Another slick trick to free up rust-seized parts such as a heat riser is paraffin, candle wax, or beeswax. I keep a few blocks of fruit jar sealing wax from the canning supply aisle at Walmart around my gunsmithing/reloading shop and some in the engine shop also. Warm up the rusty part with a torch until it's too hot to touch, and apply the paraffin block to the threads or tight fitting bits and pieces. It sticks around and penetrates better than oil does.
Jerry
