HONDA 120 TECH MANUAL
Contents
1. the engine Use a piston stop tool inserted in the spark plug hole to properly locate the piston top dead center TDC position Using a hand held electric drill rotate the engine in a clockwise direction and with a timing light check the ignition timing Honda 120 USAC 25 GX120 Tech Manual 23 10 11 12 13 Tier Il Flywheel is no longer legal You may use any key on Tier Ill Flywheel to reach maximum timing of 20 degrees Magnet and or its position may not be altered in any way Tier Ill magnet black Magnet retaining screw may not be altered any way Screw may not be replaced with larger or smaller screw No heli coiling of mounting hole Coil mounting bolts must be stock and cannot be altered in any way to advance or retard timing Coil attaching bolts will be stock mm cap screw 1 1 16 long b Therecan be no more than 3 8 of unthreaded portion of bolt that does not measure 0 230 diameter This restricts movement of coil to a position that could make ignition timing illegal Ifa coil support mount becomes stripped it is permissible to heli coil However only one leg may be repaired if both legs are heli coiled the block becomes illegal All nylon blades on the cooling fan must be intact Race day DO for any missing No metal removal or addition to flywheel is permitted Flywheel weight will be 1550 grams minimum Flywheel diameter magnet area 6 285 minimum A stock Honda spark plug cap wire end a2. Pilot Screw Check tip and Pilot seat USAC 25 GX120 Tech Manual E Once you remove the Needle Valve you can then check the Needle Valve iat dimension Remove the idle adjustment screw and then carefully pry up on the Pilot Air Jet to USAC 25 GX120 Tech Manual This is where the Pilot Air Jet is located There is not a spec for this hole The small hole in the bottom of the jet black plastic needs to be checked Also look to make sure that the o ring on the jet is there and in good condition 10 DU 10 m 2 GX 120 MAIN NOZZLE Gri uri wan Nozze Ss n GX 160 UT MAIN NOZZLE USAC 25 GX120 Tech Manual Check for restrictor if applicable and placement Restrictor must be installed between carburetor and carburetor insulator with stock Honda gasket on each side of restrictor Only stock Honda insulator gasket between black plastic insulator and head is permitted Gasket thickness 0 025 maximum Air passageway in insulator will not be altered in any way All Junior Honda s must use the blue USAC approved slotted plate only or the blue QMA plate dated 06 09 or newer Failure to use proper restrictor plate alteration of restrictor plate or improper installation of plate in designated class is cause for immediate DO and all applicable suspensions will be applied Restrictor Jr Honda Blue Plate Hole 4375 maximu
3. ble 1 This engine block must be in as cast and factory machined condition There must be addition or subtraction of metal or any other substance to the inside or outside of the cylinder block crankcase cover crankshaft rod piston pin rings flywheel or coil with the following exceptions Removal of rear gas tanks brackets is permitted b Removal of governor Governor system may be partially removed with the exception of the steel gear on the crankshaft This gear must remain intact Governor arm and shaft may be removed tied forward or altered to accommodate throttle linkage or return springs 2 Addition of brackets fittings etc to accommodate throttle linkage tachometer and temperature gauge is allowed 3 Cylinder head gasket surface may be milled butwaustbe flat milling 4 Check bore 2 366 maximum NOTE All measurements taken from the top to bottom of the bore preferred method of measurement is dial bore gauge dial calipers may be used Take Gut of service Wear Limits Parts Out of Service USAC reserves the right to confiscate 120 Honda engine parts deemed illegal or at USAC maximum wear limits EXAMPLE cylinder bore will be 2 366 maximum all measurements taken from the lop to the bottom of the bore parallel to crank 90 degrees from crank Any cylinder block that has one measurement over USAC maximum wear limits will be taken out of service If no me
