Hugonl37
It’s the nut on the outside you should have been steadying tight through all of the inside work to stop it moving.

Sorry this is a bit late but that’s how it’s assembled.
The pliers wrench hold the nut perfectly steady as the harder you grip the tighter they hold and always perfectly parallel.
Sorry I’ve been trying to dig around old photos.
The next time I work on mine I will try and do a step by step with recommended tools.
My addictive hobby and addictive day job means it’s not normal for me to recommend “cheap tools” unfortunately 😎
It’s always difficult on here to know what to say as its not possibly to judge peoples natural mechanical flair 😜 it’s therefore sometimes easy for them to make a real pigs ear out of a situation where many of the parts have a cosmetic as well as engineering function.
Of course the machine will still work with scratched paint and chipped chrome but the wife may not always approve.
Luckily i have a bullet to fire in that department as it was my wife who dive bombed my lunar scales with a rather heavy weight. Still 15 : 0 on that one with me to serve!!