remove the arm inspect the taper-- look at the arm--- if the aluminum is completely mushroomed-- replace the arm--- check the length and angle--- if it looks kinda Ok-- use a washer and grease the bolt and put it back on--
the washer should sit behind the head of the crank bolt and the lip of the inner arm --- if you are stripping the bolt or the treads on the spindle STOP-- remove and get new....my best shot-- not fact! I assume this is a square taper--- I did this once on an old bike it seemed to work.... that washer forces the arm a bit closer to the stay--- watch carefully
I never learned to ride a bike. It is my deepest shame.
Originally Posted by mtbtrek
i have a 06 trek 4900 and the left crank arm felt loose when i was riding so i tightend it and it is always loose. do i have to buy a new one or can it be fixed?
(the advice below applies to square taper cranks ONLY)
The arm needs replacing. The press fit of the taper requires very tight tolerances and the slightest deformation will prevent it from ever staying tight on the spindle again.
How can you tell if it is damaged? If it loosens up as you ride, it is damaged. There is no other explanation for loosening while riding.
Also, if you have tried to retighten it a time or three there is probably some damage to the spindle as well.. does the spindle look 'polished' at all? It should be still black (a couple tiny silver streaks on the flat face are acceptable, but anything else and you likely need to replace the spindle as well)
Retightening a damaged arm after it loosens will damage a spindle...
Attaching a new arm to a spindle that has had the old arm retightened will damage the new arm... you can go through several arms and spend quite a bit of money to fix this if you don't do it right the first time.
I usually put a drop of red locktite on the taper flats (NOT on the bolt threads) during assembly. Put grease on the bolt threads. Torque to spec or get an experience mechanic to do it. Overtightening is another easy way to damage the taper of the crank, so make sure this is done properly.