4. HONDA 120 TECH MANUAL USAC NATIONAL TECH DIRECTOR ERIC RANKINE FEBRUARY 15 2015 2015 UNITED STATES AUTO CLUB 8 HONDA 120 Tech Manual Updated February 22 2015 FROM 2015 USAC NATIONAL 25 MIDGET RULE BOOK APPENDIX 1 731 Engine Protest Rules applies to Honda and Briggs classes only Protest shall be from within the same division of class only i e Jr Sr Lt amp Hvy 120 160 Animal or World Formula only Competitors in the same division and in the same race may make a protest on an engine No protesting in Rookie Class Handlers may not protest more than one car per event and may not protest same driver than once per calendar year Honda Engines and World Formula Animal Engines may be protested for 400 00 cash only plus any applicable shipping charges if necessary No protested related inspection will be started prior to the funds being posted with the proper official This protest form and cash must be submitted to the Chief Steward or his her designee before the end of the race that the protested engine is participating in LE Checkered flag lap complete The protest can only be made during an A Main event The person protesting the motor must have their engine inspected for compliance first If the protester s engine is found illegal the protest is null and void and the protest fee will go to the club If the protester s engine is found legal the protest will continue The Chief Steward his her desi
5. GX120 EXHAUST VALVE _ _ 0022 Mnzo212 Min 0138 Mox 03870 Min 20862 Nox 20 142 Min 0 135 Max 2 458 GX 120 INTAKE VALVE 1 Check valves for dimensions and weight 2 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 USAC 25 GX120 Tech Manual 18 3 Valve weight Intake 18 grams minimum Exhaust 16 grams minimum There are two different exhaust valve combinations available to use You can not mix and match parts from the two combinations If you choose to run the UT 1 exhaust valve part 14721 2 0 000 you must use Retainer 14773 2 1 000 and Lash Cap 14781 2 1 000 If you choose to run the UT 2 exhaust valve part 14721 24 000 you must run retainer 14771 2 1 000 You can not run Lash Cap with this combination The UT2 exhaust valve part 414721 24H 000 measures 2 470 overall length HEAD 1 Cylinder head will be in as cast and in factory machined condition There may addition or subtraction of metal except for cylinder head gasket surface 2 No use of any substance to the inside of the cylinder head This includes no type of machining or grinding to increase airflow No angle milling etc or any alteration that could increase valve spring pressure 3 Ja
6. s are legal or illegal All illegal or confiscated part s shall be sent to National Tech Director All legal parts shall be returned to handler USAC 25 GX120 Tech Manual 3 7 Ifa Honda motor is found to have a valve oil seal during tech it shall be a race disqualification only 8 Spark plugs and exhaust infractions are a race disqualification only 9 Failure to go to tech and or impound will result in a race day DQ Refusal of tech shall be interpreted as an admission that the engine is illegal and a suspension from the class shall be immediate with all awards and qualifications being revoked with a six month suspension driver and handler suspension at any USAC Sanctioned event 10 For the purpose of this rule only if a handler has multiple cars competing 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 11 All membership suspensions must be sent to the National Tech Director within 5 Business Days 12 illegal Rookie engine parts shall be confiscated Honda or Animal but the suspension shall not be levied against handlers or drivers for the first offense The second offense shall result in 30 days Suspension from Rookie 13 cost to appeal a suspension is 175 plus any associated fees The appeal must be made within 3 days of the ruling UNLESS IT SAYS YOU CAN DO IT YOU CANNOT DO IT GENERAL RULES 1 6 120 UT2 ENGINES WILL BE ALLOWED FOR U
7. INGS 1 Must be stock Honda rings with stock size and configuration 2 No decreasing of ring tension by heating machining or any other means 3 Tierll Piston Ring Thickness a Compression 0 056 min b Scraper 0 056 min USAC 25 6 120 Tech Manual 27 Oil ring 3 piece lower 0 095 min i piece 0 097 min 4 TIERIIII Ring Thickness Compression 0 036 min b Scraper 0 036 min c d piece lower 0 095 min i 1 piece 0 097 min 5 The letter N or T on Tier lll piston rings may not be legible after engine has been run WRIST PIN 1 Stock Honda wrist pin and retainer a OD 0511 ref b Length Minimum 1 844 Maximum 1 864 1D 0 354 ref d Weight 23 grams minimum CONNECTING ROD 1 Stock Honda rod with no alterations 2 Connecting rod big end size 1 021 minimum 1 0265 maximum USAC 25 GX120 Tech Manual 28 3 Pin end bore is 0 5111 ref 4 Length from bottom of pin bore to top of big end will be 2 111 maximum 2 101 minimum 5 Rod weight with bolts 119 grams or 4 2 oz minimum 6 No oil grooves on bearing surface of either end CRANKSHAFT 10 THAILAND CRANKSHAFT JAPAN CRANKSHAFT Stock Honda crankshaft with no alterations No removal or addition of any metal from or to the crankshaft is allowed No balancing of the crank is allowed No oil grooving is allowed on the crank journal Governor drive gear ca not be removed Cran
