FOR delicate and proven rust removal on metals, USE White distilled Vinegar. You don't have to worry how long your parts sit in it. You don't have to, but I put a lid on the container and I did this once to hide the smell and I found it worked better.... even if your parts are not fully submerged!!!! The humidity of that stuff will rise and attack the rust in a sealed container. Throw that flywheel in it. anything can go in it!!!!! I use it and it works...
You may get black fingers from the( dead rust)while washing the part under plain water but that will come off too with soap. I use soapy water to rinse the parts off anyway.......... Feel free to YouTube search it....... Just last night I left this antique 6" ruler made by GENERAL in the USA, that was nothing less that red rust covered, and left over night submerged in the vinegar. You can use any brand btw.
DO NOT use Murotic Acid unless well experienced at it as it CAN damage your parts if you are not careful and if left too long, your parts become crisp and brittle with a tensile strength that will shock you. Just drop a spring in the stuff over night......The next day you will be able to break it into pieces with a hammer..... Muratic Acid dips last no longer than an hour for my own likes and I do get that brand new look when the part is removed! You need to really clean off the part and quickly blow dry it fro best results with an air compressor and try to avoid 'flash rust films'...then drop it into oil OR>>.. wrapping clean metal or rolling metal parts in news paper will be preserved for a long time.
The paper will absorb the humidity and the paper will dry out just as fast leaving the metal dry and protected. When I used to restore old cars, I'd lay news paper over my bare metal that I left to sit for some time. When I returned and removed the paper, it was good as new!! NEWSPAPER is the best to use... Remember, I have never had Muratic Acid damage my metal in any way so far. Try a half hour, then an hour but really never more than 3 or 4 hours max ........... I think a half hour dipped in Muratic acid would have proven better for this old ruler.........