You need a Chainwheel top hat/reducer/shim thingymajig!!!
Here they are
http://www.customriders.com/prodshow.asp?prodid=2192&cat=10&parent=1001&sid=3But you need the correct size. External has to fit your chainwheel and the internal has to fit your spindle end where the crank arms fit!!!
There might be more complications even if you get the right one.
If you chainwheel is thinner than the shim. The shim will stick out and you will not be able to tighten the chainwheel fully.
In that case either find a washer with a groove around the centre so that the top hat fits into it and allows it to be tightened up or file away at the top hat so that it isn't sitting proud of the chainwheel.
If the top hat/shim/reducer is made of aluminium then filing away is no problem. If its made of harder stuff then it could be difficult.
Someone on here will know a lot more about them.
I have 2 Haro cranks and Bof managed to sort one out and the other I had to sort out for myself!!
Alans used to sell them a couple of months ago but they are not listed now!!!
I hope this makes sense but I doubt it!!!