Silly question #26...

I'm going to remove the heater core valve and replace. Do I need to drain all the coolant out of the system before I do this or I'll create a big mess in the passenger floor? I don't mind a bit of spillage because the interior is pretty much gutted anyways. Just didn't want ALL the coolant to come out through the heater core tubes.

Your spillage will be minimal as where they enter the firewall is your highest point. It would be wise to maybe drain a little tho.

I would break the heater hoses at the firewall so you dont have any pressure when you pull the stuff out of the interior.

While I did have a little spillage. I unhooked the heater hoses on the engine side of the firewall and bent an thin aluminum cooking pan to catch the coolant and postion it to drain into an oil pan on the floor. With the heater valve open, I hooked a pump/siphon to the outlet pipe on the engine side and pumped more coolant than I thought I would be able to get into the same aluminum pan spillway. Then I put two small rubber cone shaped plugs in the heater pipes and pulled the entire assemble including the heater core from the heater box at once.