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SPECIFICALLY FOR 120 HONDA CLASS ONLY
First Issued:
March 30, 1996
Updated:
January 2006
HONDA
TECH
MANUAL
GX 120
HONDA ENGINE RULES
GENERAL RACING RULES
Jerry Mostek, QMA Technical Director
1

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   Summary of Contents for Honda GX 120

  • Page 1

    HONDA TECH MANUAL GX 120 HONDA ENGINE RULES GENERAL RACING RULES SPECIFICALLY FOR 120 HONDA CLASS ONLY Jerry Mostek, QMA Technical Director First Issued: March 30, 1996 Updated: January 2006...

  • Page 2

    Suspension for life is open to review by the QMA National Board. For the purposes of this rule only, if a handler has multiple cars competing in the Honda class (GX120 or GX160) at one race event and more than one engine is found to be illegal at that event, it will be considered to be one offense.

  • Page 3

    Inside seam of baffle must be straight edged. (NOTE: Some seams may not be parallel in baffle) You cannot cut off the threaded flange if it is to be used in Honda. It is OK to weld a washer or nut on the flange for a place to apply safety wire.

  • Page 4

    C. Rear mounting brackets for Honda fuel tank may be removed. D. The starter cup that is behind the flywheel retaining nut can be cut away to leave only the flat washer back piece that retains cooling fan.

  • Page 5

    Honda gasket on each side of restrictor. Only stock Honda insulator gasket between black plastic insulator and head. Air passageway in insulator will not be altered in anyway. All Junior Honda’s must use the blue QMA approved slotted plate only. Failure to use proper restrictor plate, alteration of restrictor plate, or improper installation of plate in designated classes is cause for immediate DQ and all applicable suspensions will be applied.

  • Page 6

    H. Main jet and main nozzle: (MUST BE TIGHT) Main jet size: maximum #65 (0.0256”) check with #71 (0.026”) no go. Main air jet: 0.0394” maximum #60 (0.040”) no go - at back of hole. Main jet access passage: 0.094” maximum #41 (0.096”) no go. Main nozzle: Main nozzle bore 0.069”...

  • Page 7

    M. Decimal equivalents of numbered size drills chart on page 20 ENGINE COOLING SHROUDS A. All pieces of the stock engine cooling shroud must be properly installed. There must be no addition or subtraction of any material from the shrouding except for the covering of the switch hole (any material).

  • Page 8

    Honda. They or their mounting position may not be altered in any manner. No heli-coiling of mounting holes. No bending of studs. Push rods will be stock Honda and will not be altered in any way. Push rod length will be Maximum 4.799”- 4.770”.

  • Page 9

    VALVES A. Check valves for dimensions and weight. Valve seating surface must be factory ground to a single angle only, 45 degrees. There will be no other angles ground on any part of valve. Valves must not be polished, lightened or altered in any way. B.

  • Page 10

    Maximum 0.265” Minimum 0.250” B. Measure from surface of head to top of valve guide. This dimension will be: 0.925” Max. Intake 0.925 Max. Exhaust 0.905 Max. C. Measure from surface of head to lowest machined area in the bowl of the port. This dimension will be: Intake: 1.063”...

  • Page 11

    All measurements taken at top of bore or very bottom of bore. “ Wear Limits/Parts Out Of Service” QMA reserves the right to confiscate 120 Honda engine parts deemed illegal or at QMA maximum wear limits. EXAMPLE: Cylinder Bore will be 2.365 Max. All measurements taken at top of bore or very bottom of bore parallel to crank, 90 degrees from crank.

  • Page 12

    CAMSHAFT PROFILE LIMITS IN TAKE EXHAUST Degrees Degrees 0.050" 10.5 to 14 ATDC 0.050" 207 to 210.5 BTDC 0.100" 26.5 to 30 ATDC 0.100" 190 to 193.5 BTDC 0.150" 45 to 48.5 ATDC 0.150" 170.5 to 174.5 BTDC 0.180 Split 0.180 Split 0.200"...

  • Page 13

    58.2 oz Flywheel diameter - magnet area: 6.285' minimum H. A stock Honda spark plug cap, (wire end at resistor), must be used. Any automotive type spark plug with ¾”reach maximum is allowed. Ignition coil air gap is no longer a tech item. Stated dimensions are recommended for safe operation will be: 0.008”...

  • Page 14

    A. Cover must be in "as cast" and factory machined condition and there must be no addition or subtraction of metal or any other substance to crankcase cover. B. Crankcase cover gasket must be stock Honda. Only one gasket may be installed with a maximum thickness of 0.025".

  • Page 15

    PISTON NOTES Piston will be stock Honda standard size and will not be altered in any way. A. Oversized pistons must not be used. B. All three piston rings must be used and installed properly. Top ring: Chrome compression ring installed with “N” or “T” on rail up. No expander under ring.

  • Page 16

    D. Governor drive gear can not be removed. E. Crankshaft drive gear should not be removed. This gear is installed by Honda to any accuracy of + 1/2 degree. If this gear is not installed to this degree of accuracy, engine may not be legal when camshaft is checked by the procedure under engine block.

  • Page 17

    NOTE: Refer to photo of crank color. CAMSHAFT Camshaft must be stock Honda with no alteration of any kind. Notes: A. There will be no additions to or subtractions from any part of the camshaft.

  • Page 18

    A. Cylinder bore will be 2.365” maximum. NOTE: All measurements taken at top of bore or very bottom of bore. 1. “Wear Limits/Parts Out Of Service” QMA reserves the right to confiscate 120 Honda engine parts deemed illegal or at QMA maximum wear limits. EXAMPLE: Cylinder Bore will be 2.365 Max. All measurements taken at top of bore or very bottom of bore parallel to crank, 90 degrees from crank.

  • Page 19

    PROCEDURE FOR CHECKING TIMING A. With degree wheel or indicator located at 0 degrees or TDC., mark both the flywheel and some fixed point (such as right hand side of aluminum block casting right above flywheel) with aligning marks. Turn the flywheel clockwise and stop at 26 degrees BTDC or 0.103”...

  • Page 20

    DECIMAL EQUIVALENTS OF NUMBER SIZE DRILLS Size in Size in Size in Size in Decimals Decimals Decimals Decimals .2280 .1590 .0960 .0390 .2210 .1570 .0935 .0380 .2130 .1540 .0890 .0370 .2090 .1520 .0860 .0360 .2055 .1495 .0820 .0350 .2040 .1470 .0810 .0330 .2010...

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