I'm adding to this old thread (which I found really helpful when mucking with the BB) rather than start yet another one about the bevel box.

While my BB was out for a clean and service I learned a few things:

Having decided to drill/tap the box for a top fill plug, I decided the right way to do this was to remove the side cover and drive sprocket before the work began to ensure any swarf was fully cleaned out. The side cover has a large diameter, though surprisingly thin, O-ring, so no gasket to disturb or sealer needed. Removing the cover (via the small internal hex screws) also removes the crown wheel and carrier (spool) and no critical settings are disrupted in the process.

I was surprised at how much gear oil remained in the BB even after being drained hot after a good run (I use a lift with the car level). Probably only a few ounces, and it appeared to have been hiding in the front pinion area. This suggests that having the nose of the vehicle elevated somewhat while draining is a good idea. Bwitched above noted his front housing had a somewhat obstructed opening to the front bearing area, though mine looked fine. I was relieved to see there was no evidence of abnormal wear, and the two short duration "flushing" stints with normal gear oil I had done left the interior very clean indeed. These days I run the Redline 75W250 Heavy Shockproof oil, the pink stuff.

As the right side port (where the breather is on my car) was 3/8" BSPP, I elected to make the new top plug hole the same so I could move the breather there. Later I decided to add a remote fill hose to the shock top area via a BSP to AN8 fitting with the breather mounted on the hose end.

I have seen photos on the forum of remote filler hoses being connected to the right side port. After having the BB open, I believe this could be a bad idea, unless you are very careful about how much oil you are adding. That right side port (center of bottom photo) is not open directly to the BB interior. Rather it opens into the center of the hollow spool shaft (top of top photo below) which carries the crown wheel and bearings. This hollow shaft has only two 5mm or so diametrically opposed holes in it (top photo again). Trying to fill via the right side port would likely take quite awhile as oil would only flow through the lowest one of those two holes or perhaps through the roller bearings themselves. If one is relying on oil coming out of the right side as a "full" indication, you could easily be fooled into thinking it was full when you had only a few ounces in there. So I would recommend using the original level plug for verification, or filling with a measured amount of oil.

For reference, with the BB set up level on the bench and being dead empty, I measured the gear oil capacity at about 10 U.S. ounces (about 300ml) by pouring gear oil slowly in the new top port with the rear fill/level plug removed. I will probably experiment with larger amounts of oil next season to measure sound and temperature changes with more oil, but am some what concerned about getting the oil level at or above the seal level as these seals are generally not designed to be fully immersed and are likely to seep oil when immersed, as others have noted.

Finally, I was surprised to see a thread repair insert in the main housing (bottom photo), as it certainly appeared as if the BB had never before been removed from the vehicle.

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Steve
Late 2012 M3W