UKH

I'm going to replace an in-situ belay at my local crag. It's 1 bolt and a bunch of pegs all equalised on cord. I'd use some 10mm static but the cord has to be thin enough that all the strands fit through a mallion and ring. 

I was going to use 6mm but worried with a knot in it and then left exposed to the elements will be too weak? 

Thin cord is going to be knackered in no time. Probably better to use thick static which isn't properly equalised but will retain strength for much longer imho. Hard to say without seeing the setup though.

If the local ethics allow bolting, why not do the job properly and install a proper belay as opposed to a mess of bolts, pegs and tat?
That isn't aimed at you, but rather a criticism of the sometimes confused world of ethics regarding in situ gear in the UK.  

The thickest rope/cord that will fit? Use a larger mallion? By adding a few more knots than usual (and fiddling with the equalisation) you can get 2 pieces of situ gear per strand of rope through the mallion and still be equalised and safe. Or use a steel screwgate glued shut instead of a mallion?

When I see 6mm cord in situ for a belay (all sorts of stuff is common in the Dolomites) that is grotty and there for an unknown length of time I absolutely want nothing to do with it.

Whatever you do, adding a tag with a date is a nice idea for other climbers in the future if you can.

In reply to FactorXXX:

It would be a shame if a simple technical question got completely derailed by a threadnaught on the ethics of situ gear, can we leave that for another day?

I totally agree with the others that if I arrived at a belay made of cord of unknown provenance, I'd very likely take a knife to it and replace it with my own version unless it was very obviously new and in good condition. Life is short enough as it is without playing Russian roulette and trusting it to thin cord that you know nothing about and that has been exposed to the elements for who knows how long.

Static rope of a decent diameter would be an improvement if at all possible. I love the idea of adding a dated tag to it too.

Like others have said, static 11mm if you can, however if it needs to be thin i’d recommend dyneema or spectra cord. It’s expensive, but if you can get hold of 5.5mm that is much stronger (12-15kn off the top of my head) than normal 6mm cord.  Best knotted with a triple fisherman’s rather than a double though as it’s quite slick and could slip through a double if not properly adjusted

cheers