Birkin Engine Mount bolt shear problem

[Doug Liedblad]

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

[slomove]

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.

[Papak]

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.

[GWise]

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.

[Doug Liedblad]

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

[GWise]

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...

[slomove]

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.

[Doug Liedblad]

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.

[Doug Liedblad]

Paul cutting bolts

Paul doing the cutting

[prabjohns]

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