Draining engine oil Partially filled intake pipe piston approaches top center of the compression stroke (both  valves closed), this liquid being incompressible, stops piston  movement. If the crankshaft continues to rotate, something  must give. Therefore, starting or attempting to start an engine  with a hydraulic lock of this nature may cause the affected  cylinder to blow out or, more likely, may result in a bent or  broken connecting rod. Figure 10-41.  Initial step in developing a hydraulic lock.  To eliminate a lock, remove either the front or rear spark plug  of the lower cylinders and pull the propeller through in the  direction of rotation. The piston expels any liquid that may  be present. If the hydraulic lock occurs as a result of over- priming prior to initial engine start, eliminate the lock in the  same manner (i.e., remove one of the spark plugs from the  cylinder and rotate the crankshaft through two turns). Never  attempt to clear the hydraulic lock by pulling the propeller  through in the direction opposite to normal rotation. This  tends to inject the liquid from the cylinder into the intake pipe  with the possibility of a complete or partial lock occurring  on the subsequent start. Proper engine warm-up is important, particularly when the  condition of the engine is unknown. Improperly adjusted idle  mixture, intermittently firing spark plugs, and improperly  adjusted engine valves all have an overlapping effect on  engine stability. Therefore, the warm-up should be made at  Engine Warm-Up the engine speed where maximum engine stability is obtained.  Experience has shown that the optimum warm-up speed is  from 1,000 to 1,600 rpm. The actual speed selected should be  the speed at which engine operation is the smoothest, since  the smoothest operation is an indication that all phases of  engine operation are the most stable. Some engines incorporate temperature-compensated oil  pressure relief valves. This type of relief valve results in high  engine oil pressures immediately after the engine starts, if  oil temperatures are very low. Consequently, start the warm- up of these engines at approximately 1,000 rpm and then  move to the higher, more stable engine speed as soon as oil  temperature reaches a warmer level. During warm-up, watch the instruments associated with  engine operation. This aids in making sure that all phases  of engine operation are normal. For example, engine oil  pressure should be indicated within 30 seconds after the start.  Furthermore, if the oil pressure is not up to or above normal  within 1 minute after the engine starts, the engine should be  shut down. Cylinder head or coolant temperatures should  be observed continually to see that they do not exceed the  maximum allowable limit. A lean mixture should not be used to hasten the warm-up.  Actually, at the warm-up rpm, there is very little difference  in the mixture supplied to the engine, whether the mixture is  in a rich or lean position, since metering in this power range  is governed by throttle position. Carburetor heat can be used as required under conditions  leading to ice formation. For engines equipped with a float-type  carburetor, it is desirable to raise the CAT during warm-up to  prevent ice formation and to ensure smooth operation. The magneto safety check can be performed during warm- up. Its purpose is to ensure that all ignition connections are  secure, and that the ignition system permits operation at the  higher power settings used during later phases of the ground  check. The time required for proper warm-up gives ample  opportunity to perform this simple check, which may disclose  a condition that would make it inadvisable to continue  operation until after corrections have been made. The magneto safety check is conducted with the propeller  in the high rpm (low pitch) position, at approximately 1,000  rpm. Move the ignition switch from “both” to “right” and  return to “both;” from “both” to “left” and return to “both;”  from “both” to “off” momentarily and return to “both.” While switching from “both” to a single magneto position, a  slight but noticeable drop in rpm should occur. This indicates  10-26