The Canadian supplier:
http://www.motorizedbicycle.ca/bicycle-engine-kits.html
sells the motor alone for $125cdn which is slightly over $100usd this week.
Last month they were selling for $115.
The streaks are from limited contact polishing and take a motorized high speed run up (+1500rpm) to show.
Turning the motor over by hand or even with a starter motor will form the streaks.
If you pulled one of those $5000 motors apart after 5-10 minutes run they would be there.
The streaking is even worse when you use moly coated pistons. Easy to look at the bore on 2 stroke rebuilds.
Factories do not use the same lubes as hand built assemblies, for many good reasons.
They typically wish to use a minimum and usually have an automatic dispersing and measuring system.
The assembly lube itself may be type of wax with an evaporative carrier fluid.
It is fogged on in a measured squirt, engine assembled, run up by an electric motor.
Better QC systems will check run up torque and compression (and oil pressure on 4t engines).
A drip of carrier fluid and wax may settle on the top of the piston with a bit of the phosphate ring coating.
The carrier fluid evaporates during shipment so that a waxy black deposit is left on the piston top.
I don't know if this is the case, but have seen it on other factory assemblies.
I spent the last 35 years working in industry. Machinist, millwright, electrician, quality control tech.
Currently I supply technical people to factories and troubleshoot difficult factory issues.
That the Chinese can build a $100 engine and ship it across the Pacific amazes me. We cannot do it.
It is not just cheap labour either. Labour is typically less than 10% of manufactured goods cost (typically 4%).
If you got the labour for free, it wouldn't take $10 off the price of these motors.
I don't mean to be an apologist for poor quality. I know why they are so cheap, and they won't be forever.
Enjoy them while they are here, for the price they are.
And the quality, or lack of it.
Steve