Well how would it be possible for a bad manufacturer to sneak a dangerously off spec product past an importer who cared enough to carefully examine every batch of product he received?

If it were me importing them, I'd make it clear to the manufacturer that a random two or three percent of the bikes would be entirely dismantled and thoroughly tested (including metallurgical analysis) to see that they met the spec. Any deviance would mean the entire consignment would be rejected. Result? Either the manufacturer would refuse to do business, negotiate a price at which he could make a reasonable profit and supply on spec product, or he'd meet the spec at the original price. Either way the goods would be delivered ok or not at all. Companies do this very successfully. There are loads of excellent and good value products coming out of China as well as a lot of dross. It depends whether the people importing actually care about what they are selling. Surely you are not claiming that the Chinese are incapable of producing quality products? It is almost certain that most of your computer was made in China for a start. My Dell laptop was and that is why the price of low end laptops has dropped between 60 and 75% in the last decade.