Otto, yes I cut a coolant filter apart when it plugged up completely after a dose of preventative stop leak. Not surprisingly it was full of black granules that I took to be stopleak. But it coule have been just more of the grit I didn't get out of the head.

1951 3800 1-ton
"Earning its keep from the get-go"
In the DITY Gallery
1962 261 (w/cam, Fenton headers, 2 carbs, MSD ign.), SM420 & Brown-Lipe 6231A 3spd aux. trans, stock axles & brakes. Owned since 1971.