Final answer:
Accurate main bearing oil clearance measurements during an engine rebuild involve cleaning all parts, coating bearings with oil, torquing the bearing cap, and using Plastigage for precise clearance measurement. The example given illustrates how friction affects force required in engine assembly, emphasizing the benefit of lubrication.
Step-by-step explanation:
When rebuilding an engine and measuring main bearing oil clearance, several steps must be taken to ensure accuracy. Firstly, cleaning all parts is essential to remove debris that could affect the measurements. After cleaning, coat all bearing surfaces with fresh engine oil; this reduces friction and imitates normal operating conditions. The next step is to install the bearings and torque the bearing cap to the specifications provided for the engine, as proper torque ensures the bearings are seated as they would be during operation. Finally, you can measure the clearances using a tool like Plastigage, which is a common practice in engine rebuilding. Placing the Plastigage on the bearing surface and then tightening the cap will leave an imprint indicating the clearance. After measurement, ensure you remove any Plastigage material before final assembly.
In the scenario provided, the student encountered a need for a 300 N force to insert a dry piston. This force is an indication of the frictional force at play when inserting the piston without lubrication, which, in this case, is equal to the normal force due to the nature of the piston entering straight into the cylinder. If the steel parts were oiled, the frictional force would reduce, requiring less force to insert the piston, demonstrating the effect of lubrication on the engine assembly process.