one of my soft plugs started leaking in my RC.
I pulled it out to change and found the inside of the block is coated with a nasty, bkack gooey slime.

I believe its some kind of stop leak.

Im afraid it will hinder the cooling. What would you guys use to clean it out with?

The last one I had to do that had goop and rust in it. So I just pulled the motor, knocked out all the plugs and washed the water jackets out with a steam cleaner. I had my radiator cleaned out, and the dude said the goop was in it also. The bad part about small blocks, is there are 2 plugs in the back of the motor, behind the flex plate/flywheel. If your luck is like mine....those are the ones that are leaking!

hi
nock out a plug or 2 on eather side you can easily get to and use a coat hanger and a garden hose , replace the plugs and coolant and drive !

plans are,,, we are going put somekind of flush in it and then pull all six of the side plugs.
going to get something like a bottle brush and Super Clean to scrub the inside the best we can and then flush real good. .
new plugs and antifreeze and hopefully all is well.

I would take a close look at the rad,most times that why it gets the bars leak...

are the plugs rusted up and thin?

you on the right track,flushing it with them out is the only way to clean it

I thought I was the only one that had the back ones leak...!@#$%pull a tranny to fix a water leak

good luck