I have a bike that is no longer in production, but it has a standard drivetrain (FSA). I had some shifting issues the past couple days, and to my surprise when I did a clean/inspect, three of the 5 chainring bolts had dropped out and the other two were finger loose.
I salvaged some bolts from my donor bike. I needed a breaker attachment to my 5mm hex to get them off. There was no residual threadlock on the threads. Now I'm unsure how hard I should tighten them onto the first bike.
Why would they have come loose in the first place? How hard should I tighten them to prevent this from happening again? Would a dab of threadlock be good insurance?
A Shimano "General Operations" manual states the tightening torque for road chainwheels is 12-14 N-m. For mountain bike chainwheels it's upped to 14-16 N-m for large and middle ring and 16-17 N-m for the smallest ring of a triple.
The website: http://si.shimano.com/#/
The manual number I'm referring to: DM-GN0001-20-ENG.pdf
Blue loctite or equivalent threadlocker is indicated here as well.
I can't accurately surmise the cause of the loose and lost bolts in this specific case. However my experience with situations of loosening bolts or part failure due to "coming apart" in various mechanical settings often comes down to incorrect or incomplete tightening of the fastener (bolt, nut, etc). Part of this process is rechecking and re-torqueing after a break-in period of normal use.
Park tool has a great reference Here
Chainring bolt- steel
Race Face&erg; 100
Jury is out on thread lock on chain ring bolts. A do not believe a light thread lock will hurt ( until you need to remove them), but also believe correctly tightened bolts should not need it. The problem is the little two prong thing used to hold the nut is unlikely to allow a decent torque.
Better to give the threads a good clean and lightly grease the threads.
I actually joined this site due to the bad advice on this post--the Shimano recommendations (~14 Nm) posted on here are about double what the torque should be for my application--ask me how I know--using such sheared the head clear off one of my bolts ; ( Very uncool...
Better advice is to consult a toolmaker's recommendations...
I recommend Park Tools listing (below)--you should consider that chain ring bolts can be made different ways and of differing materials...take multiple data points on recommendations and/or ask a LBS (local bike shop) mechanic before proceeding.
On my race BMX's I put cable ties through the chain ring bolts once they are correctly tightened as this prevents them falling out if they come loose. You need to still check them regularly to ensure that they have not come loose.
Cable ties are a good insurance policy to stop them dropping out.