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Thread: Transmission Mount Failure

  1. #1
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    Transmission Mount Failure

    The transmission brace on my track E36 M3 failed at some point, so I bought a new one to install. When I tried to torque the passenger side mount to 16 ft-lb, I couldn't get to the torque. I should have stopped, but I kept going at it. Eventually the new brace cracked at the exact same spot as the old one. I took everything apart and looked at the red poly mounts from AKG. It appears that both driver and passenger side mounts are somewhat indented. The passenger side one was worse where the brace meets the mount. It was cupped in and resulted my new brace to bow into the mount, at which point it cracked (aluminum...). Has anyone had this issue? When I contacted AKG last year about the torque spec, they recommended 30 ft-lb. I dismissed it because that is too high for the through bolts (M8 bolts) and decided to stick with 16 ft-lb. Is 16 ft-lb too much?

    I have to decide whether I should purchase different mounts or not. I have AKG red poly motor mounts, which I believe are still doing fine. My understanding is that I should try to match motor and transmission mounts. It appears that many people recommend Rogue rubber transmission mounts even for track cars. Can I install rubber mounts with red poly motor mounts?

    I have not contacted AKG yet since I want to see if this is something common first. Appreciate your help!

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  2. #2
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  3. #3
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    I have these:

    https://vorshlag-store.com/collectio...-durometer-set

    The design is different than AKG in that there's aluminum on both sides sandwiching the poly, so nothing can deform like in your case. They don't specify a torque, but says to tighten until snug against the end plate, then tighten until the poly starts to bulge, then back off a bit. In my case that's probably not far off from 16 ft/lb one way or the other. I've spent some time adjusting my poly motor and trans mount torque to get them just right. It's a bit of a guessing game.

    I would think that rubber would be fine as well, even if you have poly motor mounts. I don't see why it would cause an issue. The poly set that I got from Vorshlag are 95A on the motor and 80A on the transmission (which is similar to rubber). That's what they recommend. Poly on the motor and rubber on the trans should be similar in terms of the stiffness delta between them.
    1999 M3/2/5 - Titanium Silver - Track/Weekend Toy


  4. #4
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    Thank you for suggesting. I like Vorshlag's design in that theirs mounts have plates that cannot be easily deformed. I think I am going to go with red one (95A Durometer) since AKG's red motor mount is 75D. I think they would match up well.

    I will definitely follow their instruction. It seems that polys are finicky to install.

    Thanks!

  5. #5
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    Yeah given that you have a track car and NVH isn't really a concern red 95A should be fine. They will be noisy though.

    Good luck.
    1999 M3/2/5 - Titanium Silver - Track/Weekend Toy


  6. #6
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    You might consider this as well if you managed to crack you old tran brace
    https://www.rallyroad.net/collection...nsmissionbrace

  7. #7
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    I like the BW group N replicas motor mounts and Rogue trans mounts (or e21s with cups). Rubber doesn't suffer from memory effect.

  8. #8
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    See if you can find stainless washers for both the poly mount side, and brace side. The brace is aluminum and pretty soft. How are you measuring 16 ft-lb? 16ft-lb is pretty light and I wouldn't expect it to cause the aluminum to yield like that.

  9. #9
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    Quote Originally Posted by JitteryJoe View Post
    You might consider this as well if you managed to crack you old tran brace
    https://www.rallyroad.net/collection...nsmissionbrace
    That's next if my new aluminum brace cracks. The failure happened due to the cupping in the poly mount, and I should be okay when I install new mounts with aluminum end plates.

    Quote Originally Posted by scooper View Post
    I like the BW group N replicas motor mounts and Rogue trans mounts (or e21s with cups). Rubber doesn't suffer from memory effect.
    I thought about moving to rubbers but I am going to stick with polys for now. I will let you know when I eventually go to rubbers due to too many failures...

    Quote Originally Posted by hli View Post
    See if you can find stainless washers for both the poly mount side, and brace side. The brace is aluminum and pretty soft. How are you measuring 16 ft-lb? 16ft-lb is pretty light and I wouldn't expect it to cause the aluminum to yield like that.
    I used my 50-250 in-lb torque wrench and torqued it to 192 in-lbs. I was somewhat surprised that the brace cracked to be honest. Then again, cast aluminum is pretty soft and brittle. I am thinking of using/stacking some bigger and thicker washers when my new brace and mounts come in sometime this week.

  10. #10
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    Reinforce the brace by welding an aluminum strip across the underside.

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