You find the tdi belts flap? Even with all the extra rollers??? I have zero flapping issues with the tdi's. Its the IDI's that have flapping belts, I have added rollers to those. I was under the impression that one of the reasons IP's with larger head assmblys was due to extra reciprocating mass... Shed some light Bruce Anyways I have been using these for a while and I have been using hte stock IDI pulleys on 12 mm head pumps with no issues...