My advice would be not to do it. The AR15 was designed to be a direct-gas impingement firearm and trying to fit a gas-piston to it is difficult even when you start from scratch, like the HK416.
Trying to fit a gas piston into an existing DI rifle is much more difficult than that and ends up making a bunch of questionable compromises.
For example:
1. Op rod vs. gas tube: The AR15 has a really tiny hole in the upper receiver because all it needs to do is channel gas through a tube into the gas key.
A gas-piston AR designed from the get go to be a gas piston, still has the problem of size limitations created by the relatively small and short upper receiver; but at least they can bore the hole out a little bigger in order to allow for an op rod that is something close to other gas pistons in strength/diameter.
A conversion has to fit its op rod through the tiny M4 gas tube hole AND still leave enough room that there is no binding of the op rod (not an issue on a direct gas AR as this part doesn't move there). if the rifle gets mud/dirt/debris around this area.
2. Carrier tilt - The AR15 bolt carrier channels gas from the gas tube to the gas key and into the chamber directly in line with the bore. The gas pressure pushes the bolt carrier straight back into the buffer along the bore axis. As a result, there is no need for rails on the AR15 upper receiver (and no way to add them easily either).
When you unlock the carrier by grabbing it at the top of the gas key and pushing back, you not only move the line of force above the bore axis, you cause the carrier to tilt up in front. This has the effect of dragging your top bolt lugs across the top of the locking lugs and dragging the back of your carrier across the bottom of the buffer tube (and buffer retaining pin).
With a purpose designed AR gas piston, you can minimize this (though even here you can't get rid of it completely) by using a bolt carrier that is fatter in back and an upper that is relieved enough to provide a working tolerance despite the fatter bolt carrier. You can also make a deeper relief cut in the bolt carrier and change the buffer retaining pin so that it is better protected from the inevitable tilt - not to mention you can use an entirely new barrel extension/bolt to address tilt at the front end.
Using an existing direct-gas upper, you are very limited in what you can do and you are going to see carrier tilt - it will just be a question of whether the carrier tilt is worse than whatever DI problem you were trying to solve. Probably not since you'll be using a standard AR bolt/bolt extension and that has some engineering limitations that make it less than ideal for a gas piston.
If you really have the need for a gas piston AR, I would recommend buying an upper that was designed from the start as a gas piston rather than converting an existing DI system.
Even better, if you really are sold on a gas piston; buy a rifle that was originally designed to use one. A gas-piston AR is never going to be as reliable as something like an AK because the gas-piston AR is always going to be limited by the original AR geometry (which was designed to be for direct impingement).
Most of which is moot anyway... your average bargain-bin, direct impingement AR that is run dry by someone who doesn't even understand the system is still more reliable than 90% of people will ever notice.
Also just saw this thread, which you might appreciate:
http://www.thehighroad.org/showthread.php?t=456447
The other thing is that you need to open up the gas orifice a considerable amount to get enough gas flow to run the piston. Your best bet and my recommendation is to purchase an upper that is already finished like Primary Weapon System's. It's not worth the headache and you can't go back once you start alterations. Just my $.02
