Replacement brake disc - different size?

After much time and effort, for the first time ever, I changed the rear disk and pads yesterday. However, I'm a skint student so rather than pay the garage £90 for a disk brake I brought one off flebay (Judge me later). But now it's fitted, I have a problem. A big problem I think.
I've drawn a picture of it to help describe it.



The disk only goes half down onto the pads as its diameter is much smaller than the old one. The pads are only half being used, the disk is only half being used.
After fitting I rode round the block (500m) to test them, and obviously the brakes were only on for a bit. I've never had new brakes so don't know how they should work, but they seem to work ok. However its obviously not right/not efficient. Can I adjust the hight of the pads to fit onto the disk fully so the whole of the pad is on the disk? Is the problem described above very bad? Should I buy a new disk?
Damn me and my money saving schemes.

hi matey, im a car mechanic and fairly new to biking so i have never fitted brekes to a bike yet. however when brake discs are packed they are layered in a grease to prevent from rusting whilst in stores, did you before you fitted the disc 'clean' off this packing grease, i.e. a thinners based solution or petrol to remove this grease.

[Viney]

I cant access you rpicture at work, but by the sounds of it, you have the wrong disc!

Is the old disc and new disc the same size, if not then above.

You might also have the wrong pads. Either way it seems thats soemthings wrong. Why did you change the disc in the 1st place?

[Viney]

Originally Posted by pawl

hi matey, im a car mechanic and fairly new to biking so i have never fitted brakes to a bike yet. however when brake discs are packed they are layered in a grease to prevent from rusting whilst in stores, did you before you fitted the disc 'clean' off this packing grease, i.e. a thinners based solution or petrol to remove this grease.

Erm, thats an oil, and you being a mechanic should always specify Brake cleaner!

changed old one as it was very much in need of changing, slightly warped, burnt, too thin. Different size yes, but fits wheel correctly. cheers.

pads were brought from suzuki cresent southampton and was told they were correct for bike. Also they are same size etc as old pads.

yep Viney is correct, brake clener is the stuff. if you do not have any brake cleaner though what i have said is suitable.

Mike, did you 'clean' the disc before you fitted it?

[fizzwheel]

Originally Posted by mikebrooman

changed old one as it was very much in need of changing, slightly warped, burnt, too thin. Different size yes, but fits wheel correctly. cheers.

If you measure the new disk and the old disk are the diameters the same ?

[Viney]

Originally Posted by mikebrooman

changed old one as it was very much in need of changing, slightly warped, burnt, too thin. Different size yes, but fits wheel correctly. cheers.

If its a different size then its the worng disc and hence your problem. The discs HAVE to be the same size. Doesnt matter if it fits the wheel.