BP Blaster or Kroil (if you can find it) are excellent products and similar products are available at any auto parts store.
WD40 is not as good as penetrating oil.
Never tried it but it might work great in this application is to hold a candle against the hot threads or pivots of the heat riser. Supposed to wick into the voids and quickly penetrate and loosen frozen parts. Always wanted to try it, but I grab
a spray can of PB Blaster before thinking of it.
I definitely would do all I could without removing the exhaust manifold.
Last edited by Joe Johnston : 07-15-2017 at 12:39 PM.
Reason: spelling correction