8. SAC COMPETITION 2 Only stock Honda GX120 2 Engine and gearbox will be used in this class All parts will be stock Honda specifically made for the Honda GX120 K1HX2 U S GCO1 GCAHK and Canada GCAAT Honda OEM 120 replacement blocks may be used Must be an OEM Honda replacement block No aftermarket blocks or components 3 All stock Honda parts must be used and properly installed with the following exceptions a Governor system may be partially or fully removed with the exception of the steel drive gear on the crankshaft This gear must remain intact If shaft is removed hole must be plugged Hole can be tapped for thread or epoxy No welding b Factory air cleaner must be removed Any air filter may be attached to the outside of air filter adapter Outer wear style or equivalent can be used over carburetor only with no adapter The approved air filter adapter may be run with or without an air filter Any air filter may be used with adapter as long as there are no devices inside the air filter or adapter IE Springs not allowed Hose from valve cover must go into a catch can We are using outerwear to define a style not brand name USAC 25 GX120 Tech Manual 4 The use of air filters during qualifying at asphalt and dirt events is illegal The Senior Tech Official reserves the right to allow filters at any event that it s deemed necessary Any type of throttle linkage may be utilized Carburetor shall remain unaltered with t
9. UT2 Retainer Thickness of retainer will be Intake 0 228 minimum Exhaust 0 228 minimum Flange thickness of retainer will be Intake 0 110 maximum Exhaust 0 110 minimum From flat of flange to machined surface Intake 0 148 minimum Exhaust 0 148 minimum VALVE SPRINGS 1 Valve springs will be stock Honda springs and will not be altered in any way Use of Honda GX120 or 140 valve springs legal 2 120springs a Wire diameter 0 071 maximum b Outside diameter of spring 0 790 maximum Number of total coils 53 d Spring pressure 11 Ibs maximum at 0 812 e Stacked length will be 0 394 maximum USAC 25 GX120 Tech Manual 16 3 140 Springs Wire Diameter 0 079 maximum b Outside diameter of springs 0 808 maximum Total number of coils 7 d Spring pressure 16 Ibs maximum at 0 812 e Stacked length 0 524 maximum ROCKER ARMS PUSH RODS STUDS 1 Rocker arms will be stock Honda and will not be altered in any way 2 Rocker arm studs will be stock Honda They or their mounting position may not be altered in any manner No heli coiling of mounting holes No bending of studs 3 Push rods will be stock Honda and will not be altered in any way Push rod length 4 803 max VALVES INTAKE VALVE USAC 25 GX120 Tech Manual 17 Maroa _ 21 Min 0 121 Min 0 212 20752 Max 2395 Min 2385
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11. asurements exceed USAC maximum wear limits the part of the block will not be confiscated Handler has the right to have confiscated parts returned to them but will be rendered unusable 5 Checkstroke 1 659 Maximum 1 640 Minimum 6 Measure amount that piston is up or down from block surface at T D C This will be measured at the edge of piston top and bottom and side to side When measuring top and bottom of piston take an average of the numbers measured i e top 003 bottom 003 the piston to deck would be 000 Measurement must NOT be taken in USAC 25 GX120 Tech Manual 21 the center or relieved part of the piston This dimension will be 0 000 Max NO PISTON POP UP Carbon may be removed to check pop up 7 Install degree wheel on flywheel Install pointer in order to read degrees Locate accurate T D C this should be done with a positive stop type fixture and not established with indicator alone 8 can be checked with indicator reading off the top end of tappets or end of top end of pushrod which will provide zero clearance The inverted radius of the top of the tappet presents some problem to get accurate readings and to prevent binding of indicator stem Indicator holder and positions are very critical in this operation 9 indicator on base circle of cam Be sure that compression does not affect zeroing exhaust indicator zero dial indicator after exhaust bump ref 10 Turing engine in normal
