A gent in here recently blew his block by not mixing the coolant with the water that was stuck in the block after a flush. I also blew my new rad at it's top by topping it up at the wrong time with water in the real cold, it froze before the engine could start.
You also have to figure the block and heater core can hold about 40% pure water in them after a flush so if you top them up with 50/50 mix, you have almost no freeze protection. That 50/50 mix junk they sell is only good for topping up, it is no good to use after a flush.
I always use the correct amount of pure antifreeze first, then I top it up with water and can only usually put a quart or so water, the rest of the water was hiding inside.
86/00 CJ7 Laredo, 33x9.5 BFG AT's, 'glass nose to tail in '00, 'New' frame,wires and plumbing in '09. Carter BBD Carbed 4.0 HO in '10.
89 YJ Renegade. BBD Carbed 4.0 HO. Locked front and rear with 33x9.5 BFG AT's
Some Canadian Bush Jeep Runs and Build Photos: http://mikeromainjeeptrips.shutterfly.com
(10 new albums added Sept 16/10)