I am moving away from my GT2 belt design for Z and swaying towards a leadscrew BUT I cannot find a TR8X2 single start and for Z I demand that for my next build. So, why is it so hard to find single start? I did find one supplier on Aliexpress but I have to purchase 10 of them for 80 dollars. Heh, I only need two and the other 8 would rot as I have no wants to relist them on ebay or some other thing. Seriously, single start for Z is the way to go since you do not need greater than 30-40mm per second of travel and the accuracy is awesome AND, most importantly, the weight of Z has far less of a chance of sliding down when the power is off (yes, my Z has slid down before using the type of leadscrews OB sells).