INFORMATION ON PRODUCTION PISTON GRADE SIZE STAMP. GMC Truck GMC P RV/Van/Commercial Chassis 1994

******************************************************************** SUBJECT: INFORMATION ON PRODUCTION PISTON GRADE SIZE STAMP

MODEL: 1994 CHEVROLET AND GMC TRUCK C/K, P TRUCKS AND G VAN WITH 6.5L DIESEL ENGINE (VINS F, P, S, Y - RPOS L65, L49, L56, L57)

PISTON GRADE SIZE STAMP INFORMATION: BEGINNING WITH THE 1994 MODEL YEAR, A PRODUCTION PISTON GRADE SIZE STAMP WILL BE LOCATED IN ONE PLACE ONLY ON THE CYLINDER BLOCK OIL PAN RAIL. PREVIOUSLY, THE PAN RAIL WAS STAMPED IN EIGHT PLACES (ONE FOR EACH ENGINE CYLINDER) BECAUSE MORE THAN ONE PRODUCTION PISTON GRADE SIZE COULD BE USED IN THE SAME CYLINDER BLOCK (I.E., THE PISTONS WERE SELECTED BASED ON THE SIZE OF EACH FINISHED CYLINDER).

DUE TO MANUFACTURING IMPROVEMENTS, THE NEED TO HAVE MORE THAN ONE PRODUCTION STANDARD PISTON GRADE SIZE HAS BEEN ELIMINATED, THUS IT IS NO LONGER NECESSARY TO STAMP THE PAN RAIL IN MORE THAN ONE PLACE ON THE CYLINDER BLOCK PAN RAIL.

A 0.13MM OVERSIZE PRODUCTION PISTON IS AVAILABLE FOR IN-PLANT REWORK OF CYLINDERS NOT MEETING THE PRODUCTION STANDARD SPECIFICATION. THESE PRODUCTION OVERSIZE CYLINDER BORES ARE IDENTIFIED BY AN "S" METAL STAMPED ON THE PAN RAIL NEXT TO ANY REWORKED CYLINDER. A "J" WILL ALSO BE METAL STAMPED IN A SINGLE PLACE ON THE PAN RAIL TO REPRESENT THAT ALL REMAINING CYLINDERS ARE PRODUCTION STANDARD.

IN PRODUCTION, PISTONS ARE IDENTIFIED BY THE LETTERS "J," "S," "JT," OR "ST" PAINT STAMPED ON THEIR FACE AND WRIST PIN BOSS SURFACES. "J" PISTONS ARE PRODUCTION STANDARD AND "S" PISTONS ARE PRODUCTION OVERSIZE FOR NATURALLY ASPIRATED 6.5L DIESEL ENGINES (L49, L57). "JT" PISTONS ARE PRODUCTION STANDARD AND "ST" PISTONS ARE PRODUCTION OVERSIZE FOR TURBOCHARGED 6.5L DIESEL ENGINES (L56, L65). TURBOCHARGED ENGINES REQUIRE PISTONS HAVING AN ANODIZED TOP OF DOME TO WITHSTAND THEIR INHERENT ADDITIONAL MECHANICAL AND THERMAL LOADS.

SERVICE INFORMATION:

ALL SERVICE PISTONS FOR 1994 6.5L DIESEL ENGINES (VINS F, P, S, Y) WILL BE "T" PISTONS. REFER TO THE FOLLOWING CHART FOR PISTON AND CYLINDER BORE SPECIFICATIONS:

1994 6.5L DIESEL ENGINE PRODUCTION AND SERVICE PISTON - CYLINDER BORE SPECIFICATIONS

NOTE: CYLINDERS 7 AND 8 REQUIRE 0.013MM (0.0005 IN.) ADDITIONAL PISTON TO BORE CLEARANCE. PISTON SKIRT BORE 1-6 BORE 7-8 GRADE DIAMETER DIAMETER DIAMETER ------ -------- -------- -------- PRODUCTION STANDARD 102.865 - 102.972 - 102.985 - J OR JT 102.883MM 102.990MM 103.003MM

PRODUCTION REWORK 103.008 - 103.117 - 103.130 - S OR ST 103.026MM 103.130MM 103.143MM

SERVICE STANDARD 102.865 - 102.972 - 102.985 - JT 102.883MM 102.990MM 103.003MM

SERVICE HI LIMIT 102.904 - 103.013 - 103.026 - GT 102.922MM 103.026MM 103.039MM

SERVICE 103.399 - 103.508 - 103.521 - 0.50MM OS--0.50MM OST 103.417MM 103.512MM 103.525MM

SERVICE PARTS INFORMATION: ALL 1994 6.5L DIESEL ENGINES (TURBOCHARGED AND NATURALLY ASPIRATED) ARE SERVICED WITH "T" PISTON (I.E., "JT," "GT," OR "0.50 OST"). THESE SERVICE PISTONS WILL HAVE "JT," "GT," OR "0.50 OST" PAINT STAMPED ON THEIR FACE AND WRIST PIN BOSS SURFACES. SEE BELOW FOR SERVICE PART NUMBERS.

PISTON AND PIN ASSEMBLIES SERVICE STANDARD "JT" P/N 12550059 SERVICE HI LIMIT "GT" P/N 12550061 SERVICE 0.50MM OVERSIZE "0.50MM OST" P/N 12550062

PARTS ARE CURRENTLY AVAILABLE FROM GMSPO.

FIGURES: 0

GENERAL MOTORS BULLETINS ARE INTENDED FOR USE BY PROFESSIONAL TECHNICIANS, NOT A "DO-IT-YOURSELFER". THEY ARE WRITTEN TO INFORM THOSE TECHNICIANS OF CONDITIONS THAT MAY OCCUR ON SOME VEHICLES, OR TO PROVIDE INFORMATION THAT COULD ASSIST IN THE PROPER SERVICE OF A VEHICLE. PROPERLY TRAINED TECHNICIANS HAVE THE EQUIPMENT, TOOLS, SAFETY INSTRUCTIONS AND KNOW-HOW TO DO A JOB PROPERLY AND SAFELY. IF A CONDITION IS DESCRIBED, DO NOT ASSUME THAT THE BULLETIN APPLIES TO YOUR VEHICLE, OR THAT YOUR VEHICLE WILL HAVE THAT CONDITION. SEE A GENERAL MOTORS DEALER SERVICING YOUR BRAND OF GENERAL MOTORS VEHICLE FOR INFORMATION ON WHETHER YOUR VEHICLE MAY BENEFIT FROM THE INFORMATION.