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Reviving this thread and adding a question
The original 60 series heater core O-rings 96710-01016 have been superseded to 96711-19016. Out of interest, that is the same O-ring that seals the pump cover to the rear housing on a later, 4-bolt type power steering pump (in the US I think these are only found on the FJ62).
View attachment 3221757
Anyway, I have cleaned up the ports on my heater core and the flanges of the pipes which go through the bulkhead in citric acid, and polished them up to a shine. They look nice and smooth but still seem to have some light dimpling or texture from, I assume, corrosion.
View attachment 3221758
I'm not sure whether to expect to the O rings to take up this surface, or should I help them by putting the tiniest smear of silicone RTV on the flanges and ports (not the O ring).
Any ideas which is more likely to seal?