[Radio Wrangler]

Ferrite is rather brittle. The core in that article is not gapped, which some manufacturers code as oL in their markings (EG Siemens/Epcos)

Shimming the centre post does indeed gap the centre, but it also gaps the outer as the stuff is so stiff. So you get two gaps in the magnetic circuit. With the gap, the AL value of the core will be much lower than the ungapped use would give, but also the saturation current is improved.

They make ready-gapped cores where just the centre post is ground (below the level of the outer) and this makes the core self-shielding. Helpfully, they printed the AL values on them too. Watch out though, some cores were ground equally on both halves, others got the whole gap on one half only. Usually these had a nut for an adjuster glued in on the non-ground half. You could bodge an ungapped core from two bottom halves!

David