Sazerac wrote: ↑Tue May 11, 2021 5:41 pm
... it could also be a variety of other things like head manufacturing variations,
Large variations in porosity of head castings seems - to me - to be a major contributing factor in the onset and extent of SCS.
Partial paste from TSTMNBN:
I have pulled a few other sets of heads with good decks but the coolant history of those 928s wasn't as well known.
And I've pulled a couple of 'bad' heads that had had "my" Prestone in them for a some years but with unknown coolant change histories for a decade or two before getting Prestone.
About the only thing that I'm pretty sure of is that using Prestone Long Life and never changing it isn't BAD if the 928 just sits.
If there's any common thread - in *my* observations - about good vs. bad head decks it is that the 'bad' head decks came from 928s with a history of being "run up to temperature once a month" rather than just sitting and with no evidence of coolant changes or evidence of maybe one coolant change.
Whereas the good decks had evidence of coolant changes and/or just sitting for years with no cycling.
One other item I've learned of lately: there are vast differences in the porosity of the cast aluminum in the heads. All these 'cheap aluminum' castings(*) are somewhat porous. Some are much more porous that others and porosity is never uniform. I *suspect* that porosity variation near the deck surfaces may have a lot to do with how bad or good some of the heads are after 30-40 years. Why? Crevasse Corrosion.
(*) Talking with the Head Tech at my local Porsche dealer a week or two ago, he reminded me that when Porsche went from cast AluSil crank cases to cheap aluminum with silicon-impregnated bores (e.g. Lokasil) that some of the engine cases were so porous that they leaked engine oil *through* the case, into and out of bolt holes. That gives some idea of how porous cheaply-cast cheap aluminum can be...
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