Reamers without coolant holes worked better than those with holes.
I used full carbide valve-guide reamers with internal coolant holes on an NTC machine for machining cylinder heads in the two-cylinder engines that go into my company's one-ton vehicles. The 6.982-mm-diameter reamers kept breaking as they worked down the 45-mm-long cast iron valve guides because of a wall-thickness issue.
I realized that using a reamer with a left-hand helix (right-hand cut) would provide automatic chip evacuation, and I decided, instead, to try external coolant and the full carbide reamer that didn't have coolantthrough holes, but it had grooves on its O.D. that would transport coolant to the cut. Not only did tool breakage stop, but the reamers without coolant holes were less expensive.