Chapter  3—ENGINE  MAINTENANCE danger that the piston will strike valves that may be open and extending into the cylinder. Make certain that any gears removed are replaced in the original   position.   Special   punch   marks,   or numbers (figure 3-24), are usually found on gear teeth that should mate. If they are not present, make identifying marks to facilitate the correct mating of the gears later. Bearing, bushing, and gear clearances must be properly  maintained.  If  bushing  clearances  exceed the  allowable  value,  the  bushings  must  be  re- newed. The allowable values for backlash and bushing clearances should be obtained from the instruction manual for the engine involved. Usually, a broken or chipped gear must be replaced. Care should be exercised in determin- ing whether a pitted gear should be replaced. BLOWER  ROTOR  GEARS One of the most important parts of a root type blower  is  the  set  of  gears  that  drive  and  syn- chronize the two rotors. Satisfactory operation depends on the condition of these gears. Worn  gears  are  found  by  measuring  the backlash  of  the  gear  set.  Gears  with  a  greater backlash than specified in the applicable technical manual are considered to be excessively worn and, if not replaced, will eventually cause extensive damage to the entire blower assembly. A  certain  amount  of  gear  wear  is  to  be expected, but scored and otherwise damaged rotor lobes resulting from excessively worn gears are inexcusable. It is the duty of the engineering force to inspect the gears and lobes, and to measure the clearance  at  frequent  intervals.  During  the  inspec- tion, it will be possible to measure accurately the values of backlash. These values should be re- corded. By observing the rate of increase of wear, it will be possible to estimate the life of the gears and to determine when it will be necessary to replace  them. Lobe  clearance  can  be  found  by  determining the  difference  of  the  maximum  and  minimum rotor lobe clearance at the same distance from the center. To find the maximum clearance, hold the rotors so that there is maximum clearance between the two rotor lobes. Then, with feeler gages deter- mine the value of the rotor lobe clearance. (See figure  3-25.) Figure 3-26.—Checking the backlash of blower rotor gears. The minimum clearance is found in a similar manner except that rotor lobes are held in such a position as to take up all slack and backlash. The difference of the two clearance readings is the value of the backlash of the rotor lobes. Since a change in lobe clearance is normally caused by wear  of  the  gears,  the  gear  clearance  must  be checked. The most direct method for checking gear clearance is by the use of feeler gages. (See figure  3-26.) Any gear set which has excessive lash or shows any sign of fracture must be replaced with a new set. Since blower drive gears come in matched sets, gears   from   different   sets   must   not   be interchanged. CHAIN MECHANISMS In some engines, chains are not only used to drive  camshafts  and  auxiliaries  but  also  to  drive such  parts  as  rotating  supercharger  valves.  Con- necting links for two types of chains are shown in figure 3-27. Note that the connecting pins in one are secured by cotter pins, while the joint pins shown in the other are riveted. The principal causes of drive chain failure are improper   chain   tension,   lack   of   lubrication, sheared cotter pins or improperly riveted joint pins,  and  misalignment  of  parts,  especially  idler gears. Chain drives should be checked for any symp- toms  of  such  difficulties,  in  accordance  with  the instructions in the appropriate engine manual. The tension  should  be  adjusted  as  required  during 3-39