I have always been told that we fit two clacks in the oil delivery from a mechanical lubricator as "extra security" against water getting into the oil reservoir. But, if this is the case, why don't we fit two clacks in boiler feed pipes? Could it be that the real reason we fit two clacks in the oil supply is that the clack at the pump outlet is to enable the pump to work and the clack at the other end of the delivery pipe is to prevent the pipe from emptying whilst the locomotive is stationary?? (This would mean the cylinders would be starved of oil when the engine first starts until sufficient running has been done to fill the pipe). Barrie Purslow Warrington U.K. MODEL ENGINEERING DISCUSSION LIST. To UNSUBSCRIBE from this list, send a blank email to, modeleng-request@xxxxxxxxxxxxx with the word "unsubscribe" in the subject line.