Motorcycle Repair: Comparing Bikes, stroke motors, dirt bikes

Question
Hello eXpert Mike,

My name is Nagy and I am a fellow expert in Electronics, home theatre and satellite Pay-TV.
I live in Australia and I have an import business.
Due to the local demand for Dirt Bikes, I have been communicating with Chinese manufacturers as to source from them.
In return I was confused with all the jargon they used which I doubt that all end-users would know but since they affect the price greatly I would appreciate all help in deciphering them.
So basically, number of engine strokes, number of valves, alloy vs steel parts, normal or upside down absorber, water vs air cooling.
I understand it could be too much for you to discuss and write but it will be equally helpful if you attach some links for me to read, I'm good at it.

THANKS HEAPS

NAGY

Answer
For what you are wanting to know, it would be helpful for you to get some books or look on the web for some info on bikes and how they operate.
Here is the answer to your questions

Engine strokes: There are two strokes and fours stroke motors. The two stroke fires on every other stroke, the four stroke fires on every fourth stroke. The Two stroke has to have oil mixed someway with the fuel, Four stroke doesnt.
Number of valves: Some bikes have one intake and one exhaust valve, some have two of each. It all have to do with movement of fuel and exhaust and how fast it can move. Generally, the more valves, the more efficent the motor is.
Alloy vs steel: Alloy is lighter, but flexes more. Steel is heavy, will rust, not much flex.
Upside down forks: It is jsut what it implies. The forks are redesigned to work just backwards of the normal forks. Personnaly, I dont know if there is an advantage or not, but I do they cost more.
Water vs air: Water cooling keeps the motor a little cooler, works great when the bike is not moving, but cost more, you have to maintain a raidator,  and the motor is heavier. Air cooling is cheaper, lighter motor, no radiator to maintain, but when its just sitting there idleing, it tends to get hot.
Hope this helps
Mike