Measuring the Piston:
The piston is measured in diameter at 10-15mm from the bottom of the piston skirt on both axis. They are also visually inspected for damage and wear. Small chips, scuffing of the piston, carbon buildup, and scratches can effect the pistons life and operation. The measurements are done to determine if the piston has been worn, tapered or become oval in shape. all of these can effect the engines operation.
Inspection of the cylinder bore:
The Cylinder bore is inspected for wear and damage. The cylinder bore can scuff and score from the movement of the piston, this can cause more damage to the piston or piston rings and to the hardening layer in the cylinder bore. Wear is measured with a bore gauge at 6 positions on the bore. At the bottom middle and top on both the y and x axis, this is to check for taper and ovality of the cylinder bore. The top of the bore is the most worn, and one axis of the bore is worn more than the other because it is the thrust side of the crankshaft movement. This is also used with the measurements from the piston to calculate the piston clearance.
Visually inspected for damage and wear. Piston rings can snap and cause a lot of damage to an engine, also if they do not work correctly pressure from the combustion chamber can go past the pistons leading to poor combustion. Carbon deposits can build up behind piston rings and restrict there movement and operation.Ring side clearance: Checked with a feeler gauge, if a worn or wrong ring is fitted then they do not work properly.
Groove depth: Carbon buildup can reduce groove depth.
Ring end gap: Measured by putting the piston in the cylinder with the ring on and measuring the gap. If the ring gap is too big (from wear) compression can be lost.
Worn piston ring
Con rods are under huge strain and can suffer twisting and bending from the forces of piston movement and transmitting linear motion to rotational motion. If a con rod is bent or twisting it can increase the amount of wear on the engine and piston, or even break under strain. Bending can be repaired to a certain degree. Twist is measured on a flat face with a special tool.
Bent con rods from immense strain.
The crankshaft is visually inspected and measured to determine if it is damaged or worn. As the crankshaft has a lot of force applied on it, It can suffer wear and surface damage. Cuts on the crankshaft can damage the con rod bearings. and if the wood roff key is damaged, components may not work properly. The Crankshaft journals are measured for wear taper and ovality. If the journals are worn, the crankshaft may not spin correctly or the con rods travelling on the big end journals could wear more aggressively. Measurements are taken (by micrometer) at each end of the journal and at both axis. Crankshafts are also measured for Deflection. This is the 'twisting' of the crankshaft that comes from the torque applied to it over time. It is measured with a DTI gauge and small measurements are recorded.
A lot of force is needed to break a crankshaft.
The cylinder block is visually inspected for cracks, corrosion and general damage sustained during use . A crack or corrosion can compromise the strength of the block and make it degrade faster. The Distortion of the cylinder block face is measured with a straight edge and a feeler gauge across different parts of the face. If the Cylinder block face is distorted then the cylinder head may not sit properly or combustion could be lost.
Broken bolts or threads can make the block weaker.
Timing gears, sprockets and belts.
Because the timing gears are essential to the running of the engine, they are important to check for damage and wear. Worn gears could upset the timing or break. A worn belt or chain has the potential to wreck an engine if it breaks. Chains are checked for worn links, corrosion or tension, and belts are checked for fraying, cracks,corrosion, brittleness and tension.
A broken Cam belt
And the results, valves left open can be bent by rising pistons.
Core plugs are checked to ensure they are doing their job, plugging holes in the block made during casting, and also provide a pressure release if the water in the jacket freezes. If they are corroded they could leak. If they fall into the water jacket they can obstruct water flow.
The oil pump pumps oil from the sump to parts of the engine that need lubricating. If the oil pump breaks the engine could seize due to lack of lubrication. The clearances of the pump impeller and housing are measure with a feeler gauge to ensure they are in manufactures specifications and are not leaking.
A rotary oil pump that has broken its housing.
Bearings and caps
The crankshaft and con rod bearings are important as they help reduce wear on the engine and increase its lifespan. They can be replaced instead of larger parts. Bearings are checked for visual damage and oil galleries are not clogged.
Main bearing and big end bearing oil clearance
Measured with a palsti gauge, the plastigauge measures the gap for oil to pass through and lubricate the bearing, if the gap is too small the bearing could seize.
The thrust movement of the crankshaft is called crankshaft float. this reduced by thrust bearings. The float is measured with a DTI gauge. Too much float can wear the crankshaft or result in poor operation.
Con rod play
Con rod play is the thrust movement between the conrod and crankshaft big end bearing. A feeler gauge is placed in the gap to measure the thrust of the con rod. if the gap is too big the con rod can be under more stress and the piston can be worn more.