BA7 nuts are tiny. The thread is 2.38mm diameter. Admittedly, there are smaller nuts, but I have had so many problems with the BA7, that I do not want to even contemplate the even smaller ones.
If I drop a BA7 nut, I have about a 50% chance of seeing it on the floor. There must be a small fortune in BA7 nuts on the floor of my workshop, or wherever they bounce to.
The steam engine which I am currently building has several hundred of these tiny fasteners, and many of them are in inaccessible cavities, at least relatively inaccessible to my 65 year old fingers.
The more accessible BA7 bolts and studs can have nuts fitted with the assistance of a 4mm jeweller’s tube spanner. I added some usefulness to the tube spanner by turning its outside wall thinner, to decrease the space it occupies, but even so, there are many locations where no tube spanner, however modified, or open ender, or needle nosed pliers will reach, and fingers are required.
So, I had a brain wave yesterday, about a method of starting small nuts on relatively inaccessible studs and it works! This might not be an original idea, but it is to me.
It requires a sharp needle, on a handle, with an appropriate bend near the end of the needle. The sharp end of the needle is exposed. In my previous life I was a surgeon, so I have a supply of medical needles, and they are ideal. A syringe makes a good handle.
The nut is placed on the needle, (carefully).
The needle point is placed in the centre of the end of the stud or bolt, carefully to avoid the nut slipping off prematurely, and the needle is angled so it is in line with the stud. The needle needs to be sharp, so it does not slip off the end of the stud.
The nut slips down onto the stud, and it can be spun with a finger tip until it attaches to the stud. The needle is then (carefully) placed away, and the nut is tightened down by whatever means are possible.
This method requires some dexterity, but it can change an impossible task into a merely difficult one.
Ps. If you use medical needles, make sure that they are new. Some diseases like hepatitis can be transmitted by needle stick injury.