Have 2012 xj sc. My last 2011 xf had the head gasket blow. End of that car,
Preemptively what would it cost to have Jaguar replace all the plastic fittings that allow water to run through them, with aluminum fittings.

 

Call your local dealership (or wherever you get your parts or service) and ask them.
Otherwise, you'll get a wide range of guesses & un-related (to your location or parts/labor) anecdotal general responses as available that don't apply to your vehicle, location and the day you have it done.
The only real answer to your general & vague question (replace the plastic fittings with aluminum? Only in a couple specific possible instances) is that it would cost much less than if the weakest one fails and destroys the engine, which in-turn, often destroys the entire vehicle.
I think I've seen some recent posts where someone asked for references in your area, and someone responded with a shop they've used. If you look for that post & contact that shop, they could give you a realistic answer.

 

Yes we have had a couple of those failure prone plastic parts redone in lifetime Aluminum but as posted above not all of them...yet. But no Jaguar dealer is going to install aftermarket parts and provide a warranty.
So I don't think you will get the Al. replacements except at an independent garage.
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