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Simple way to gain some Downpipe clearance

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When you’re doing this with the original DSM parts, you are not using the portion of the shaft with the hex or shoulder. It becomes a straight, hollow shaft threaded on the outer diameter the whole length of the piece.

Like you said - ancient thread, so I’d have to look at it again to be certain, but the question I would have is whether that portion of the Subaru fitting interferes with the oil filter mating flush with the housing when installed.

If it is in the way, surely you could turn that portion of the shaft down. But then, what do you gain? Spending $10 for a part you need to modify to work versus modifying a part to work? I suppose there is the benefit of saving your original part, just in case.

Seems the Internet shows “close nipple fitting” or “male to male shoulder nipple fitting” as possible search terms for a piece without a shoulder, but I’m not finding one very quickly with the needed thread.

And for those reading in the future, this isn’t 7-bolt specific. It would work the same for the 6-bolt water cooled housing.
 
To me, it seems that the advantage of using the Subie fitting is more threads into the oil filter housing because it's a tad longer than just the threaded portion of the stock bolt. You can also tighten it with a socket or wrench. If you use a shortened stock bolt, it seems that the best you could get is hand tight. The fitting ends up being a tad shorter on the filter side, but despite that and the hex, both 1g and 2g oil filters fit just fine.

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If the filter sits flush and seals, I don’t see a problem at all. And that would be a very nice find for future DSMers!

I’d probably red loctite the fitting into the housing to keep it from backing out.
 
After playing around a bit more just thought I should add a disclaimer that it's all just test fitting with spare parts, and that I'm not currently running it on the car. Regarding the filters, the smaller is a K&N and the center is more recessed and doesn't appear to make contact with the hex portion when hand tightened (Fram would be similar). The larger is an old Purolator and there *is* contact because the center is not as recessed, and that may affect how well the O-ring seals.
 
Figured i'd add a bit of useful info on this. I did this mod yesterday and added a set screw per the following instructions.

Couple things to note: Depending on what size bolt you use to retain the modified m20 shaft, you may not be left with enough threads to adequately hold the oil filter on. I tapped mine for a m8 bolt because I felt if I ever wanted it removed there would be a risk of snapping a m6 off in there from galvanic corrosion and I also already had a m8 tap out on my work bench. I threaded the shaft in as minimal as possible to cover the newly drilled and tapped hole and it only left enough threads sticking out to thread a 2g Fram Ultra oil filter on 2.5 turns before the oring made contact. Long story short, drill your retaining bolt hole as close as possible to the filter and use a set screw rather than bolt (to allow you to get closer to the filter from lack of a bolt head). I plan to buy the piece linked above by RTM FWIW.


EDIT: The threaded portion on the factory shaft is ~ 1-1/4" long so the RTM part would definitely allow the same procedure above and increase thread engagement.
 
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