TM 10-3930-653-14&P (2)   BORING AND HONING (1)   Determine   proper   oversize   according   to   extent   of cylinder  bore  wear,  measure  outside  diameter  of  piston (long  span  across  skirt  of  piston)  and  rebore,  leaving 0.02 mm (0.0008 in) for honing, so that piston to cylinder clearance is within 0.025 to 0.045 mm *0.0010 to 0.00 1 8   in).      Measure   cylinder   at   normal   temperature.      If reboring of one cylinder, is required, rebore all cylinders at same time. Fig. 4-52.  Measuring Long Span of Piston Skirt Fig. 4-53.  Honing of Cylinder Block (2)   Calculating individual cylinder bore sizes: Piston skirt outside diameter (measured value): A Piston to cylinder clearance: B B = 0.025 to 0.045 mm (0.0010 to 0.0018 in) [at 20°C (at 68° F) Honing allowance: C C = 0.02 mm (0.0008 in) Cylinder bore: D D = A+ B-C (3)   Cylinder boring precautions Cutting depth of cutting tool should be approximately 0.05 mm (0.0020 in).  Do not cut quickly all at one time cut gradually. Measure    cylinder    bore    carefully,    since    bore    will change due to heat generated by cutting. To avoid deviations or distortions due to cutting heat, bore cylinders in following sequence of 24-1-3. Mark pistons according to applicable cylinder so that pistons  are  properly  applied  to  correct  cylinder  and  so that they are not interchanged (mixed). (4)   Measure bore and check taper and trueness (out-of- round) condition after proper finishing by honing. (5)   Measure piston to cylinder clearance. Fig. 4-54.  Measuring Clearance between Piston and Cylinder For   this   measurement,   Insert   piston   from   reverse side (with piston pin removed) and insert a feeler gauge 0.04  mm  (0.016  in)  so  that  it  projects  on  thrust  side  of piston.      Then   apply   a   spring   scale   to   gauge   and   lift vertically.  If spring scale reading is 0.5 to 1.5 kg (1.1 to 3.3 lb), clearance is normal. Oversized piston Piston size Diameter mm (in) 87.185 to 87.235 STD (3.4325 to 3.4344) 87.655 to 87.715 0.50 O.S. (3.4514 to 3.4533) 88.165 to 88.215 1.00 O.S. (3.4711 to 3.4730) 88.665 to 88.715 1.50 O.S. (3.4907 to 3.4927) 4 - 33

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