Big buggers, aren't they?
For a 17mm nut, the proper torque for that depends on the bolt grade. Ace Hardware grade should see 38 ft. lbs. and Grade 8 is 50 ft. lbs.
Here's a Handy Chart for yah. Just click on the blue highlight.
I would also put some locktite blue on them, or a flat washer and lockwasher against the nut, or toss those plain nuts and get some 17mm nylon insert stopnuts (sometimes called "Nylok nuts) instead. Those shouldn't loosen up.