avoid cheap chinese farmjacks. go for Hi-lift or jackall. they're not that much more to buy and as you risk ya life and limbs everytime ya use one. You don't want some cheap scrap that fails on you when you need it most.
I had to dump a disco by getting the jack to collapse because it was stuck in the air on a farmjack that wouldn't come back down. Not something ya wanna be doing at all really.