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Birkin Engine Mount bolt shear problem

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  • prabjohns
    replied
    See! I helped! Doug didn't do everything! (only 95%....)

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  • Doug Liedblad
    replied
    Paul cutting bolts

    Paul doing the cutting
    Attached Files

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  • Doug Liedblad
    replied
    Originally posted by GWise View Post
    Where are you buying your fasteners from? Please don't say Lowes or Home Depot...
    Bought from HERE

    The broken bolts were 12.9 also and plated. I know about Hydrogen Embrittlement but don't know if it applies in this case. Both of the forward bolts broke in the same place.

    When I first put my SVT engine in I mounted the oil pressure sending unit directly to the block with a pipe nipple. Don't do that as the vibration breaks whatever you thread into the block with the weight of the sender on the end. I have since mounted the sender remotely with a braided line to the block. The point being Zetecs vibrate a lot which breaks things. I don't know if this applies.
    Last edited by Doug Liedblad; September 4, 2015, 05:02 PM.

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  • slomove
    replied
    As long as the bolts are not over-tightened, they should easily hold the engine in place in terms of static tensile or shear load. At a reasonable torque of 40 ft-lbs for such a bolt, the tightening force is almost 3 tons (for a single bolt). I don't think another 240 pounds (distributed over 4 bolts) for the engine weight will make any difference. Something else must be at play to break them. That is why I don't like the fact of 2 bolts only with the long leverage of the mounting bracket because the 2 mounting points become a pivot and any force applied offset to that pivot axis will bend the bolts. With that leverage the stress and/or fatigue limits of the bolts can easily be exceeded. After all, the mounts have to take up the changing reaction forces of the engine applying torque to the drive shaft as well as vibrations.

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  • GWise
    replied
    Yup - re-read it now and I see "cut to length." my apologies.

    Another thought is to using a lower grade bolt where ductility is preferred over strength. Maybe try an 8.8 this time considering 10.9 made it 4000 miles vs. the shorter life of the 12.9's?

    Where are you buying your fasteners from? Please don't say Lowes or Home Depot...

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  • Doug Liedblad
    replied
    That's just his terminology. Paul is a movie music editor not an ME. Plus he's English. All we know is it broke, probably in tension as the bolt broke inside the block about 2 threads, but I too am not an ME so you may have more insight.

    The threads are rolled threads, by cutting he means we took a longer bolt and cut it to length to try and get the weight supported by the nonthreaded portion as much as possible.

    If you can tell the failure mode by a close up picture Paul may still have the broken part.

    Doug
    Last edited by Doug Liedblad; September 3, 2015, 01:35 PM.

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  • GWise
    replied
    How can you tell that the bolt in photo 6 failed in shear vs. tension?

    I assume you have to have the bolts cut to get the right shank length? Cut threads are inherently weaker than rolled and based on that photo it could have failed in tension due to the cut thread.

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  • Papak
    replied
    I am building a Birkin wide body kit right now. The chassis's being sold currently are built for the Duratec motor. The mounting consists of a reinforced horizontal tube welded to the frame rail on each side and a three armed mount ending in a 1آ½" tube with a polyurethane bushing inside of it. The mount attaches to the block with M10 bolts and the bushing end holds an M12 bolt that extends outward through the frame mount tube. I will try to upload a couple of pictures tomorrow. I replaced all of the stock 10.9 hardware with 12.9. After several discussions with Tom Carlin, he has not heard of any issues with the current mount and has a fair amount of track time on a couple of cars with this configuration.

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  • slomove
    replied
    I have not heard about other similar failures, but then, I did not follow all posts.
    My motor mount dampers are just the standard cylindrical sandwich types with studs like the pic below, seated vertically in a metal cup. They hold up for a few years and I had two to delaminate the end plate a while ago, maybe because they had been drenched with oil for a long time. But they are known to do that. I replaced them with the engine swap as a preventive measure. They are only a few bucks each and it takes half an hour to replace.
    The newer style like Paul has are captured with the bolt and can probably not fail easily.

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  • Doug Liedblad
    replied
    Are other Birkin owners with the newer mounts having broken motor mount to block bolt breakage or is it just Paul?

    What are the symptoms of failure of the cylindrical damper motor mounts?

    Paul's looked OK but I also wasn't looking for issues there.

    Doug

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  • slomove
    replied
    I have the old ones. Not sure if they are still available. Tom Carlin or Dick Brink might have them.
    Unfortunately, the chassis mounting points have changed with the engine mounts as well. But I would guess that is easy to modify.
    As mentioned earlier, This type of engine mount used the cylindrical dampers with vulcanized end plates. They do have a limited life but are easy to swap when they fail.

    I can try take pictures how that looks on my engine. Should have done that 2 weeks ago before when I had the exhaust and intake off.

    Edit: I tried taking the pictures but there is just too much junk in the way...sump guard, oil hoses exhaust and intake.
    Last edited by slomove; September 1, 2015, 05:48 PM.

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  • Doug Liedblad
    replied
    Do you know who's Birkin is the newer one?

    Paul might consider contacting Birkin and asking if the have the engine mounts that bolt to the block in three or four places.

    Easier than building your own.

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  • slomove
    replied
    Wow, I have not looked in detail at how the newer Birkin motor mounts are designed. I guess they overdid it a bit with the "added lightness". I my car they are pretty heavy duty welded sheet metal fabrications with a 5mm plate attached to the block using 3 or 4 bolts. I can imagine that a 2-bolt design tends to flex and fatigue over time with that stiff bushing damper. If that happens again I would consider welding on an additional strut to a 3rd threaded hole in the block (there are ample).

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  • Doug Liedblad
    replied
    You can't really tell from photo 6 but the bolts broke off a thread or two inside the hole.

    I am not sure the full size shank will make a difference but perhaps they weren't tight enough. The broken bolts were a plated grade 12.9 Allen head, the replacements are the standard black oxide finish.

    Did you check them to be sure they are still tight?

    We also need to thank neighbor Ray for supplying the angle drill and a good hard drill bit, without which the job would have taken much longer.

    Doug
    Last edited by Doug Liedblad; August 31, 2015, 10:45 PM.

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  • prabjohns
    replied
    Huge thanks to Doug for spending so much time working on my engine mounts with me on Friday.
    FullSizeRender(4).jpg
    Removed the left side engine mount due to concern that the locktite I had used would make the bolt difficult to remove if they sheared again.
    The bolts came out a little too easily. Slomove might be tempted to say that I sissy torqued them....He'd be right!
    Re-tapped the threads to remove the locktite and clean them upFullSizeRender(2).jpg
    Doug cut new shanked bolts to lengthFullSizeRender(3).jpg
    Installed the engine mount and torqued the bolts to 45lb.ft, (well Doug did.)
    Re installed the exhaust and headers. Lastly, quick check of the right side engine mount bolts and then done.
    First bolt fine, second bolt...oh dear... Sheared!! I guess it had been taking too much load since the other side sheared off.
    FullSizeRender(6).jpg
    After some skillful drilling by Doug, out it came. Phew!
    FullSizeRender(7).jpg
    I will be keeping a regular close eye on them now to make sure it doesn't happen again.
    Last edited by prabjohns; August 31, 2015, 11:56 PM. Reason: spelling

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