Looking to build some confidence after having snapped off a bolt twice in the past year, causing a major headache.

I just got a good deal on a nice shiny outer chainring for my 1950's Monark Blå Blixten, will look nice with my new pedals - the old one is still working, but the tooth are starting to look nonexistent - and the shine is long gone. The inner chainring tooth are in better condition (the outer one is all I could find anyway) so I'll be replacing just the big ring.

So what I'm worried about is snapping the 70-year-old bolts, I really don't want to end up there. Not again

It's a Simplex, this image is from VeloBase - the model is not a 100% match but the bolts are the same type: regular hexagonal up front and a strange looking round-shaped one (with a groove in the middle) in the back.






Anyone been down this same road - any pointers?

I haven't attempted to open yet because I know my day'll be ruined if I snap something in there, so figured it's better to plan ahead/sleep on it.

I have a decent 9mm wrench close by with a shaft of about 12mm / 5 inches to give me a little torque and I'm thinking I'd get some penetrating oil from the local hardware store, let it sit a few hours and then just hope for the best. I do not know if I should apply some heat as well, never done that.

Can I just apply torque from the outward side and nevermind that strange looking nut (?) on the inside, or do I need to grab on to that groove on the inside bolt with something as well?