"Core plugs" are also known as freeze plugs and are designed to pop out when the coolant freezes, preventing damage to the engine block (see the Calix heater technical article for more details).
Gruß Bertil
Skoda 5E5 CZDA + Mini R50 W10 + VW ID.3 + Fiat Ducato 250 + 161 DX
*** Technische Anfragen per PN werden von mir nicht beantwortet! ***