I'm guessing it was a different plug to the one you changed before ?
If so it's a sign that someone hasn't replaced the coolant at the correct interval or maybe hasn't used a corrosion inhibitor at all in the past.
With anti-freeze anti-boil you must mix at the suggested ratio otherwise you do not get full corrosion resistance. The same goes for corrosion inhibitor. You also need to change at the correct interval which is 2/3 years for traditional type coolants (IAT) and longer with OAT, HOAT etc. Check the brand you used for more details.
I've seen pictures on AULRO of cylinder heads where the aluminium has corroded in the waterways.
The block being cast iron will rust but should be OK. The core plugs/Welch plugs being thinner will rust out first.
I keep a log of when things like coolant was changed (too many vehicles to remember all of them) and I need to do my Defender & Series I in the very near future.
Colin
'56 Series 1 with homemade welder
'65 Series IIa Dormobile
'70 SIIa GS
'76 SIII 88" (Isuzu C240)
'81 SIII FFR
'95 Defender Tanami
Motorcycles :-
Vincent Rapide, Panther M100, Norton BIG4, Electra & Navigator, Matchless G80C, Suzuki SV650
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