Technical Revisions For Junior Honda UT3 Engine for 2021 Season

Written on 11/18/2020

The items listed below in red have been revised for the 2021 USAC.25 race season:  



Honda 120 UT3 –Basic Build 

Specifications For Use in USAC.25



The following steps are general specification guidelines. If there is an area not mentioned below, USAC approval is required prior to going beyond these specifications.


This tech sheet applies to the new Honda 120 UT3 engine. The current Honda 120 UT2 specifications apply unless listed below. All UT3 parts must be as factory produced unless specified below. All parts can be compared to stock known parts during tech inspection.


1)   The cylinder block must be the Honda 120 UT3 version only. This block does not have the cc listed on top of block. The only modification allowed to the block is the milling of the deck surface. After milling and with the piston installed, the piston must not protrude above block deck surface. Technical measurement will be from the block deck to the machined spot face on top center of the piston. the measurement must not exceed 0.000


2)  The Honda carburetor, number 99L must be used for Jr. and Sr. Honda classes.  The 99L carburetor must use the GX120 Japan, UT1 and UT2 main nozzle in place of the UT3 long version nozzle. The standard Honda 160 carburetor can be used for Heavy Honda class as outlined in UT2 tech manual.


3)   The head gasket must be a Honda UT3 steel shim gasket with a minimum thickness of .008” 


4)  The current UT2120 and 140 valve springs must be used.  An optional spring shim with a maximum thickness of .030 will be permitted under the intake valve spring.  Valve spring shims will not be permitted for use on the exhaust valve


5)   The cylinder head must be Honda factory supplied UT3 with no modifications. Both the TKI and the ATA head castings are approved.  Addition or subtraction of material is not legal. 


6)   Honda factory supplied UT3 aluminum push rods must be used  – 4.790” max length.


7)   Honda factory supplied UT3 valves must be used . Alterations to valves will not be permitted.


8)   Honda factory supplied crankshaft must be used. The UT3 crankshaft stroke must be 1.700”min / 1.720”max.


9)   The Piston, piston pin, piston rings and connecting rod must be Honda factory supplied UT3  parts.


10) The ignition timing for the UT3 must not exceed 17 degrees maximum for Senior Honda and Heavy Honda. For the 2021 race season the timing for the UT3 120 Junior Honda will be 20 degrees


* For the 2022 race season the UT3 120 Honda timing for Senior Honda and Heavy Honda will change to 20 degrees.



11) The Honda factory supplied UT3 flywheel with a green magnet must be used.