12. ected but switch assembly must remain intact in crankcase 11 Gearbox may be rotated to any desired position 12 On Off ignition switch may be removed and hole covered any material no welding 13 All pin measuring gauges are plus tolerance 14 Exhaust oxygen sensor or temperature sensor attached to any part of Honda exhaust system is illegal 15 Valve seals are illegal 16 Cryogenics of any Honda part is illegal 17 Taking parts out of service Reference Wear limits in Engine Block Internal section 18 DQ only Not suspension for Exhaust Air Filter Adapter Spark Plug 19 Shroud may be factory RED or BLACK 20 Honda gaskets for the intake carb side cover that are green on one side and tan on the other in color are legal 21 All seals must have th stock spring installed inthe seal and in its proper locaton TECH PROCEDURE Modifications or machining of any parts except the gasket surfaces between the cylinder head and block and main jet in order to bring them to stated minimum or maximum specs blueprinting is not legal 1 External visual check of engine for required components Pipe and muffler shrouds and sheet metal and level sensor this can be partially observed from outside 2 air cleaner must be removed Any air filter be attached to the outside of air filter adapter Outerwear style or equivalent can be used over carburetor only with no adapter The approved air filter adapt
13. er may be run with or without an air filter Any air filter may be used with adapter as long as there are no device s used inside the air filter or adapter LE Springs not allowed hose from valve cover must go into catch can We are using outerwear to define a style not brand name 3 Theuse of air filters during qualifying at asphalt and dirt events is illegal The Senior Tech Official reserves the right to allow filters at any event that it s deemed necessary 4 Any Type throttle linkage may be utilized Carburetor will be unaltered with the exception of the black plastic piece on upper end of throttle shaft this is the only part in the carburetor that can be altered Main jet is Non Tech Removal of material from the black plastic piece is allowed No additional material may be added to any part of the carburetor i e bolt in black plastic piece tubing on throttle stop 5 Rear mounting brackets for Honda fuel tank may be removed USAC 25 GX120 Tech Manual 6 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 7 They keyed end of the ring gear shaft may be shortened drilled and tapped machined for snap ring 8 Heli coiling threads for shrouds all valve cover existing throttle mounting holes oil drain and fill holes one of the coil bolts and side cover bolts is allowed Dowel holes ate not to be modified or relocated al
14. gal This is best measured using a 0 452 rod type gauge with 0 429 flat area to be used as go gauge Air vent holes on the side of the main nozzle must not be plugged Main nozzle must not be fastened into the carburetor body by anything other than the main jet It must not be epoxied or positioned by any other means Pilot jet 0 0138 maximum 79 0 0145 no go Pilotairjet 0 050 minimum 1 25mm 0 0492 go Pilot screw no spec Pilot seat diameter 0 035 maximum 61 0 039 go Tip of pilot screw 0 019 minimum Float bowl vent 0 148 maximum 31 0 120 go Needle valve seat 0 065 maximum 51 0 067 no go The butterfly screw the butterfly and the throttle shaft must not be removed from the carburetor Any evidence of tampering will be a disqualification and suspension Honda 160 carb for Heavy Honda MUST USE Honda part number 16211 ZE0 000 Insulator for the Heavy Honda class This is the standard GX 120 Insulator No modifications allowed BE65B BE65Q Carburetor Bore Intake end Maximum diameter 0 951 ref Throttle end maximum diameter 0 710 Carburetor venturi bore 0 523 no go This measurement is best made with no go gauge but may be made using a telescoping gauge as a no go Mainjetis nentecls Must be an OEM Honda jet Brass or Silver Jets can be drilled to any size 9 Main air jet 0 0587 maximum 53 0 059 no go at back of hole USAC 25 GX120 Tech Manual 13 h Mai
15. gnee will hold the protest money until the protested engine has been inspected for legality The protested engine shall be tagged marked and sealed as soon as it car comes across the scale if it has not been sealed prior The protested engine as well as the engine of the protested party shall be immediately taken to impound and or presented to the Tech Director for inspection Engine must remain in impound and in the possession of tech officials throughout the entire process including shipping to USAC Headquarters or designated tech inspection station and the transferring of funds Both protester and protestee have the option to be present at the time of inspection USAC 25 GX120 Tech Manual 2 9 Any protest that is withdrawn will be assessed a 50 00 fee that will be paid to the host club 10 If the protested engine is found to be illegal the motor must be completely torn down to check for additional illegalities The Tech Director must confiscate all illegal parts and related parts from the protested engine and shall immediately forward them to the USAC Headquarters If engine is found illegal protest money minus 50 plus any shipping cost will be returned to the person filing the protest 11 Refusal of protest destroying or withholding of parts or any other lack of cooperation in this protest or inspection process shall be interpreted as an admission that the engine is illegal and shall subject the driver and handler to the conditions set fo
