First you need to use the right tool to not destroy them. If after they are powder then you need to go looking for core or slugs for coils of the same value which is not that easy to do. I took some cores of the right thread off a Sony VCR board but they must have been much higher reluctance as the coil only went in about 1/4 the way and then has to be lacquered in place. It did work but it is always better to have the right slug. I try not to damage them. You might find coils that are adjustable but breaking the slugs just makes a big headache for yourself. The record EQ coils do not do much anyway and the Bias traps are usually pretty close. I use the ceramic adjustment tools I got off E bay- they fit more perfectly.