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Engine Nomenclature

There are several types of “China Girl” engines on the market and though they may look the same, there are many differences once they are inspected internally.

The GT-5 type motors have an excellent FM80 connecting rod and is a good reliable motor. The true PK80 SilverJet half-breed motor vibrates less than the GT-5 type. These two engine types will have similar low speed engine performance, however the silver jet engine will have better high speed performance due to the better balance of the crank.

The China Girl 66cc/80cc 2 cycle engine has basically 3 engine geometrics and could be broken into separate classes ie: PK80 – GT5 – Half Breed.

Here is a way to measure and know if you have the 110mm or 115mm Con Ron length without taking motor apart. Stick a screw driver along the side of the rod and measure. If you measure around 80mm then you have the 110mm Con Rod, if you measure around 85mm , then you have the 115mm con rod.

Another way to tell: If you have a 40mm stroke, the piston will likely come to the top of the barrel on TDC (or close to), if you have a 38mm the piston is likely to sit a little low in the barrel at TDC.

This is important to know since the top ends are designed around the con rod length.


NT Carburetor:
Bore: 14mm
Float level (48cc and 66cc): 21mm
(Measured with carb upside-down and gasket removed, from gasket seating surface to highest point on float when float is held level)

Main Jet:
48cc: 0.5mm
66cc: 0.7mm or 0.79mm  Thread is .5mm

If your engine is 4 stroking below 3/4 throttle, adjust the needle. Above 3/4 throttle, smaller jet.

To read a spark plug properly what you want is a distinct burn line about half way down the tip and a heat line on the electrode just as it approaches the threads above the gap between the insulator and the body. You should also see a color change in the skin tone of the ceramic looking down in the plug about half way up then its damn near perfect. (prevents fouling) and doesn’t load up with carbon.

If you engine is running too hot first check the jetting. If you are too lean the motor will be much hotter. A properly running two stroke can have exhaust temp of over 1000 degrees.


two-stroke engine
A two-stroke engine, in this case with an expansion chamber illustrates the effect of a reflected pressure wave on the fuel charge. This is essential for maximum charge pressure (volumetric efficiency) and fuel efficiency. It is used on most high-performance engine designs.



Type A = 15/16″ Bottom of piston to center of connecting rod. Used on 38mm rods. Also on specialized ZAE 50 rods 38mm stroke – If you have this rod and crank, double check

Type B = 1 1/16″ Bottom of piston to center of connecting rod. Used on 40mm rods.





Working area





Header 6

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Cylinder Stuff

Cylinder bodies comes in 30mm and 40mm intake manifold stud spacing.

31MM Intake Stud spacing: Compatible with Flying Horse and Mega Motors Engines.

40MM Intake Stud spacing: Compatible with Grubee SkyHawk Engines. Note: Will not be compatible with Manic Mechanic after market intake manifolds.

All 80/66cc cylinders are bored at 47mm, use 8mm cylinder head bolts with a 44mm space square between the cylinder head bolts. Cylinder is approx. 81.5mm in height and the exhaust bolts are spaced at 45mm.



Piston Stuff

66/80cc Piston #2

Part Number (See Parts Manual): 2-25 or D2-35

Only compatible with the Flying Horse Bike Engine Kit*

Piston Measurements:

Height: 49mm

Width: 42mm

Skirt Width: 19mm

Skirt Height: 5mm

Piston Pin Port: 5mm

Height from Skirt to Piston Pin: 5mm
66/80cc Piston #4

Part Number (See Parts Manual): 2-25 or D2-35

Compatible with most Grubee Skyhawk Bicycle Engine Kits*

Piston Measurements:

Height: 50mm

Width: 47mm

Skirt Width: 23mm

Skirt Height: 10mm

Piston Pin Port: 10mm

Height from Skirt to Piston Pin: 5mm
66/80cc Piston #5

This piston fits most 66/80cc Flying Horse, Mega Motors, and RAW* engine kits.

Piston Measurements:

Height: 50mm

Width: 47mm

Skirt Width: 19mm

Skirt Height: 11mm

Piston Pin Port: 5mm

Height from Skirt to Piston Pin: 7mm



—–Notes Only——


The GT5 cranks are ALL 38mm stroke (37.66 actually but hey, whos keeping track?), and the con rods are 110mm and 115mm (80mm eye-to-eye and 85mm eye-to-eye). This is why the wrist pins of the Type A/D and type B motors differ by 5mm (Type A/D has a 115mm [85mm] rod with the wrist pin center 21mm from the crown, the Type B has a 110mm [80mm] rod with the wrist pin 26mm from the crown). This sets both the pistons at exactly the same height at TDC and BDC. a 2mm longer stroke (40mm stroke) will give you 1mm lower at BDC and 1mm higher at TDC. This also means the jug needs to be at least 0.4mm taller (to make a deck height of 0.1mm), and the ports need to be separated more to achieve the same timing. That said, the port maps are identical last I saw, the only difference is the Type A/D motors’ ports are 1 or 2mm LOWER than the grubee’s. That means the transfers aren’t nearly as open as they should be, the exhaust is never fully opened and opens later (good for low-end power! this is what gives it it’s grunt!) and the intake opens more fully.  Grubee (Type B) ports are at least 1-2mm higher up.


when you increase compression, the motor runs hotter, as such, the slant head (depending on ambient, I live in the tropics) does not have the ability to stay cool. The 38mm stroke engine does not bring the piston to deck height, as such they use the slant head to raise compression (compared to straight fire). The straight fire head on a 38mm stroke engine without mod will lower the compression. 38mm stroke engine generally needs to have the barrel decked to be able to run an effective squish band.

—- End Notes—-











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