16. he exception of the black plastic piece on the upper end of the throttle shaft This is the ONLY part of the carburetor that may be altered removed of material from the black plastic piece is allowed No additional material may be added to the carburetor i e bolt in black plastic piece or tubing on throttle stop Stock Honda fuel tank must be removed Recoil starter must be removed Pull cup may be cut down for washer Must original cup Exhaust Stock Honda muffler will be removed Mounting flange may be cut off of muffler and used as adapter flange Any transition from the D shape of the exhaust port to round must take place with in the thickness 0 250 max of the flange This applies to all exhaust systems No steps or tapers allowed grid marks are allowed past 25 flange area No suspension for exhaust flange or pipe infraction just disqualification If an after market flange is used maximum allowable flange thickness will be 0 250 If slip on type flange assembly is used pipe stub will be maximum 0 880 outside diameter tubing with maximum overall length of 1 500 Pipe stub must be inserted into exhaust pipe at least 0 750 and will have minimal exhaust leakage b All USAC 25 midget mufflers to be used will be 4 to 8 hp Briggs amp Stratton part number 294599 or equal equivalent Muffler will be internally unaltered except that the round cup shaped baffle may be welded to the perforated baffle without moving
17. indicator after exhaust bump 0 050 ref Maximum valve lift will be checked from the top of valve spring retainer Valves may be adjusted to zero clearance or shims may be installed to create zero clearance This may dictate making special shims as it is difficult to insert feeler gauge blades so as not to interfere with indicator contacts on retainer Valve lift Intake 0 245 maximum Exhaust 0 255 maximum USAC 25 GX120 Tech Manual 14 CYLINDER HEAD HEAD GASKET VALVES SPRINGS 1 2 3 Remove cylinder head Head gasket thickness 0 040 minimum thickness of inner rim Thickness of the head will be measured from valve cover surface to head gasket surface on the side at a position in line with the upper intake and exhaust flange bolt Minimum 2 911 MINIMUM 2 911 Remove valves Inspect retainers for alteration that would increase valve spring pressure Both intake and exhaust must have stock Honda retainers Exhaust valve only can have lash cap and corresponding retainer All valve oil seals must be removed The use of valve seals is illegal and will result in applicable suspension per Honda suspension program USAC 25 GX120 Tech Manual 15 UT1 Retainer Thickness of retainer will be Intake 0 228 minimum Exhaust 0 241 minimum Flange thickness of retainer will be Intake 0 110 maximum Exhaust 0 070 minimum From flat of flange to machined surface Intake 0 148 minimum Exhaust 0 165 minimum
18. it s original location No drilling holes in the baffles Inside seam of baffle must be straight edged NOTE Some seams may not be straight Threads will not be removed from the muffler It is ok to weld a washer or nut on the flange for a place to apply safety wire and or spring Exhaust pipe will be a maximum of 1 000 outside diameter with a length of 20 0 to 26 0 including a threaded pipe coupler welded to the end of the pipe in order to screw muffler in place so that muffler may be removed for inspection Pipe coupler will be a standard unaltered 4 NP threaded coupler Length will be 1 000 minimum to 2 250 maximum Exhaust pipe length will measured by using a small diameter hose inserted though pipe to measure overall length Flange and coupler will be included in the overall length when measuring pipe No coating of any type may be applied to the interior of any part of the exhaust system The intent of this rule is to have all of the exhaust pass through the muffler All measurements are to be taken with the component pieces in the same position as they were installed and on the car 9 Steel or Stainless are the only materials allowed for exhaust pipes USAC 25 GX120 Tech Manual 5 9 Choke butterfly and shaft must be removed Only the top hole may be filled only with silicone or old shaft cut down No addition or subtraction of material in the bore or venturi of the carburetor 10 Oillevel switch may be disconn
