Honda GX 120 Tech Manual
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Honda GX 120 Tech Manual

Honda GX 120 Tech Manual

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See also: Tech Manual , Owner's Manual
SPECIFICALLY FOR 120 HONDA CLASS ONLY
First Issued:
March 30, 1996
Updated:
March 12 2014
© 2012 Quarter Midgets of America, Inc.
HONDA
TECH
MANUAL
GX 120
HONDA ENGINE RULES
GENERAL RACING RULES
Kevin Augustus 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 Kevin Augustus QMA Technical Director First Issued: March 30, 1996 Updated: March 12 2014 © 2012 Quarter Midgets of America, Inc.

  • Page 2: General Rules

    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 3 Material may not be added to throttle stop area of black plastic piece or carb body. C. Rear mounting brackets for Honda fuel tank may be removed.
  • Page 4 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 5 Main jet and main nozzle: (MUST BE TIGHT) Main Jet Size: maximum #70 (0.028) no go. Jets must be stock no stepping or funneling of jet No Go must not go thru jet. Main air jet: 0.0394” maximum #60 (0.040”) no go - at back of hole. Main jet access passage: 0.094”...
  • Page 6 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). Starter cup may be altered to be used as washer retainer for the cooling fan. 1.

  • Page 7: Valve Springs

    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 8 No alteration that could increase valve spring pressure. Note: Cylinder Head gasket surface is non tech Effective 4-1-11 Thailand heads will no longer permitted on the GX 120 engines. Any legal Japanese GX120 head may be used. New Gx 120 casting number 13 heads are available from HPD).

  • Page 9: Engine Block

    ( Japan heads may be identified by the casting mark shown in photo above) Measure from surface of head to lowest machined area in the bowl of the port. This dimension will be: Intake: 1.064” maximum Exhaust: 1.010” maximum Thickness of head. This will be measured from valve cover surface to head gasket surface at the side at a position in line with upper intake &...
  • Page 10 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 2.366 Max. All measurements taken at top of bore or very bottom of bore parallel to crank, 90 degrees from crank.
  • Page 11 0.150” 162 ½ to 167 ATDC 0.150” 44 ½ to 47 ½ BTDC 0.100” 180 ½ to 185 ATDC 0.100” 26 to 29 ½ BTDC 0.050” 197 ½ to 201 ATDC 0.050” 9 to 12 ½ BTDC FLYWHEEL, FAN AND IGNITION SYSTEM Caution should be used when removing flywheel.
  • Page 12 Flywheel weight will be: 1550 grams minimum Flywheel diameter - magnet area: 6.285’ minimum A stock Honda spark plug cap, (wire end and resistor), must be used. Any automotive type spark plug with ¾”reach maximum is allowed. Note Tapered seat spark plugs are illegal .

  • Page 13: Crankcase Cover

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

  • Page 14: Wrist Pin

    OLD STYLE TIER III RINGS A. Must be stock Honda rings with stock size and configuration. Compression and scraper rings may not be collapsed in grove. B. No decreasing of ring tension by heating, machining or any other means. Ring thickness:...

  • Page 15: Connecting Rod

    D. Governor drive gear cannot be removed. E. Crankshaft drive gear should not be removed. This gear is installed by Honda to any accuracy of + ½ 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 16: Camshaft Specifications

    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. B. Compression release will remain intact and unaltered. Lobe center angle will not be altered by any means.
  • Page 17 A. Cylinder bore will be 2.366” 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 2.366 Max. All measurements taken at top of bore or very bottom of bore parallel to crank, 90 degrees from crank.
  • Page 18 Tech officials have the right to tech any or all cars in any class at their discretion. Tech Officials follow the same chain of command as all officers of QMA – as follows: Local – Regional – National I.E. Regional tech officials can tech at any event at their region and National Tech Officials can tech at any event in QMA.

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