Originally Posted by
ftwelder
I personally would look over all the related parts very carefully. It sound like you have an damaged or excessively worn component.
Are the arms pretty tight with no cotters?
If you put a long bolt through the cotter hole is it tight in the bore and square to the arm?
This ^^^^. Are you absolutely certain, given the beater nature
of your project, that these are the original cranks ? Often cranks
get swapped out by guys who are not up to speed on differing
diameters and interchangeability.
Otherwise, since you are using new cotters and seem to understand
the process well, I got nothin'.