Respooling 120 film on to a 620 Spool
Although the 620 film is the same as 120 film, the spools are different. The 620 spools are slightly shorter and have a smaller diameter. Do not use 120 film in 620 cameras because it will jam and may snap. It is possible to re-spool some 120 film onto 620 spools in a darkroom or changing bag. With a little preparation, it will be easier than you think. The secret is to keep each spool as tight as possible whilst transferring.
The process is quite simple. Spool some 120 film on to another interim 620 spool, then spool it back onto the final 620 spool. To keep the film tight, it is important to anchor the backing paper tabs onto the spools while the transer is taking place. This is quite difficult to do in the changing bag so a little preparation will be helpful.
In the light, unwrap a small amount of backing paper from the 120 roll. Put a clip on the roll to keep it tight. Thread the tab of the backing paper onto a 620 interim spool going from the wider slot towards the narrow slot. Make a few turns of that spool to secure the backing paper. Put a clip on that spool to keep it tight.
Get a thin strip of paper and put a small amount of tape on the end which will be used to stick to the tab. Thread this through the 620 spool so that when you pull the tab though the spool it will go from the wide slit to the narrow slit. This will make it much easier to pull the backing tab through the spool slits. You might like to put a small bit of tape to attach the paper to the spool so it doesn't fall out too early.
The following procedure must be carried out in complete darkness.
Put the items in the changing bag. Take off the clips from the spools and begin to wind the backing and film onto the interim 620 spool. Try to keep both spools tight and as close as possible. When this is complete, put a clip on the interim spool to stop it unwinding. Pull out a small amount of backing and use the paper and tape to pull the tab though the 620 spool slot. Start winding onto the 620 spool keeping the backing as tight as possible.
After a few turns, the free end of the film will present itself. You will have to look out for this as it often simply curls itself around the interim spool. When you find the end, tuck it carefully into the 620 spool keeping everything as tight as possible. Continue to respool the film onto the 620 spool. Only touch the backing paper and not the film. Eventually you will reach the place where the film is attached to the backing paper. Check to see that the film will lay flat. If the film will not lay flat, you will need to untape the film and reattach it to lay flat. If you have kept everything tight, you will not need to do this.
Continue to respool until you get to the end. Put a clip on the film to stop it unravelling. Take the film out the bag and put some tape on it to keep the film tight.
Job Done - Actually not quite as easy in the dark. If you have some old 120 film, you may want to have a practice in the light first.
Using the film
It is important to realise that the film you have prepared will not be wound as tightly a commercially produced film. Store, load and unload your film in much reduced light to avoid light leaks.
Good Luck