Connie

The load-spreading explanation makes sense of something that confused me — all the clips failed on the same day rather than one at a time, which I had put down to a bad batch.

The one thing genuinely worth understanding for question 2 is carbon and unburnt fuel.

At idle the combustion is cooler and less complete than under load. Over many hours that deposits carbon in the exhaust path and lets a small amount of unburnt fuel past the rings into the oil, thinning it slightly. Neither is a problem in a single event — this is a cumulative thing, the reason short-trip-only cars get sooty and taxis used to foul plugs.

The fix is free and takes twenty minutes: take it for a proper drive. Get it fully up to temperature and give it some sustained load — a dual carriageway rather than a crawl through town. That gets the exhaust hot enough to burn off what was deposited and drives the fuel dilution back out of the oil. Do it today rather than next week.

That single drive is most of the answer to question 3.

Pitch is the distance from one thread crest to the next, along the length of the screw. Two bolts can both be M5 and have different pitches, and they will not go into the same hole — which is exactly what you felt when it started and then bound.

Every metric size has a coarse pitch that is the default, and one or more fine pitches that are not. For M5 the coarse pitch is 0.8mm and the common fine one is 0.5mm. Written properly they are M5×0.8 and M5×0.5, but shops routinely print just M5 on the coarse one because it is what nearly everyone wants.

The useful table, because these are worth knowing by heart:

If a packet says only M5, it is coarse. If your hole is fine-pitch — common in anything automotive, and in adjusting screws where precision matters — you have to ask for it by name.