Thinking about replacing my 2.25 main sheet top block with Harken Carbos. Anyone else do this? What size (40mm/57mm)? My main sheet is 5/16 Yale Light.
Chris,
I did that on my 17 - used the smaller block on the boom and the larger Ratchamatic block on the traveller. It's an awesome setup for that boat since you have to let out a bunch of sheet in a tack. The sheet itself is 5/16 Marlow Excel Marstron - similar to Yale Light, but not as durable and way less expensive. I think the line cost all of $18.
Anyway, I'm going to try that setup on the 16 this weekend. I sold the 16's main system (stock line w/ regular Harkens)with the 17. I'll let you know how it works, but I've already got a pretty good idea. 
Matt
Chris,
I'm running a 29mm triple with 5/16" Yale Light on the boom of the F14.
Bob 
Chris,
While you are ordering your new blocks, why don't you order some tramp cleaner, too?! 
I use a set of 57mm Carbo blocks on my H-16. I use the 57mm triple as an upper block and a 57mm triple with ratchet, cam, and becket. I use a 3/8 Samson trophy braid mainsheet and like the blocks. They are much more durable than they look like, they have survived 2 seasons use without a single issue.
Chris,
you know we are always experimenting with new stuff. many teams in Europe have tried those carbon blocks, but are not really convinced because there is no real advantage (other than that the dealer has a better turnover).
The harken blocks have avery similar performance.
But, if you insist: Take the 57mm for the bottom and the 40 mm on the top, and tie the top blocks to the boom which allows more rakes. But then we have to turn over to the rudder discussion...
Fritz