Bicycle Mechanics - crank bolt problem on bmx cranks, flattend threads!

hello i have a problem on my bmx profile cranks i recently boght it from somone and the only thing rong with it was it needed some new bolts because the threads had been flattend on the previouse once. So i found some crank bolts on ebay that came from amercia and said fits profile dk and oddsey cranks so obviously i boght them £15. now i have the same problem! i try and twist the bolts in but the threads are starting to flatten again! could somone please tell me why this is happerning i dont know weather to screw this bolts in mega tight nd just leave them there could someone tell me if this would be ok? cheers i have a pic below of the perviouse and the new crank bolts i boght... cheers.:twitchy:here is the old crank bolts...http://i105.photobucket.com/albums/m225/super-cagking-colby/IMG_0811.jpg

the new one...
http://i105.photobucket.com/albums/m225/super-cagking-colby/IMG_0810.jpg

sorry for bad picture quality.. thanks.

I have a hunch that there is a chewed up section of threads in the spindle, which shears the tops off the bolt threads as you screw in. If you can find a thread chaser the correct size, run it into the spindle and it should clean up the gnarled section. If you can't find a thread chaser, a hardened steel bolt with a groove cut lengthwise along the thread using a file or hacksaw could work as a chaser. Good luck.

Whatever you are threading those bolts into has damaged threads. It's possible that the tapped (threaded) piece can be cleaned up using the appropriate tap and still have good enough threads to be serviceable, but it's also possible that the piece has been ruined. The damaged threads were likely caused by the bolts being crossthreaded (installed at an angle which caused the threads in both parts to not mesh properly) by the previous owner.

My suggestion would be to take the threaded part to a bike shop (or machine shop if one is available) and have them clean up the threads with a tap. Throw those olds bolts out as they are worthless now and get some new ones. With the threads cleaned up, attempt to install the parts and torque to spec. If everything goes ok, you're good to go. If you it turns out the bolt keeps spinning and won't torque to spec, then you'll need to look into some options for repairing that component.

As a general note, if a fastener won't thread on easily by hand something is wrong. Do NOT attempt to forcefully thread something together using tools as you'll likely damage something. It's a lesson that just about everyone who spends enough time working on mechanical things learns the hard way.

thank you i have looked inside th axle and the threads seem pretty clean to me but i shall take to to a bike shop soon as possible.. thanks

cheers i dont think i have a thread chaser but i could prehaps borrow one from somwear. thanks.