China Girl Engines

February Update:
Many retailers are flooding the market with cheaper / illegal 48cc motors that look like a 66cc PK-type engine. They are definitely not the 66c motors pictured in ads / online listings. The "Only Black" or "Only Silver" replacement (no kit) motors are really a low-performance HT 1.6 hp @ 5K version. Ask me how I know :)
1. Many of these 48cc motors do not have the required EPA non-compliance sticker.
2. These are disguised by a rectangular - slant head; measured bore is actually 40mm,
    with a 38mm stroke. The castings are ROUGH. Don't be fooled!
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Main Article

Without getting too technical there are two primary types of China produced HT,
two-stroke bike kit motors, aka the 'China girl'.

The original de-rated 'Happy Time' or HT bicycle motor: 48cc 1.6 hp @ 5,000 rpm.
Bore & Stroke: 40mm X 38mm = 48 cm3.
Early 48cc 'China girl' HT kits were introduced to the US in 1999.
Importer, Don Grubee did much to popularize bicycle motor kits.

History:   "In 1999, after using an engine on a bike in China, Don Grube brought a 48cc kit home to the USA for his own use. He installed it on an old 15 dollar used bike and one trip around Lake Heifner in Oklahoma City proved he had a winner.  Folks loved the bicycle power idea and the unique engine "putt-da-tee" "putt" "putt" sound! Unexpectedly, Don never made it home with his engine powered bike.  A new name was born when an observer noticed and said: “I want to buy one like that, where did you get that cute little "Wing Ding"? The man bought it for cash money on the spot!!!.
The idea to go into business became reality. < Gru-Bee inc. > was founded and incorporated in 2001. Star-Fire & Skyhawk brand engine kits were distributed in the US out of a warehouse in Oklahoma City USA until 2005. "

This motor is usually a round finned, small head version compared to the larger 66cc 
Skyhawks and PK-80 rectangular / slant head styles.
Their main distinguishing feature: 6mm cylinder / case studs.
Early HTs use bolt-on flywheel balance weights & suffer from heavy vibration. 
Both engine types possess chromed cylinder bores.
Caution: if you encounter such an engine it must be labelled as to its true EPA status;
"This gasoline engine is not in compliance with EPA regulations".

48cc Silver - 66cc Black compared

Late rectangular / slant head motors include Skyhawk, Grubee, Stinger, PK-80 RT-80 are nominally listed as "66cc."
66cc is a marketing ploy. Actual swept volume must be within the limits of federal bicycle law. Piston: 47mm Stroke: 40mm
The latest 66cc (actually 69cc) motors appear similar to a 48cc and contain important differences like better rods / bearings, needle bearings; 8mm cylinder studs, longer stroke, Speed carburetor.
Power transfer / clutch details remain virtually the same as 48cc models.   

PK-80 Balanced Motor

The '66cc' China PK-80 & Skyhawk, Dax-RT motors offer better materials and many are 'balanced'.
That is, crankshafts are full circle & hand drilled to reduce (offset) vibration.
Otherwise external castings are similar, with later units appearing slightly 'cleaner.'
The larger engine has black allen head case screws.
Generally the later engines will rev to 9000 rpm which means they're rated 2.75 hp. 
That's still only 0.65 hp. per cubic inch stock.

China HT motor advantages: 
A simple design, low initial outlay. Kits are relatively lightweight, cheap and easy to install. Carburetors are simple and fairly reliable. They're good on gas, with fuel consumption ranging from 120-150 mpg. There's an active aftermarket providing upgrades.
A 48cc motor will rev faster than a 66cc unit. Actual HT power tops out at 1.9 hp @ 5500 rpm, depending on oem jetting / exhaust.
Kit equipped bikes are fun to ride.



China HT Disadvantages: 
Lack of real data, sketchy dependability, poor quality, vague / lacking power figures.
Inferior materials, no warranties and tricky marketing continue to dog the MB world. 
Two strokes are messy, they exude black sooty "stuff" from their exhausts.
Lastly, you have to mix fuel.