Difficulty scale for "how to" articles

If you don't have much experience working on cars, adjust the difficulty and time up.  If you are experienced working on cars, adjust the difficulty level down.  Do you have a safe, level space to work?  Do you have free time to deal with possible complications?  Are you the kind of person who double checks their work and does research beforehand?  If you think you can do a good job by skimming instructions or do not have a factory service manual then don't work on your car!

For the price of taking your car to the dealership for 1 major job, you could buy a set of tools that will let you to work on your cars forever.  Be thorough, read the tips on this site and the factory service manual a few times until you are familiar with every step, double check your work, and you should be able to complete the work successfully.  Here are some useful tips to get started: 1000 answered questions: garage organization and mechanic's tips

Disclaimer: this website and any information you find here, both in the FAQ or forum, is not a substitute for the factory service manual or the supervision or work of a certified mechanic.  Any work you do is at your own risk.  Take all normal safety precautions in addition to those listed in the factory service manual.  See the Terms of Use Agreement for the full legal disclaimer.

Difficulty scale:
This job can be done by almost anyone and should always take less than a few hours including prep and clean up.
Examples include: changing the headlight bulb, car detailing

2.  Factory service manual is required.  The entire job should take less than 4-5 hours including prep and clean up.
Examples include: oil, coolant, transmission, or brake fluid/brake pad changes, changing the wheels/tires.  

3.  Factory service manual is required.  This job requires research to be done correctly.  If there are no complications, plan to take up to a full day including prep and clean up.  With complications, plan for up to a weekend.  A machine shop is not needed but you may need specialized tools.  It can be done by someone with no prior mechanical experience as long as they follow the instructions in their factory service manual.
Examples include: changing a wheel bearing, removing the transmission, timing belt change.

4.  Factory service manual is required.  This job requires more time, research, and requires specialized tools.  These projects will take more than a full day for most people including gathering parts, complications, prep, and clean up.  This is a challenge if you have only basic prior mechanic experience and it will take more time, but it can be done as long as you follow instructions, do your research, and plan ahead.  If you have only basic mechanical experience and successfully complete this project, upgrade yourself from "total newb" to "can speak from experience".   If you have some solid experience working on cars and a well stocked tool kit, go for it.
Examples include: cylinder head replacement, clutch change, removing the engine

5.  Factory service manual is required.  This is the limit of what a shadetree mechanic can do in their driveway.  This job requires outside help, such as from a machine shop.  You should have some prior basic mechanical experience or do research beyond the factory service manual.  Ask around for tips and common problems.  There is a high probability of complications and delays and it will take multiple weekends.
Examples include: rebuilding the engine, converting a donor gasoline car to diesel with "bolt ons"

5+.  If you don't already know how to do this job, don't bother asking how because you need to understand the basics just to ask the right questions.  There will be at least a few major delays and complications.
Examples include: custom engine tuning or building, custom turbo fabrication.