19. kshaft drive gear should not be removed This gear is installed by Honda to any accuracy of 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 Keyway location must not be altered in any manner Measure thrust to crank gear side 81340 Min The only cleaning method allowed is on the flywheel side of crankshaft for the purpose of removing calcium rust etc from the exposed end of the crankshaft This is permitted only from the seal groove out to the end of the thread of the crankshaft where the flywheel bolts on Only a wire wheel may be used in the cleaning process The use of Scotchbrite sandpaper or any other compounds or abrasives is illegal No material may be added or removed from crankshaft Crankshaft main journal at flywheel and gearbox ends may not be altered in any way USAC 25 GX120 Tech Manual 29 CAMSHAFT 1 Camshaft must be stock Honda with no alteration of any kind 2 There will be no additions to or subtractions from any part of the camshaft m 3 Compression release will remain intact and unaltered 4 Lobe center angle will not be altered by any means 5 Lobe profile will not be altered in any way 6 Camshaft specifications INTAKE EXHAUST Heel to heel 0 864 0 869 Heel to heel 0 865 0 870 Heel to Peak 1 079 1 093 Heel to peak 1 081 1 095 Length thrust flange to thrust flange minimu
20. m 3 142 maximum 7 Cam bearings are 0 547 0 551 and unaltered under 0 547 minimum will be TAPPETS 1 Tappets must be stock Honda with no alterations 2 diameter 0 909 minimum 3 Stem diameter 0 312 minimum USAC 25 GX120 Tech Manual 30 Base thickness 0 076 minimum 0 090 maximum Length 1 181 minimum 1 220 maximum Weight 16 grams minimum ENGINE BLOCK INTERNAL 1 The engine block must be in as cast and factory machined condition and there must be no addition or subtraction of metal or any other substance to the inside or outside of the block except for cylinder head gasket surface may be milled to achieve 20 00 piston pop up No angle milling of the cylinder surface Must remain flat Cylinder bore will be 2 366 maximum NOTE all measurements taken at top of bore or very bottom of bore Wear Limits Parts Out of Service USAC reserves the right to confiscate 120 Honda engine parts deemed illegal or at USAC 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 Any cylinder block that has one measurement over USAC maximum wear limits will be taken out of service If no measurements exceed USAC maximum wear limits the part of block will not be confiscated Handler has the right to have confiscated parts returned to them but will be rendered unusable Cylinder bore will n
21. m Restrictor thickness 0 0625 maximum No slotting of holes on any Honda restrictor Insulator gasket thickness 0 025 maximum Check for any alterations or worn parts that would allow additional air into engine holes slots perforations spacers loose bolts warped flanges etc Two stock Honda intake gaskets may be used between Carburetor and plastic insulator Carburetor identification number a Jr Honda and Sr Honda 6 UT2 carb BE99A b Heavy Honda BEA5O gd BESAD sain noz 16146 2H6 WEO ra Check carburetor for alterations Upper choke shaft hole may be sealed with silicone type sealer Honda 120 carb for Jr and Sr Honda a BE60B BEGOR UT2 carb BE99A b Carburetor Bore Intake end Maximum diameter 0 950 Throttle end Maximum diameter 0 632 d Carburetor venture bore 0 456 no go This measurement is best made with a no go gauge but may be measured using a telescoping gauge as a no go Mainjetin cacburetoc wilLbe Mon Tech Must be a Honda OEM jet Brass or Silver Jet Jet may be drilled to any size No other modifications f 9 Main air jet 0 0394 caaxienum 60 0 040 no go at back of hole Main jet access passage 0 094 maximum 41 0 096 no go USAC 25 GX120 Tech Manual 12 i Main nozzle bore 0 069 maxieauen 50 0 070 no go Main nozzle will be checked with a NOGO gauge 429 maximum If gauge goes over main nozzle carb is ille
22. n jet access passage 0 0942 maximur 41 0 096 no go i Main nozzle bore 0 02327 maxienuen 47 0 078 no go Main nozzle will be checked with a no go gauge 0 424 if gauge goes over main nozzle carb is illegal This is best measured using 0 519 rod type gauge with a 0 424 flat area to be used as a go gauge k Airvent holes on the side of the main nozzle must not be plugged 1 Main nozzle must not be fastened into the carburetor body by anything other than the main jet It must not be epoxied or positioned by any other means m Pilotjet 0 0135 maximum 79 0 0145 no go Pilotairjet 0 0478 cnaxienuem 55 0 052 go o Pilot screw no spec Pilot seat diameter 0 0365 maximum 610 039 no go q Tip of pilot screw 0 020 minimum Float bowl Vent 0 418 maximur 31 0 1200 no go s Needle valve seat 0 0685 maximum 31 0 1200 no go t The butterfly screw the butterfly and the throttle shaft must not be removed from the carburetor Any evidence of tampering will be a disqualification and suspension ENGINE COOLING SHROUDS 1 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 Remove engine cooling shrouds Remove valve cover Zero dial
