I've been having some shrouds made for a high tank 1970's boat, 2825, the existing ones are 3.0mm diameter and the ones made for me are only 2.5mm, which I'm not confident about.  have things in the world of SS improved so 2.5mm now does what 3.0mm used to, or is the trend towards thinner shrouds of a new make up, different number of cores vs outer strands or something. The construction looks different to the main halyard, for instance. A definitive spec for the wire would be ideal, but opinions also interesting.
Thanks.

2.5mm is fine, and is what all the new boats use BUT their shrouds don't have to go through the deck. If your shrouds have to pass throught he deck i'd get them replaced.

Also if they are 2.5mm chances are they are dyform which i think is 7 strand wire. It does not like turning corners and again is unsuitable if your shrouds go through the deck.

You want 3mm 1x19 wire.

Thanks, that makes sense - the chandlers were sure 2.5 mm would be OK, but were possibly less familiar with older boats - the shrouds pass through a SS lined bush and turn through about 30 degrees. Perhaps a 30mm sheave would be better. Currently the last 450mm of the shroud is a separate replaceable section. I'll try and count the strands at the cut end to be sure, but thats been a big help.

If the detachable bit is either 1x19 or better still the more flexible 7x19(??) you'll be fine with 2.5mm dyform as the main bit.

Sheaves are unecessary on a shroud levered boat, and at £20 each at least not cheap!!

Well, I've discected the old shrouds and they are indeed 3.0 mm, made from 19 single strands (1 x 19?) and the main and jib halyards are the 7 strands made up from too many small filaments to count, but probably 19,(presumably 7x19) and as you say the flexibility is dramatically different.  

I think the new detatchable 2.5mm sections might also be 1 x 19, but armed with a more informed perspective I'll ask the chandlers what they made them from and if 3.0 7 x 19 is an option.

The old shrouds had a few strands broken at the crimp ferrules and I suspect that more may have been ready to give up at a critical moment.
The main halyard was down to only 2 strands of 19 filaments.

Thanks again,

One thing that you can do straight away is get rid of the wire for the main halyard - almost everybody know uses 4mm string either in vectron or dynema (less good).  Much easier on the hands, you can see the wear and change the halyard over end to end each year.

Hi Richard, I found some Dyneema cored 4004 at a good price to use as the main halyard - will probably leave the Jib halyard as the 7 - 19 wire as it is still in good condition and will tension OK.
Good luck for Sunday

I wouldn't worry about fitting sheaves instead of the through deck bushes. All the boats of that vintage that I have seen, including mine, have bushes. 
Well done with the refit. I have just refurbished 2789 and hope to go to Brightlingsea from Portsmouth at the end of May.
Rgds Peter

Sorry Kieron, the penny should have dropped that it was you!  Leave the jib halyard as wire and agree with comment not to bother with sheaves.  I have these on 3489 and still had to replace lower shrouds this year.