Mat asked: I'm interested in fitting 3 link & screw link couplers to my OO gauge models to bring about more realism, plus I'll be using these as the main coupling. I've read up on a few sites and Smiths seems to be the coupler people recommend. What do you think?
Well Matt, my first thought is ARE YOU CRAZY !
DO YOU WANT TO SPEND ALL YOUR TIME POKING AND FIDDLING WITH TINY LINKS WHILE SLOWLY GOING MAD AND BLIND ?!?!?!
But that's not all that helpful. Besides, many excellent modellers have used 3 link couplings and say they work very well. Certainly nothing looks better hanging on a British wagons buffer beam. For all that I love Spratt & Winkle couplings, I have to admit that aren't exactly realistic. The Americans have it much easier with knuckle couplings.
We Brits though are made of sterner stuff and if you want chains then you should have them.
Now I have to preface this by saying that I have never used 3 link couplings in 4mm scale other than as ornaments decorating the end of locomotives. Looks have always been more important than operation in this instance. Therefore you need to take all my suggestions with a pinch of salt.
Smiths couplings are fine. In fact I have stuck them on locos and they look very nice. As far as I know they work too. Make sure however that the chain goes easily into the hook. If it doesn't drop in easily when you'd fiddling on the workbench, you'll really hate them on the layout.
Iain Rice writes about these in one of his books (the detailing RTR one I think) and he recommends Exactoscale or Ambis Engineering hooks slightly opened out combined with EM Gauge Society chain. This stuff, as seen in the photo attached to a resin hook on a Silver Fox loco, is quite fine and slightly larger than scale. That should make it easier to use. It's also steel so you could pick it up with a magnet. My little 3 link use (one someone else's model) experience tells me that bigger chain is easier to use and doesn't look any worse than the dead scale stuff.
When you make your choice, stick to it. At least that way you avoid incompatibility problems. We have all been on layouts where there have been the odd rogue wagon what won't couple with some of the others. Working a train set is hard enough without having to worry about that sort of thing.
Whatever you do, fit up a couple of wagons and practise coupling before doing anything else. Ideally test them on a real layout too with all the access problems this will give you as you try and reach around buildings, signals and station awnings. If you are still building, it's probably worth seeing how you need to adapt the model to avoid too many problems. And while you are at it, getting bucket loads of light on it too so you can see the things. And don't make the baseboard too wide or you'll never reach across it !
As far as I've seen, most people use a small torch with a hook taped to it as a coupling pole. Again, you'll need to experiment a bit to find what works for you.
I hope this helps a little. If you are really keen then perhaps having a chat with people using the couplings at a show (they might even let you have a go) would be wise. Most will be only too happily to help and tell you some of the bad bits as well as the good ones. And if you do go for chain, well at least you can surreptitiously poke recalcitrant locos when "the hand of God" is at work !