Fun with a boroscope - II: installing structural foam in the front subframe cavity

[martin.oconnor]

So do you basically inject a full tube of epoxy into each side of each front mount. So hole b and c...and the same on the other side? So you would need to buy 4 total tubes...

Yup - 4 total for the two front mounts, and another 4 for the rears. So, about $700 worth of goo (each tube is about $85).

It's a very easy job - and factory holes can be used to inject the structural foam for all four mounts. (If I were doing it again I would not bother to drill the middle hole into the rear cavity to inject seam sealer since I don't think the foam even reaches the middle.)

[cozmo kraemer]

So do you basically inject a full tube of epoxy into each side of each front mount. So hole b and c...and the same on the other side? So you would need to buy 4 total tubes...

I did plates, and poly bushings for the SF...which was after I had already epoxy foamed both of the rear mounts per the BMW TSB. At this point I dont know if I need to do the fronts, but I guess it isn't a terrible idea.

[nuc1]

Yeah - I have no idea why BMW specified the procedure for the left rear only, particularly since all four mounts can crack (though the left rear and right front seem to be more prone to it).

BMW has not exactly been impressive here. Similar failures occurred on the E36, so a lot of effort was clearly not expended into solving this problem properly.

Probably due to how the driveshaft/rear diff spin and cause torque on the RACP. So they threw in some rough numbers, a bunch of assumptions and then bounced that off of the $$$ to fix the cars under recall. And voila, one mount only.

Mike

[old///MFanatic]

Yeah - I have no idea why BMW specified the procedure for the left rear only, particularly since all four mounts can crack (though the left rear and right front seem to be more prone to it).
BMW has not exactly been impressive here. Similar failures occurred on the E36, so a lot of effort was clearly not expended into solving this problem properly.

Yep, BMW E36s had the front mounts iirc. So BMW put a brace up in there and then on the E46s it moved to the rear being the main problem area.

[martin.oconnor]

Yeah - I have no idea why BMW specified the procedure for the left rear only, particularly since all four mounts can crack (though the left rear and right front seem to be more prone to it).

BMW has not exactly been impressive here. Similar failures occurred on the E36, so a lot of effort was clearly not expended into solving this problem properly.

[old///MFanatic]

Yes on a car that has cracks it can absolutley move to all four mounts. But if a car doesn't have cracks or welds giving out and it's preventative only is what my question was based on.
Most people aren't aware that BMW has the foam/epoxy method listed for the left rear. Or that research shows the front right mount often cracks before the right rear.

[martin.oconnor]

Very interesting, thanks for sharing.
I know you're doing them all. Wonder since BMW's TIS Repair calls for the rear left for the epoxy, and other info suggests the front right breaks early. Wonder if doing the front right and back left would be better than doing just both rears??

Given that all four can show cracks it is probably best to do them all.

Take a look at this recent video from Redish Motorsports (a UK shop specializing in M3 RACP repairs) showing cracks at all four corners: https://www.youtube.com/watch?v=d8bMX2NF9e0

While the structural foam is not cheap (about $170 per corner), it is probably best to do all four while you are in there.

[old///MFanatic]

Very interesting, thanks for sharing.
I know you're doing them all. Wonder since BMW's TIS Repair calls for the rear left for the epoxy, and other info suggests the front right breaks early. Wonder if doing the front right and back left would be better than doing just both rears??

[martin.oconnor]

In a recent post I outlined how I used a boroscope to examine the rear cavity on the RACP before injecting structural foam to strengthen the rear subframe mounts.

The boroscope images in that post clearly showed that the rear cavity is completely enclosed and that pre-drilled holes at the top of the cavity can be used to inject structural foam to surround the left and right rear subframe mounts.

After examining the front cavity with a boroscope, it is clear that a similar same approach can be used to surround the left and right front subframe mounts with foam.

Here is a picture of the left top side of the RACP (facing towards the rear of the car).



All four holes are factory drilled. Holes A and D are tapped for the rear left seat hinge brackets, B is tapped to hold the rear left seatbelt, and C is unused and has a factory plug.

The left front subframe mount sits between holes B and C, though is slightly closer to hole B

In inserted the boroscope into hole C to examine the left side of the cavity.

Here is a view of the left front subframe mount inside the cavity. The top and bottom of this mount are welded to the ceiling and floor of the cavity.



Here is a view that also shows the underside of holes of A and B. A red ZipTie is inserted into hole B.



Unlike the rear cavity, which extends uninterrupted for the width of the car, the front cavity appears to have a panel in the middle, though it has a circular hole in it.

So, it is not reallly neccesary to inject seam sealer into this middle section to seal off both sides. (In fact, I don't think it really is needed for the rear either since the foam is unlikely to reach that point even when cured.)

Here is a view from hole B towards the side of the car.



As can be seen, the cavity terminates immediately after hole D.

The goal of foam injection process is to bond each mount to the RACP by completely surrounding it with foam.

For the left front mounts, holes B and C above make the most sense as injection points.

After injecting one 420ml container into hole B, I inserted the boroscope into hole C to see where the foam ended up.

Here is a picture of the left front mount immediately after the injection of structural foam into hole B.



As can be seen, the foam is beginning to surround the mount even before the curing expansion process - which is exactly what you want to see.

After foam is injected on the other side of the mount it will be completely enclosed - and bonded to the RACP permanently.

For completeness (and to verify that the cavity is symmetrical side-to-side) I also inserted the boroscope into the right side of the cavity.

Here is a view of the right front subframe mount inside the right side of the cavity.



And, finally, here is the right front mount after foam is injected on one side of it.