TM 9-2815-237-34 1-8.  METRIC  SYSTEM The  engines  described  herein  contain  metric  components  and  require  metric  common  and  special  tools; therefore, metric units in addition to standard units will be used throughout this publication. In addition, a  metric  conversion  table  is  located  on  the  inside  back  cover  of  this  publication. 1-9.   MANDATORY   REPLACEMENT   PARTS The maintenance instructions contained herein make reference to removing and discarding piece parts such as:  gaskets,  lockwashers,  cotter  pins,  O-rings,  seals,  etc.;  these  items  should  be  considered  mandatory replacement items and replaced with new parts during assembly/installation. Section  II.  EQUIPMENT  DESCRIPTION  AND  DATA 1-10.   DESCRIPTION The engine used in the M998 series vehicles is an eight-cylinder, four-cycle, liquid-cooled, naturally aspirated diesel engine. The 6.2L engine has a compression ratio of 21.5:1 and develops approximately 150 horsepower (112 kW) at 3600 rpm. The 6.5L engine has a compression ratio of 21.5:1 and develops approximately  160  horsepower  (119  kW)  at  3400  rpm.  For  proper  orientation,  the  left  and  right  sides  of  the engine  are  as  viewed  from  the  rear. 1990  and  above  6.2L  and  1994  6.5L  engines  have  the  following  new  and  revised  parts: *Cylinder Heads. The 1990 cylinder heads have been revised to meet 1990 emissions standards. Precombustion Valves. The 1990 engine is equipped with reverse throat precombustion chambers. Exhaust Valves. The 1990 exhaust valves are coated with chrome to prevent burning and to increase   durability. *Exhaust Valve Stem Seal. The 1990 engine is equipped with a new exhaust valve stem seal that is  not  available  for  previous  model  engines. Cylinder  Head  Gaskets.  The  1990  cylinder  head  gaskets  have  new  circular  combustion  seals  and are  made  of  a  non-asbestos  material. Fuel Injection Pump. The 1994 fuel injection pump has a TP sensor and a revised calibration procedure. Fuel Injection Pump. The 1990 fuel injection pump has a new governor and a revised calibration procedure. *Fuel Injection Nozzles. The 1990 fuel injection nozzles are shorter and are orientated ten degrees from  the  vertical. *Fuel Injection Lines. The 1990 engine is equipped with high pressure fuel injection lines. Fuel Injection Line Brackets. The 1990 fuel injection line brackets have been revised to provide clearance  for  the  injection  nozzles. *RPM  Sensors.  The 1994 engine is equipped with two rpm sensors. One is for STE-ICE-R and one is for  the  engine. *Fan  Cut-Off  Switch.  The 1994 engine is equipped with a fan cut-off switch which replaced the kick-down   switch. *These  1990  parts  are  not  interchangeable  with  parts  for  previous  model  engines.  Differences  in maintenance  procedures  are  identified  where  applicable.  Refer  to  engine  decal  number  on  left  rocker arm  cover  before  ordering  replacement  parts. 1-2