Can I use my wrench/spanner which has a box end / 12-point spline, and connect a breaker bar using a 12-point spline bit to it to get extra leverage.
Or is it more common to use extending bar/pipe over the wrench?
Is the first way possible, or would the line of torque be off-centre? Or would the spline bit / socket connection make this not practical?
The open end on a wrench is created to take only so much force before the jaws start to spread. If you need a Swede bar on the wrench, dispense with using the wrench.
Using a Swede bar on open end wrenches guarantees enough spread that they start rounding bolts after they've been permanently deformed. Wrenches are sized (jaw thickness and length) so the maximum torque using normal force will be about the range the bolt should be tightened to if you were to consult a bolt torque chart.
For similar reasons, you probably shouldn't be using Swede bars on box end wrenches.
For difficult to loosen nuts and bolts, you need a socket on the breaker bar and the socket directly on the bolt, preferably a 6 point socket so you don't round the hex.
Breaker bars are used because they're longer (more torque) and have no ratchet mechanism to break. You use them with a socket because the socket fully surrounds the nut and won't splay under the force (though I've split both 6 and 12 point sockets).
The 6 point socket usually is physically stronger than a 12 point and has larger engagement area (less likely to round the hex), though the newest technology is spline sockets which are designed to engage both nuts and bolts 1/3 the length of the flat back from the point and potentially are less likely to round off hex fasteners.
If you must use an extender (Swede bar), use it on the breaker bar as it is the most likely device you have that will take the excess torque. If the breaker bar breaks (I've done that as well) you need to graduate up a size on the drive (3/8" to 1/2") and try again. 3/8" drive stuff is for 5/8" and below, use 1/2" drive for 3/4" and above.
Consider using penetrating oil and mild heat if these fasteners are large and refusing to move. Also, how much rust buildup is there?
If I understand correctly what you are describing then the spline bit in the end of the wrench would be connected to a standard ratchet or another wrench of some sort.
Torque by definition is a measure of Force x Distance considering a single Torque arm. The longer the distance the less force is needed to cause rotation over a fulcrum. When you place a pipe over the wrench or jimmy a bar in the wrench to give yourself more length, you are actually creating a second torque arm where the start of the second arm bears the full force against the end of the first arm. They are not contiquous so this force could cause a break at the weakest contact point of either arm.
In what you propose, the transfer of force at the weakest point between two arms will likely be the horizontal bit extending off the wrench, or in the case of using a gear action ratchet, the gear teeth or ball bearings.
With that being said, most geared ratchets probably have a maximum safe torque limit advertised somewhere on the package. A typical 1/2" ratchet would probably be in danger at just over 100ftlbs of torque. That isn't much and I am certain the teeth on a spline bit can take considerably more than this. I think it would be worth a try as long as you do not attempt this with a ratchet and you cannot find a pipe.
