SHAFT ASSEM. (L. H. - 20 Teeth) 4N7978 - Caterpillar

Scheduling a Top End Overhaul
Top end overhauls are scheduled according to the recession of the valve stems. This measurement provides an accurate indication of the rate of valve wear. This measurement can be used to predict when a cylinder head requires replacement.To determine a baseline, measure the projection of the exhaust valve stems after 100 to 250 service hours. The baseline is a reference for subsequent measurements. Continue to measure the projection periodically.Plan for the top end overhaul as the valve stem projection approaches the maximum limit. Perform the top end overhaul when the valve stem projection has increased by a total of 2.3 mm (.09 inch). Do not allow the projection of the exhaust valve stems to exceed this limit.Note: Generally, cylinder heads wear out at different rates. Sometimes, servicing the cylinder heads at different times may be the most economic decision. This decision depends on the valve stem projection of the individual cylinders. However, this decision must include the costs of more downtime that is caused by this procedure. Perform an economic analysis to determine if cylinder heads should be serviced as a group or divided into smaller groups.Note: The generator or the driven equipment may also require service when the engine overhaul is performed. Refer to the literature that is provided by the OEM of the driven equipment.Top End Overhaul Information
A top end overhaul involves servicing the cylinder heads and turbochargers. Also, some other engine components are inspected.Top end overhauls require more tools than preventive maintenance. The following tools are needed for restoring the engine to factory specifications:
Torque wrenches
Dial indicators
Accurate measurement tools
Cleaning equipment
Rebuilding equipmentCat dealers are equipped with these tools. Cat dealers can provide a flat rate price for a top end overhaul.Unexpected problems may be found during a top end overhaul. Plan to correct these problems, if necessary.
Build up in the cylinders from excessive oil consumption
Build up in the cylinders from contamination of the fuel
Plugging of the aftercooler from coolant that is poorly maintained
Plugging of the aftercooler from contamination of the inlet air
Degradation of the oil cooler from the hydrogen sulfide in the fuel
Deterioration of exhaust manifold componentsThe following definitions explain the terminology for the services that are performed during an overhaul:Inspect - Inspect the components according to the instructions that are in Caterpillar reusability publications. Refer to Guidelines for Reusable Parts and Salvage Operations, SEBF8029, "Index of Publications on Reusability or Salvage of Used Parts". The guidelines were developed to help Cat dealers and customers to avoid unnecessary expenditures. New parts are not required if the existing parts can still be used, reconditioned, or repaired. If the components are not in the reusability guidelines, refer to the Service Manual, "Specifications" module.Rebuild - The component can be reconditioned to comply with reusability guidelines.Replace - The service life of the part is exhausted. The part may fail before the next maintenance interval. The part must be replaced with a part that meets functional specifications. The replacement part may be a new part, a Cat remanufactured