ok so here we go......

flushed the radiator the other day and man OH man was that crud BROWN!! my car is a 66, and only owned by ONE lady before me, she bought it from a lot in '69.

looks like no one flushed for 20 years. I took off the thermostat housing and replaced the old 160F thermostat with a 180F unit since I removed all the AC. when I looked in the water passage it was SO dirty and crusty and rusty, looks like tons of buildup. I do plumbing part-time and it looked like OLD drain pipe, like cast iron with TONS of build-up.

what is this? can this cause problems? what should I do? etc. etc. etc?

caesar

Like cast iron pipes, cast iron engines / heads get the same rust scale build-up if you do not keep antifreeze or rust inhibitor in the coolent.

The scale will get loose and clog the cores of your radiator so you my want to run a screen or coolent filter in the upper radiator hose to catch the crud befor it enters the radiator.

You can do some aggressive engine cooling system cleaning to get this stuff out. The most effective way is to remove the rear freeze plug, thermostat and the water pump and use coat hanger wire to probe around and break-up the scale as you flush the passages with water.
DD