Here is part of a post from Gus Fischer over on the M14 Forum. Also check out: Organizational Spare parts Kit from TM9-1005-223-20
HTH
Bill
Spare Parts I Recommend for the M-14 
Folks as one who was both a standard Infantry Weapons Repairman (Armorer) when we still used M14’s as our main battle rifle and as a National Match Armorer who walked the lines of many different ranges throughout the world for well over two decades and as one who works on semi auto M14’s for civilian shooters primarily for the last 11 years: I’ve got a slightly different view on what spare parts one should kept on hand from seeing what parts break or wear out the most often. Spare parts to me are those parts that break or wear out most often. I often seen parts listed as “spare” parts that I would call either “repair parts” or “rebuild” parts and these are not required everyone have as “spares.”. I would like to point out that I recommend you buy them in the order listed as well.
So here’s my take from my experience.
Level One – Spare Parts (Minimum spare parts for any use of the rifle)
1. Op Rod Spring (replace every 2,000 rounds)
2. Extractor
3. Extractor Spring and Plunger (comes as an assembly)
4. Ejector and Spring (comes as an assembly)
5. Trigger Pin (These don’t wear out that quickly, but if you take the firing mechanism apart, you can lose this part rather easily.)
Note: If you shoot NM competition, I most heartily recommend you install a new Extractor Spring and Plunger (assembly) and a new Ejector spring or Ejector Spring and Plunger (assembly) every year to ensure you won’t have serious problems on the range. This is more in the way of “insurance” than absolute necessity, though. Also keep and eye on the extractor for wear and change it before the indent for the extractor plunger wallows out or you get a noticeable “saddle shape” indentation wear (not just finish wear) on the post/pin of the extractor. If you replace the extractor when you see this at an annual inspection, you should not have problems for the whole shooting season/that year)
If you have up to three rifles, the only parts you would need more than one to take care of them would be the op rod springs.
Level Two –Spare Parts (Less commonly used parts. Good for up through three rifles.)
6. Firing Pin
7. Bolt Stop spring
8. Hammer
9. Trigger and Sear Assembly
10. Bolt Roller and Retainer spring (If you have a G.I. bolt, bolt roller and spring AND if the receiver is properly cleared for the bolt roller – then these parts become repair parts rather than spare parts. If you have an SAInc bolt, then these should be considered spare parts.)
Level Three – Deep Level Spare Parts (Most people will not need these very often, if at all. Good for up through three rifles)
11. Elevation Pinion
12. G.I. Magazine Catch, spring and pin (If you have a G.I. firing mechanism, then these parts become repair parts rather than spare parts.)
13. Safety and spring.
14. Front Sight Screw
15. Trigger Guard
Level Four – Basic Repair Parts
16. Flash Suppressor Set Screw
17. Flash Suppressor Pliers (Doesn’t do any good to have a spare flash suppressor nut unless you have a set of these pliers.)
18. Flash Suppressor Lock Nut aka “Castle Nut”
19. Hammer Spring Housing
Edited to add: 20. Bolt Stop and Bolt Stop pin
21. Gas Plug and ONLY if you don’t have a G.I. Gas plug in your rifle
Please notice I have not listed parts like the op rod, gas cylinder, gas piston, flash suppressor and other things. Folks, these parts last longer than one or two barrels and they are thus in the REBUILD parts category. I truly believe it is a waste of time and money for most people to buy and stock these parts. If a person wishes to buy them now for many, many years of use, then there is nothing wrong with it.