I look at engines as just another "kit" to put together before it can be used.

It is very common to find chips, old coolant, and/or tapping oil that were never properly cleaned off the parts before it was put together, and doesn't make a very good first gulp the engine takes on its first fire-up.

Loose screws from the factory should be a total imbarresment for the company, but probably is the most common manufacturing defect.

Maybe that is why some instructions advise checking the tightness of all the screws after its first use...they are too ashamed to say "check them but only after we are ruled out as the culprit".

Heat expansion and contraction on model engines is common for making steel screws to loosen or freeze up, but finding them two turns out after you find the engine doesnt run the first time isnt good.