Try several shots of PB Blaster as opposed to Liquid Wrench, letting
them have some time to soak in.

Supposedly it has much smaller molecules that penetrate better.  Was
recommended on the list a couple of years ago by a number of people and
helped me get some stuff off that other products hadn't loosened.  I use
nothing else now.   Nothing is perfect on exhaust parts though.

And there have been many warnings to be very careful and judicious in
use of anti-seize compound on the O2 sensor installation.

--
Sam Walters
Baltimore, MD

89 Syncro GL
85 Westy Weekender
84 Vanagon, original owner, soon to be retired, just too many problems

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