Bicycle Tutor

Hi,
I'm trying to replace the fork of my bike - the current one doesn't have shock absorber. I took the new fork, which has shock absorber, from another bike I have, but when I had both forks out of the bikes I realized that their tubes (the parts that go into the frame) have different sizes. The tube of the new fork is thinner than the hole of the frame. Is it possible to change these tubes (although I haven't seen any screw to unscrew - they seem welded to the rest of the fork)? If not, does somebody sell something like a "tube adapter"?
Thanks!

PS: As I'm neither a native English speaker nor a cycling expert, I might be using a rather confusing terminology, so I attach a clarifying sketch



   

Some forks do allow you to change the "steerer tube" on the fork although most do not and it may be a little expensive.

Better solution is to get a new headset that will adapt a smaller fork to a larger frame. Here is a link to some on Cambria Bike:

http://www.cambriabike.com/shopdisplayproducts.asp?Search=Yes&Field=&Keyword=headset%20reducer

You may have to search around a little to find the right size combination.

I think there are also adapters around that would let you use the suspension fork and the headset it was mounted to and use both those items on the new frame. I don't have a source to recommend for that though.