23. ot be bored oversize Cylinder bore will not be re sleeved Cylinder bore position will not be moved or tipped in any manner There will be no polishing sandblasting or glass beading to any interior surface Machined surface of block down to thrust face of cam boss 3 228 minimum 3 240 maximum Machined surface of block down to bearing face 3 430 minimum 3 442 maximum Oil level sensor will be intact and unaltered Wires may be externally disconnected or cutoff USAC 25 GX120 Tech Manual a Tech officials have the right to tech any or all cars in any class at their discretion Parts in question that need further review must be sealed and boxed up at the track in front of the handler The handler and tech director must also sign a slip indicating that they both acknowledge the part is in question The part must then be shipped to the USAC National office at 4910 West 16th Street Speedway IN 46224 USAC 25 GX120 Tech Manual 32 Decimal Equivalents of Number Size Drills Size of Size of Size of Size of Dril in Dri in Dril in Dri in No Inches No Inches Inches No Inches 1 E rm moo 2 210 2 1570 2 095 E B 210 1540 aum 4 2090 P as m 5 2085 5 a 0829 E 5 200 aem o aman 7 2m0 auo a m D E auos b D 1960 aso ao m p w 1935 s E o E n 1910 E s r1 n 2 E E 1850 s an
24. pan UT1 and UT2 heads are all legal but some of the original UT1 heads had grinding and filing marks located in the ports Heads with grinding or filing will NOT be legal It is your responsibility to make sure your motor s head hasn t been ground or filed HPD has Japan 13 cylinder heads available 4 Thickness of the head will be measured from valve cover surface to head gasket surface on the side at a position in line with the upper intake and exhaust flange bolt No angle milling of surface Minimum 2 911 5 Cleaning of the combustion chamber or changing the finish of such chamber is illegal USAC 25 GX120 Tech Manual 19 INTAKE AND EXHAUST PORTS 1 Ports will be in as cast and factory machined condition and there must be addition or subtraction of metal or any other substance to the inside or outside of the cylinder head No alterations of any kind to be made to the intake or exhaust port This includes any grinding polishing etching sand blasting or glass beading to interior surface Valve seats must be a stock single 45 degree angle Multi angle valve seats are not permitted Valve seats must not be replaced Intake and Exhaust ports at valve Intake Maximum 0 752 Minimum 0 745 Exhaust Maximum 0 675 Minimum 0 665 ENGINE BLOCK USAC 25 GX120 Tech Manual 20 Must be OEM Honda Cylinder block No aftermarket blocks Honda Performance Development has Honda replacement blocks av
25. rotation clockwise facing flywheel take reading at specified opening Readings must fall between specified degrees on the following chart 11 Check max lift at intake and exhaust CAMSHAFT PROFILE INFORMATION INTAKE DEGREES EXHAUST DEGREES 0 050 105 14 0 050 2065 2105 BTDC 0 100 265 30 0 100 1895 1935 0 150 45 49 ATDC 0 150 1705 1745 BTDC 0 200 71 76 0 200 144 148 BTDC 0227 MAX LIFT 0 229 0 200 135 141 0 200 705 745 BTDC 0 150 162 167 0 150 445 485 0 100 180 185 0 100 26 295 BTDC 0 050 197 200 0 050 9 125 BTDC USAC 25 GX120 Tech Manual 22 FLYWHEEL AND IGNITION SYSTEM Caution should be used when removing flywheel Do not hit with hammer or other heavy objects Service manual show flywheel to be removed with commercially available 6 puller Another method is inertia type knocker that threads onto crankshaft end 1 The transistorized magneto ignition is fixed at 20 degrees and may not be altered in anyway a Firing must not exceed 065 or 20 degrees BTDC Quick check Turning flywheel clockwise if the leading edge of depression of flywheel rim where the magnet is mounted is not still under the right hand coil leg at 065 BTDC it is probably illegal and should be checked further PROCEDURE FOR CHECKING TIMING 1 Ignition timing is to be checked with a degree wheel and a fixed pointer mounted
