Valve guides.

kerinorton said:

4.2 l Jag engines would never do more than 60,000 miles without rings and exhaust valves [ valve stem wear ]. And probably wouldn't do much more than that before cracking heads as well. But you know what the original Jag owners were like. Drive it till its stuffed and couldn't care less, until it came time to pay the bil,l then the shit would hit the fan..

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Before the war (WW2), the british military got tough with bike makers - and all suppliers of vehicles.
They stipulated they wouldn't take any bike that couldn't pass their 10,000 mile test if the engine/drivetrain couldn't survive.

This required most makers to do some serious testing to meet this spec.
Cylinder material, rings, pistons, big ends, bearings, sprockets, clutches, you name it.
These tests involved considerable mud, dust and water exposure, so some of it was sealing and dustproofing things, etc.

The original Spitfire RR Merlin engine had a design life of 20 hours of combat use. (full throttle).
With experience, this was extended out - considerably.
Much of it of necessity...

https://en.wikipedia.org/wiki/Wright_R- ... ex-Cyclone

By this point reliability had improved with the mean time between overhauls at 3,500 hours and specific fuel consumption in the order of 0.4 lb/hp/hour (243 g/kWh, giving it a 34% fuel efficiency)

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Prewar there was considerable nervousness about (airliners) using anything with only 2 engines,
they just weren't reliable enough.
Trimotors rule OK ?

"Developed before World War II, the R-3350's design required a long time to mature before finally being used "

We diverge, muchly though....