Let me start by saying that the fill, inspection, and drain plugs on the Turbo-II's R box don't make much sense to me. Regardless of what Mazda was thinking when they designed it this way, here are all of my notes about them.

There are 4 (rather than the typical 2) holes in the transmission: A fill, inspection, and 2 drain holes.





-Fill Hole: (1/2" NPT, plug: tapered threading, 14mm/9/16" square head - 16mm/5/8" 12pt socket will fit, 24.4-39.3 Nm/18-29 lb ft)
Name says it all. The hole is on the side of the main body of the gearbox, on the driver's side. It's the highest of the holes, and pretty difficult to stick fluid in conventionally with the transmission in the car. The hole isn't recessed, so you can stick any style of plug head here instead of the original square head. The female threads are not tapered, but the original plug's threading is. The tapering is the way the plug normally stops and seals: the square head of the plug doesn't stop against the exterior of the gearbox like a hex/socket/any flanged head and a conventional 1/2" NPT square-headed plug will simply continue screwing in until it falls into the gearbox. If you use a replacement plug with an internal hex/internal square/external square head, make sure the threading is tapered.



-Inspection Hole: (1/2" NPT, plug: tapered threading, 14mm/9/16" square head - 16mm/5/8" 12pt socket will fit, 24.4-39.3 Nm/18-29 lb ft)
Also on the side of the main body of the gearbox, on the driver's side. It's slightly lower than the fill hole, and used as a gauge for how far to fill the transmission. Once fluid reaches the bottom of this hole, the gearbox is full. The hole is exactly like the fill hole: not recessed and plenty of space for a different style of plug. Likewise, be mindful of the tapered threading on the plug.




(Note that the top left plug, which came from the car, has tapered threads compared to the normal plugs. The fill and inspection plugs are identical and interchangeable)

-Side Drain Hole: (3/8" NPT, plug: tapered threading, 12mm square head - 14mm 12pt socket will fit, magnetic, 31.2-46.1 Nm/23-34 lb ft)
Again, on the side of the main body of the gearbox, on the driver's side. It's at the very bottom, on an angled protrusion near the transmission mounts. This is one of your drainage options, but you should pull both plugs when draining the fluid anyways. The angle the hole is at limits the space around it, but has enough so you can stick any style of plug head here instead of the original square head. The threads are not tapered, but the original plug's threading is. The tapering is the way the plug normally seals: the square head of the plug doesn't stop against the exterior of the gearbox like a hex/socket/any flanged head. A conventional 3/8" NPT plug will simply screw in until it falls into the gearbox. If you ever round this plug, be prepared to go straight to the dealership: a 3/8" NPT tapered thread 12mm square-head magnetic drain plug is not exactly an industry standard.



-Bottom Drain Hole: (M18x1.50, plug: 24mm hex head, magnetic, 39.3-58.3 Nm/29-43 lb ft)
Of all of the plugs in the transmission, this one is the most conventional. It uses a fairly common metric threading (for a transmission fill/drain plug anyhow) and is in a normal location, towards the front of the transmission. The plug does not have tapered threading and relies on an aluminum crush washer for sealing. A crescent guard hugs the hole (presumably to prevent the transmission from being rested on the plug), but leaves enough space for an impact socket to fit around the plug. Again, when draining the transmission, make sure to remove both drain plugs for good measure. Remember to clean the magnet of debris when the plug comes out.

 

Beautiful thing here!
Trans Fluid change is on my soon-to-do list.
Thank you.

 

Good writeup. For reference, here is the service manual fill procedure:

 

the turbo trans has this setup, because 5th gear is an add on, and 1-4 live in the case under the bolt on cover, and 5th lives in the housing behind it. if you saw an Rx2/3/4/5/repu version, which are 4 speeds, they don't have that extension, so they fill normally

 

I question your reference to NPT (National Pipe Thread). Metric pipe is BPT (British Pipe Thread). It comes in both straight and tapered variety just like American pipe threads. We just call them straight pipe thread. From what I remember the corresponding NPT, like 1/4", is smaller in diameter than BPT and also has a slightly different thread form.

 

Makes sense. I always wondered why the main body of the transmission was segmented like that.

Valid point. I'm not very well versed with pipe threading (as evidenced by the hour and a half I spent at Autozone and O'Reillys trying to find a metric size that would fit this plug), but the 1/2" NPT plugs I got from Autozone fits it perfectly.



The threading also matches perfectly



The head sizes are slightly different, but that's not a game changer. So it might be that the holes are BPT, but the NPT plugs work well enough. Might need to use teflon or some other sealant for good measure.

Also, I need to amend some of my earlier statements in the write up: while fiddling with the fill/inspection holes to verify TonyD89's statements, I found that the female threading is in fact tapered. The threading itself does not taper into a smaller diameter, but the threads become more shallow cut. So, feel free to use whatever plug fits.

Also, thanks to arghx for the FSM diagram. Although it seems as though my fill and inspection holes' locations are flipped compared to where the diagram says they should be. Perhaps it's just a minor difference between USDM and JDM transmissions.

 

If it works it works Teflon tape is a good idea. It can be hit or miss sometimes with the metric pipe thread stuff when it comes to the Japanese. Sometimes they did use NPT.

On a side note, my ex-wife had, way back when we first met, an old Nissan Pulsar. The thing leaked transmission oil (manual) and when I hunted it down it was coming from the fill plug. Someone had tightened the tapered pipe plug too much and cracked the case right at the plug. After all, it's not that thick of a die-casting.

Something to think about.