I have the Ronstan Orbit auto ratchets on the spin sheets and they work pretty good, came with the boat. The grip is great. They are switchable between full ratchet, auto, and no ratchet, which seems great until you find they will switch themselves from one to the other, mostly from auto to full ratchet. So on jibes when you think the ratchet will disengage it doesn't, which really slows down the kite coming across, especially because they have so much grip.

On all the other boats I sail I use Harken autoratchets and they are dependable and work great. Just set the adjustment all the way out and you're at the perfect setting.

If i was replacing the spin blocks on my Tiger I'd go Harken.