rs aprilia 125 top end?

If you mean the small end bearing, you'll hear intermittent or continuous tapping noises or if you've got piston and/or bore wear it'll make a high pitched rapid tapping/rattling noise (piston slap) when running under little load or just at the point of giving it some beans, a rebore to next oversize plus oversize piston is the only option here. If its the small end you may have had lubrication failure which you'll need to find out why.

Ted
April 2011