MODELS 640 AND 641 FORK SHACKLE BOLTS
It has been found that when the set screws are tightened on the front fork shackles, the shackle tightens onto the threads of the fork bolt before it draws up on the diameter of the fork bolt itself. This has caused a number of fork bolts to break.
When a fork is being repaired or a fork bolt is being replaced, we would suggest that the threads in the shackles be relieved so that when the shackle is tightened up, it will tighten onto the bolt before it draws tight onto the bolt thread.
To do this, the shackles should be all removed from the machine and drawn together slightly with the set screw. A tap should be run thru the threads until they have become sufficiently relieved.
To determine the proper amount of clearance at the threads, saw off an old broken bolt and place it into the part of the shackle which does not have threads and tighten the shackle adjusting screw until the shackle is tight on the bolt. If the threaded end of another bolt can be screwed into this shackle, loosely with the fingers, the threads in the shackle then are sufficiently relieved so that when the shackle is tightened up in assembly, it will clamp fast to the bolt diameter and not hold onto the threaded part of the bolt.
If the shackle is allowed to hold fast to the threaded end of the bolt and be loose on the diameter of the bolt, when there is a strain placed on the fork it carries the whole load on the small threaded end of the bolt which oftentimes causes them to break.
A 7/16 x 20 tap should be used for tapping out the shackle thread and as the shackles are hardened, care should be taken in tapping, going over the threads several times, if necessary.
INDIAN MOTORCYCLE COMPANY