Originally Posted by norcalsvx

some info

Differential Breathers:

What's a differential breather and what do they do? On most vehicles with axles and differentials, there is a need to vent the axle housing to the atmosphere to allow for the air inside to expand and contract with temperature. If this were not done, the pressure in the axle would build up as it heated up, possibly causing gear oil to be forced past the oil seals. Similarly, when the axle cools, the pressure inside drops and if the axle were in water, this could suck water in past the seals, contaminating the oil.

So correct this potential problem, the factory usually installs a breather to allow the pressures to equalize. The Toyota breather consists of a nipple that threads into the top of the differential housing and a small cap that loosely covers the vent hole. The cap is designed so that as the vehicle moves, the cap jiggles and allows pressure to vent to the atmosphere. When the vehicle stops, it usually seals up to prevent contaminants getting in. One problem can come up with this design that if you suddenly dunk a hot axle into cold water and stop, a fairly strong vacuum is created inside as the air and gear oil cool, and this can cause water to be pulled in. A very common fix for this problem is to replace the stock breather with a fitting to which a hose is attached and run the hose up to a higher point on the vehicle, like the firewall or air cleaner. This allows the axle to breathe freely in or out to prevent pressure differences from occurring.

So, one might be tempted to simply pull off the cap and clamp a hose onto the end. While this will work (for a while) the exposed fitting is very short and irregular and keeping a hose attached is difficult. So, various other types of fittings have been used to replace the stock fitting. One commonly used fitting is a vacuum hose nipple similar to what is used on the engine. Another popular fitting is the Nissan differential breather part # 38323-C6010; cost around $4.50/ea.

However, the threaded hole in the Toyota axle is nothing more than 1/8" NPT:

NOTE:
Officially I would think the thread is either M10-1.0 or 1/8" BSP but its close enough for gov't work
NPT = National Pipe thread Tapered, and it is 0.407" (10.2mm) OD with 27 TPI (threads per inch)
M10-1.0 is 10mm OD and 1.0 mm thread pitch (or 25.4 TPI)
BSP = British Standard Pipe thread, and it is 0.407" (10.2mm) OD with 28 TPI (threads per inch)
So you can see the three are VERY close and for the few threads in the housing and the fact that pipe threads are tapered (meaning the male thread starts out small and gets larger in diameter as it threads in, female thread is the opposite) an 1/8NPT thread fits fine for this low pressure, low stress application, YMMV.
For an even better fit, run a 1/8" pipe thread tap down into the housing, apply grease to the tap to hold the few metal shavings it'll cut.