26. rth in the Suspensions Program 12 Anyteched or protested engine block or part which are deemed to be over maximum wear limits in one or more spots but is under maximum wear limits in other spots is subject to confiscation but not DO able 13 Reference to Confiscation due to Wear Limits in Engine Block Internal Rules of both Manuals 14 If the engine is found legal 400 will be given to the person whose engine was protested 732 Engine Suspension Rules Handlers and drivers guilty of having an engine declared illegal at technical inspections shall be disciplined as follows 1 First offense up to 30 days and or 4 race suspension for handler and driver from participating in the respective class at any USAC Sanctioned event 2 Second offense within one year of first infraction up to One year suspension for handler and driver from participating in the respective class at any USAC Sanctioned event 3 Third offense within two years of last infraction Suspended for life from USAC s 25 Midget division 4 Suspension for life is open to review by USAC 5 Suspension shall begin immediately 6 Illegal Honda Animal and World Formula part s shall be sent within five Business days to the USAC office or designee for review The Tech director has 48 hours to determine if the part s are legal or illegal If the part s are determined to be legal it shall be returned to handler Handler shall be notified if part
27. so one carburetor mounting bolt 9 Honing and deglazing of the bore is allowed 10 Lapping the valves is allowed 11 Blocking air flow No device may be used that will or appear that it may impede air flow into the engine cooling system This may require that the engine be run at a speed above idle by the tech personnel at the scale after the car has qualified or raced CARBURETOR HONDA 120 INSULATOR D 4 HONDA GX120 CARBURETOR FOR JUNIOR amp SENIOR HONDA USAC 25 GX120 Tech Manual 7 0375 mas is area canbe eine nole 80125 Mae Remove Carburetor Air cleaner adapter ID 2 250 maximum Length 1 375 maximum Flange thickness 0 375 maximum Flange ID 1 000 minimum ID hole size straight walled flat bottomed and parallel with carburetor using existing air cleaner mount holes Restrictor Plates a Red Rookie Honda Red plate with single hole 3125 maximum b Blue Rookie Honda Blue plate with single hole 4375 maximum c Jr Honda Blue plate with single hole 4375 maximum plate legal only with a date of 06 09 or newer d The use of approved USAC plates is recommended USAC 25 GX120 Tech Manual The Pilot Air Jet hole is just inside of this brass piece This needs to checked with the proper no go gauge The hole at the end of the arrow is the Main Air Jet hole and will be checked using a pin type no go gauge Float Bowl Vent
28. t resistor must be used Any automotive type spark plug with 34 reach maximum is allowed No plug indexing washers allowed If temperature sensor is used under spark plug factory washer may be removed USAC 25 GX120 Tech Manual 24 GX 120 FAN GEAR BOX AND RING GEAR Gear box may not be altered in any way Maybe rotated to desired position 2 gear may not be altered in any way with the exception of the keyed end of shaft that may be shortened drilled and tapped or machined for snap ring grove No other machining drilling grinding etc to ring gear Key way may be cut deeper 3 Ring gear may not be altered in any way including polishing or use of any compound or abrasive on gear shaft where bearing rides 4 gaskets maximum between gear box halves CRANKCASE COVER 1 Remove crankcase cover 2 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 USAC 25 GX120 Tech Manual 25 3 Crankcase cover gasket must be stock Honda Only one gasket may be installed with maximum thickness of 0 025 4 Critical dimensions are thrust face of camshaft holder and position of crank bearing Placea straight edge over crank bearing and cam boss thrust face These surfaces should be level b Maximum tolerance will be 0 005 There will be no alterations to crankcase cover This incl
29. udes any alteration to crank bearing and camshaft holder position and height in an attempt to alter valve timing PISTON WRIST PIN AND PISTON RINGS 1 Remove rod and piston triangle or boss on top of piston must point toward push rods 2 Piston wrist pin and rings must be absolutely stock and not altered in any manner Max 0 049 p 2 350 Min 0 043 noas 072228 Maxi 2 358 n 238s LLL Max 2 363 Min 2 357 Max 1 815 Wi aos fed testae Lh Vin nass Max 2352 Mex Oeo Min 2348 Mexo2aso Minoos j l N Mino 2342 GX120 PISTON 13101 2H7 020 USAC 25 GX120 Tech Manual 26 TIER PISTON TIER II PISTON PISTON 1 Piston will be stock Honda standard size and will not be altered in any way 2 Oversized pistons must not be used Allthree piston rings must be used Topring Chrome compression ring installed with or T on a rail up No expander under ring b Middle ring Oil scraper ring installed with or T on rail up No expander under ring Bottom ring Three 3 piece oil rings are allowed 4 Piston may not be knurled grooved or coated 5 6 Piston weight 106 grams minimum 6 Total Piston weight with rings pin and clips 121 grams minimum 7 Total Piston weight with rings pin clips complete with rod and bolts 263 grams minimum 8 drawing for dimensions R
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