400 BB build issues

actually i did clean the radiator using the distilled vinegar method and hot water, got some crap out but not alot it was pretty clean, I was in the process after the breakin to flush the block and heads to git rid of any remaining crap left behind, so motor, trans and front k-frame are all out, not fun by yourself btw

Well, using vinegar will only do so much...I meant taking it to a radiator shop where they physically melt of the braze on the side tanks, physically brush out the cores after a caustic bath, and rebraze the tanks on and pressure test it.

Once upon a time, I installed a new top end (heads, cam, rockers, intake) on the 306 in my Mustang...it didn't run hot, but a little warm on the 3 core radiator I installed after first putting that engine in several years before. I did the swap, and during break it, it puked a little more than I liked, and constantly ran hotter than I liked...probably about 205-215 on a hot day. I had a good cooling fan and I thought it was just a little dirty cuz the coolant in there was a little off color. I did a flush with the flush kit you buy at the store, it puked out a bucket full of slime and I thought I was good. Car still ran hot. I took the rad out and had a shop clean it. When I picked it up, the guy behind the counter told me it had a 60% core blockage...everything below the top 6-8" of the cap was filled with calcium.

New 50/50 mix of prestone and distilled water dropped my temps to 185-195 on the same Texas summer days with the same fan. So, if yours is still puking rusty water, you might have more crud in there than you know.

You're positive there's no oil/water mixing anywhere?