even 100/500. And this is before the bouncing and swinging dynamics factor in.

    Use the lift eye cast into the block at the back. The front will be a lot more forgiving; a M8 should suffice.
    Yes. Analysis with the assumption of an equal loading shows unsafe bolt loads. Uneven loading only makes the situation worse.

  10. #10
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    Ok, so I got the engine in the car. But I chickened out and used the traditional lift points. I had a leveler with the hoist so, I was able to install the engine without removing the hood or the front end.

    The only problem with the traditional lift points are the engine rotates to the passenger side. This meant I couldn't keep the passenger side motor mount on, I had to remove it to clear everything.

    I have to say though, when removing just the block and the transmission, the knock sensor bolts made the operation much smoother. I had the engine out in 10 minutes. I was discarding the original motor anyway so I wasn't concerned about damage to threads or bolt.

  11. #11
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    I have to say "Stephen Max" those are great calculations. I am electrical engineer not a mechanical. Here is what I found, which could be wrong.

    If the thread depth is 2 times greater than the bolt diameter then the threads will be stronger than the bolt. The bolt becomes the weak point of failure. Is this a true statement?

    When I looked at bolt strength tables the lowest grade M8 bolt was listed as 1800 pounds proof load. That number seemed high.

    The engine and transmission together must weigh around 700 pounds.

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