M42 oil filter housing passage questions

[TakiM3]

I am guessing that it is an oil return for the long cap-securing bolt. When you remove the bolt, oil will run down into the bore hole. You can see some of the threads through the round hole in the pic of the rectangular port. If oil is forced out of there, it will fill up that void area, and just vent back into the main dirty-oil reservoir once it is full. Given what is below it, I do not know why they bothered with the little round hole through which you can see threads.

That tapped hole seems to go all the way through into the larger open area in the housing, and the return for that seems to be all the way at the bottom of the timing case, where oil can drain back into the pan below the regulator valve/plunger assembly. Since the long bolt goes into a through-hole, I am really not sure why they would have bothered with the upper relief.

So it is part of ensuring that you cannot hydrostatically lock the bolt and break the filter housing or blow out the gasket. That is my guess, anyway.

This is as good a guess as any bmwman91. Still perplexing though, especially since you can apply some logic and figure out the flow of the oil throughout the rest of the system.

[bmwman91]

When you install the cap and long bolt, how far down do the threads engage? If the random-seeming hole is left partially open, the upper rectangular area will then have oil flowing down into the larger open area, and then to the small opening at the bottom of the plunger assembly. Maybe it serves to lubricate it in its bore?

[TakiM3]

When you install the cap and long bolt, how far down do the threads engage? If the random-seeming hole is left partially open, the upper rectangular area will then have oil flowing down into the larger open area, and then to the small opening at the bottom of the plunger assembly. Maybe it serves to lubricate it in its bore?

That's certainly a plausible explanation. I'll put the bolt in and take a photo and post it later.

[TakiM3]

So I pulled the top of the oil filter housing and long bolt off, and filled it with water, and as expected it drained out from the triangular hole into the rectangular area. No surprises there. I filled the clean side of the filter housing (the central tube), and again, as expected the water drained out of the bottom of the bolt hole and the hole into the rectangular area. Next, I put the bolt in without the lid, and screwed it down, and then filled the tube with water. This time no drainage out of the bottom holes at all.


Top hole into rectangle, the threads is the long bolt can. Be seen.


Bottom hole. The bottom of the bolt can be seen.

So, I’m still clueless as to why the triangular port seems drain dirty oil into the rectangle with seemingly no drainage out of it. Seems like a recipe for a leak to have standing oil in there with seemingly no outlet....


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[TakiM3]

Also of note, this rectangular cavity of all three engines I've opened is full of dirty, sludgey oil, not surprisingly.

[bmwman91]

Yeah, it is definitely a bit of a mystery.