From memory as I have not fitted a disc one for probably 20 years, they do not need anything to seal
Make sure the surface area is clean, put the disc in and then tap the centre of the disc, this will slightly expand the disc giving a water tight seal
Remember when these were fitted in the 50's there was no silicone sealer's around. The core plugs rusted through quickly as antifreeze was only put in during the winter. The core plug was a safety feature, it popped out if your engine froze (no antifreeze)and prevented a cracked block
Peter
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