OK, so this is the deal... this oil tank is a flat-ended cylinder, about 6.5 inches diameter, about 12 inches long... essentially a "custom" style harley big twin oil tank, complete with the flat cutout on the bottom of the left side to clear the top of the primary cover on a harley big twin, and the "battery tray" cut out of the back of it (I'm not using this battery tray... my battery is mounted below the oil tank, behind the transmission, on its own little tray welded to the frame).
Turns out I thought the tank had 3 ports on the bottom (right side of the tank), but it actually has 4 ports. One is the drain, and next to that is the supply. Then, more towards the center is a port that the oil pump return was connected to. What I couldn't see until I disconnected and moved stuff out of the way is that 4th port, hidden behind there, that was plugged. Drained the tank and using a mirror and one of those cheap "endoscope" USB cameras I determined that the port the oil return was connected to, and that 4th port, both have tubes inside that go to the top of the tank. In fact, the port the oil return was connected to is just a straight tube, up to the very top of the tank and open at the top, and that 4th port is a tube that goes 3/4 up towards the top of the tank, then takes a 90 degree turn and extends all the way to the left side of the tank where it opens. So, it seems the 4th port really should be the oil return, and the PO had the oil return connected to what is supposed to be the vent.
So I need to get another 1/8 NPT to 3/8 hose barb fitting. I need to connect a hose to the "vent", and do something with it.
That's where I'm not sure what might be best.
Simplest thing is just connect a hose, leave it open, and tie it off somewhere.
But I want to do something that addresses the crankcase vent, the nipple on the back of the transmission/engine, AND the oil tank vent. Don't like that transmission port just open to the air.
I suppose I could just connect the oil tank vent to the back of the transmission... But I don't think that's right... that would result in excess pressure delivered to the transmission, which I'm not sure will be handled correctly.
I know not to connect the oil tank vent directly to the crankcase vent - both need to release pressure. I could use a "Tee" to connect them, with one leg of the "Tee" open (with a filter, I suppose). But that doesn't address the transmission.
I could use a "Tee" and connect the oil tank vent to the transmission port, with one leg of the Tee open. That would vent the oil tank, and if anything actually comes out of the oil tank, it could/should drain down into the transmission (that's an oil "return" line, as I understand it, anyway. But I still have that crankcase vent with just a short little tube and filter, just waiting to get soaked and drip all over.
I could use a 4-way "Tee", to connect the crankcase vent, the oil tank vent, the transmission port, and the 4th leg being open (filter)... hoses routed so as to cause any liquid oil that happens to collect in these hoses to drain to the transmission.
That's what brings me back to the idea of the oil "catch can"... used instead of the 4-way "Tee"... Crankcase vent and oil tank vent going to the "in" of the catch can, drain of the catch can down to the transmission, and "out" of the catch can to an open hose (with filter on the end).
Just hoping for advise on whether any of you have specific advise for one of these approaches over the other, or perhaps a reason why my idea has a problem I haven't considered?
Final thought - This tank really isn't ideal - eventually I'd like to get another solution that fits better... in part because the air cleaners for the carbs are right up against it, right now... almost blocking the oil fill cap.
Included pics of the tank - mounted, and pulled out, plus the open area where the tank was.
