Shimano Hollowtech left crank arm getting loose After 2 years of good use, left crank arm on my FC M542 crank set has started to get loose quite often (cca 500 km). It is usually indicated first by few now an then click-clanks somewhere in the region around BB. When checked, the bolts are loose, and the safety plate loses it's "nose"

As far as I can see there are 2 visible problems:


The spline on the crank arm shows some wear, but they do not seem completely damaged.
One of the fixing bolts is not original Shimano one M6x19, but M6x20 from hardware store with filed head so that it can be inserted in crank arm. It is also missing washer.


I intend to buy original bolt with washer. Other than that, is there something I can do to delay changing the whole crank set? 

Maybe using some threadlocker like loctite 242? Where would I apply it on the bolts or on the spline? If it is applied to the bolts, does that mean that I have to use more force while tightening them, or just use force as usual?

Anything else?
Thread lock will help hold the bolts on. I would apply it to the bolt thread, and not the actual crank arm thread. You will need more force to remove them when you do come to remove the crankset. But they shouldn't need any more strength to put them on. Just tighten them to the recommended torque setting (it should have it written on the crank arm).

Fundamentally though, it sounds like you have a few problems which only a new crankset and BB would solve.

I got a Tiagra Hollowtech chainset on ebay for around £30-£40. It might cause more harm than good trying to threadlock it all together, and give you more problems when trying to get it off and replace it later down the line.