Yamaha YZFR-3 Overbore +2 mm Engine Kit Pistons/Cylinder/Gaskets/Velocity Stacks 339.43cc


This engine kit is the Approved Overbore Kit  for MotoAmerica 

*** Special Pricing for all MotoAmerica Teams and riders*** Must provide Current MA License

Spears Racing Piston Kits have been winning races and Championships all over the world for over 4 decades. Spears Racing pistons have been in several of the race wins in close course racing and in road circuits in Europe and has won the Isle Of Man TT Twins race several times.



1 in stock

SKU: SP12-320 MotoAmerica Engine Kit Complete Categories: , , , Tags: ,


This Yamaha YZFR-3 Overbore Engine Kit 339.43cc is assembled by Spears Racing to allow the Yamaha YZFR-3 to be more competitive with the Kawasaki Ninja 400cc, This engine kit is the “Approved Overbore Kit” for MotoAmerica. This kit will build more engine torque and be able to allow more air into the engine with the Spears Racing Velocity Stacks.

The Spears Racing Overbore 339.43cc Piston Kit was designed by Spears with a team of engineers from JE Pistons to be the best product on the market. When JE Pistons makes these race pistons they use their own developed proprietary tooling, dedicated cutters and unconventional fixtures to ensure that each piston is produced to the highest quality. Our 5-axis machining techniques allow them to create our Spears Racing piston. Along with this they use computerized electronic inspection equipment to make sure that each piston is  manufacture to the tightest tolerances in the industry. Spears Racing continuous committed to excellence to advance as the industry changes. This commitment has resulted in more forgings specific to certain applications, proprietary treated wrist pins and piston rings manufactured only for us. Each of these aspects has been put through stringent research and development stages to ensure that our product will exceed your standards.

At the forefront of performance, piston manufacturing has been a result of continuous research and development both in the dyno room and on the race track winning championships all over the world.

The stock NEW OEM cylinder is bored ,coated with a  proprietary Nickel Silicon Carbide (NSC) cylinder plating process and cylinder services. This NSC process is so superior to factory plating that Millennium Technologies is often used by professional race teams all over the world to re-plate brand new OEM cylinders. Millennium stands behind our plating with a LIMITED LIFETIME WARRANTY.

Spears Racing Overbore 339.43cc Piston Kit: This is the new 3 Ring FSR Slipper Skirt forging Piston made for Spears Racing  by JE Pistons. This 2mm over kit includes pistons, rings, lighter wrist pins, cir clips and weighs 139g per piston. The compression is 13:5 to 1. At the forefront of performance, piston manufacturing has been a result of continuous research and development of unique materials, piston designs and technologies. Utilizing State of the Art CNC machines for manufacturing, along with purpose-designed forgings, while being able to maintain exacting specifications on each and every piston – holding tolerances to millionths of an inch. This is the new forging that is lighter than stock OEM Pistons.

These pistons also have had the domes (tops) of each unit brushed ,this will insure that all of the sharp edges of the dome and not creating more heat in the combustion chamber.

Piston Terminology for Spears Racing Pistons:

ANTI DETONATION GROOVES (CONTACT REDUCTION GROOVES) Also know as contact reduction grooves, which limit the piston/cylinder contact during high temperature and high RPM. These grooves also protect the top ring by disrupting detonation waves

ACCUMULATOR GROOVE V-shaped groove machined in the 2nd ring land that adds to the volume between the top and second ring. This groove accumulates residual gasses from combustion which alleviates top ring flutter and premature second ring wear, improving ring seal.

DOUBLE PIN OILERS Double Pin Oilers (opposed to single pin oilers), in conjunction with our annular reservoir, can add twice the amount of oil from the cylinder wall to the wristpin bore. This is a standard option on any piston that receives forced pin oiling.

PIN FITTING The pin bore is precision honed to attain an exact pin clearance. Depending on the application clearances typically range from .0003 to as much as .003 between the wristpin and bore.

Cam and Barrel Extensive research and development has been done to find the optimum skirt shape for each piston. Having the correct cam and barrel on a part promotes the following:
1. Tighter clearances
2. Less noise
3. Better ring seal
4. More power
5. Durability
CUSTOM LASER LOGO/SERIALIZATION Each Spears Racing Piston is custom laser marking with the Part number, Job Batch and the Spears Racing Logo.

Each Kit comes with a custom plus 2mm over  head Gasket to a special thickness . Spears Racing’s designs are specifically engineered for extreme applications including professional competition.

Spears Racing Piston Kits have been winning races and Championships all over the world for over 4 decades. Spears Racing pistons have been in several of the race wins in close course racing and in road circuits in Europe and has won the Isle Of Man TT Twins  several times.


Spears Racing recommends using a surface gap spark plug when installing these pistons, You will need to look at your current spark plugs  USA NGK CR8E (recommend using NGK R0045Q-9 ) European NGK LMAR8A-9  (Recommend using NGK R0465B-10 )


“Preferred Performance” Pistons Kits By Spears Racing

Kit Includes the Following:

New Bored Re-Plated cylinder

Pistons x 2

Wrist Pins x 2

Piston Rings x 2

Cir Clips x 4

Custom 2mm over head gasket

Spears Racing have developed new, innovative Yamaha YZF-R3 Velocity Stacks.  Their patent pending revolutionary design is backed by real dyno testing and our Corbin Custom Design engineering staff.  Our stacks will improve torque/HP and smooth out power delivery.  This allows the rider a truer feel for the throttle which enables more accurate throttle input, improves confidence to get on the throttle sooner and boosts rider confidence.
The OEM Air box of the stock “commuter bike” Yamaha R3 contains two different length velocity stacks, one long, one short and two small intake tracks which severely limit the air intake into the engine.  This kit replaces both stacks with our own Spears Racing shorter velocity stacks and instructions to modify your current OEM air box.  These shorter velocity stacks not only allow more air into the engine, but they also smooth out the increased flow of air to really maximize the benefit.

The Spears Velocity Stacks can make the power band stronger through the complete power and starts a separation from the stock run at 9800 RPM’s and continues to make more power to the red line instead of falling off at 10,700 and we don’t lose any important midrange power. The increase from 10,500 to the rev limiter is massive both with RWHP and Torque.

YAMAHA-R3-Velocity Stack INSTALL


Dyno proven gains don’t lie! Developed in house on simulation software, the race track and hours on the dyno.


Yamaha YZF-R3 Velocity Stacks Kit Contains:

2ea. Air box manifolds
2ea. Spears Velocity Stacks
1ea. Silicone 1oz. Tube