These are a good, economical replacement for kingpins - much cheaper than the fluoride coated ones... an essential for any serious driver!
I have got a couple of sets of these in, because in a crash, I have lost one of these already as Kingpins are so small.
They seem to offer very similar performance to the fluoride coated king-pins Tamiya do. I'd recommend wiping a very small amount of high-quality grease (like Tamiya CeraGrease HG) around the top to give a really smooth movement around the collar they fit into. Don't put too much on as you will clog it - literally just wipe a tiny amount on. I use a small piece of sponge to apply it.
Also, another tip - care should be taken not to over-tighten kingpins when fitting them. If you over-tighten then you will be putting un-due resistence into the steering and putting stress on your servo. It's a careful balance between screwing in the king-pin to make sure it stays in and going too tight. I'd pop=off the tie-rod between the steering and the servo to check this.