Is there a trick to use SET KEY command when the index expression contains BINTOC() ?

I've created an index based on 2 integer fields:
BINTOC(INT1) + BINTOC(INT2)

I want to use the SET KEY to see only some fields that match the 1st value

I'm trying this way with no success:

SET EXACT ON
SET ORDER TO indicenovo
SET KEY TO BINTOC(lnValue)

where lnValue is an Integer

But if I create an index using PADL instead of BINTOC it works perfectly:
PADL(int1,6,"0") + PADL(int2,6,"0")

What am I doing wrong ? I'd prefer to use BINTOC() because this way I can reduce the index file size...

TIA

Cesar