Sram x1, never had one before and it has worked loose. Do I just tighten the 8mm bolt (seems too simple, what stops it falling off?) how do I remove the crankarm? Just loosen the 8mm and ignore the 10mm? It all seems too simple?

Yes, it’s all in the 8mm. Ignore the 10mm unless it’s managed to work itself loose somehow.

I never understood what the 10mm bolt was for when I had sram cranks

also I found they needed LOTS of torque to do up tight enough, but were actually surprisingly easy to remove after that

I never understood what the 10mm bolt was for when I had sram cranks

It’s a self extracting crank bolt.

The 8mm bolt pushes against the 10mm bolt to pull the arm off. Some square taper cranks had a similar setup to work instead of a crank puller. My x1 cranks had a little wobble untill done up tight.

So I just need to get the 8mm bolt up to torque again. Sounds like they weren’t installed correctly by the lbs, or they loosen up slightly after initial use?

Haven’t had mine (XX1 cranks, but I’d imagine they’re the same) loosen up, but they need to be done up pretty damn toight in the first place to remove play.

48-54 nm when fitting. Has to be about the tightest bolt I ever do on a bike

Don’t remeber seeing one come loose and have to use a big bar to undo them often
Some have a collar on the left hand arm to adjust the pre load some have shims and a wavy spring washer