There is a lot of misinformation out there on this subject. The reason for this is because the clutch and how it is wearing is not an easy thing to inspect. You have to go by how it feels and that is very subjective and also depends on the clutch you installed and any other mods you may have installed at the same time like a lightweight flywheel.
If you want to just see a quick and dirty break down of the main types of clutches and their break in procedures and pros/cons then click here.
There are 3 main theories when it comes to breaking in a clutch.
#1 Break it in hard and fast and just beat on it from the start. This can work okay in certain circumstances depending on the parts you changed and the kind of clutch you are using. If it is a ceramic clutch you can usually start beating on those very soon after install. If you are installing an OEM style or stage 1 organic clutch and a brand new flywheel I would avoid the hard and fast method. This combo needs time to bed in to be smooth. I cover this more below.
#2 Drive 500 miles in the city with stop and go traffic. This more often than not lines up with what the clutch manufacturer suggests as well. You need to do at least 500 city miles or roughly 1000-2000 highway miles before your clutch will be mated to the flywheel and pressure plate surfaces. This does not mean that at mile 501 you can blast it and go WOT. This means that on average you should be clos to done breaking the clutch in by this mileage. How many highway miles you do greatly affects it as highway mileage does nothing for breaking in the clutch.
#3 Then there’s the idea that what really matters are heat cycles. A single heat cycle is when you go out and drive the car and get everything heated up to normal operating temperature and then park the car and let it cool off. I have heard you need ~10 heat cycles to bed a new clutch in but from personal experience you will need more than that.
How Much Slip is Too Much?
You will hear anything from “give it a few good slips and it’s ready to go” to “try not slip the clutch at all if you can”. Those and everything in-between are true because it again comes down to the exact type of clutch you have. With a ceramic clutch usually you really do only need a few good slips to bed it in, but those and maybe sintered iron are the only ones. You likely won’t deal with sintered iron as that is usually for race applications and they don’t care if it lasts more than a few races.
Slipping the clutch is normal and you need to slip it a bit to get started usually. When you replace your clutch this is still true. Drive it normally and slip it a bit to get going. Where you run into issues is with excessive slipping of the clutch. This causes excess heat and when the clutch is brand new you can glaze the surface and if you’re lucky you will end up with a clutch that doesn’t hold the rated power, if unlucky you will need a new clutch again in short order.
I have a New or Resurfaced Flywheel, What Now?
You have to take some more care when you have a brand new flywheel in the car. First of all make sure the flywheel is balanced correctly. Most of the time the manufacturer does this and you can tell it was done because little circles will be drilled out of the flywheel somewhere. This means they removed little bits of weight to balance it. If you do not see this you may need to find a machine shop to balance your flywheel. This is important to have done and should only cost you about $60.
The reason why you want to take more care when you have a new flywheel on the car is similar to the reason you need to bed new brake pads in when you have new or resurfaced rotors. The flywheel needs time to have the material from the clutch deposited onto the surface and to build up an even layer of this material and “learn” the other surface as it works in.
With a lightweight flywheel this can change things even more depending on the type. When you have an aluminum flywheel you should be more careful and try not to overheat it during the break in. They are much more prone to warping and If you are in this situation then definitely follow whatever the manufacturer tells you to do.