Heavy damage items, especially on profile planes. Then you start the "rob Peter to pay Paul" routine....

Hello The classic missing needle often occurs where people remove the needle from an engine that lives in a box, so it will fit in without poking a hole in the box lid. It gets lost and you get a new engine in the box with no needle like an ETA 29 I have, at least reproduction NVA's are being sold on Ebay but pricey.
Probably more common are the broken or bent needles in used engines that are missing , the owner probably intended replacing, but never did.

Missing prop drivers especially on splined or loose keyed shafts are annoying too. (I epoxy them on the worst offenders to save loosing them when using an electric start as they often shoot off to never be seen again).

Regards Gerald

    I think the reason for the missing parts is that even though there may have been more  people participating in the hobby back then, it doesn't mean that most of them knew what they were doing with engines!! If they couldn't get one to run they took it apart to "find the problem"!! When they couldn't find anything, they shoved the parts into the box and set it aside. The same reason is why a lot of these old engines don't have hardly any time on them. The previous owners either couldn't get them to run correctly, and they survived a first or second flight crash intact. I had a great discussion with Al Kelley from Merlin Glowplugs about this several years ago.
  Type at you later,
   Dan McEntee