Tools needed: Torx-10, Torx-15, Phillips-driver, thermal paste, spatula for paste

I marked all important parts red (see pictures).
First of all you need to put the iMac on a soft surface (I used a wool blanket, although this is not a good solution, as static electricity can damage electronic components.)

1. First you must remove the "User Access Plate", simply remove the 4 screws with the phillips driver. Photo below is with User Access Plate removed. Airport (802.11b) card and USER (sodimm) RAM slot shown. (Apple's Customer Installable Parts (CIP) guides already cover installing the Airport card and RAM in the "User" slot there.)

2. Remove the marked screws with a Torx-15. Then you can (carefully !!) take off bottom housing. It's a little bit tricky because there is a connector (see photo below - it's on the right side, when you look straight ahead onto your iMac) which fits pretty solid, but when you just pull strong enough on the bottom plate it loosens (but gently, please)

2.1 Now you can replace/upgrade the "factory" Internal (full-size dimm) Memory module, if thats all, go to step 6. (FYI - iMac G4/700-800MHz use PC133 ram (typically up to 512MB modules available). The iMac G4 1GHz-1.25GHz models use DDR333 RAM, up to 1GB dimms.)

One note to the iMac-TFT-take-apart-article. I wasn't able to get the drives out until I removed the little black plastic bar that is glued onto the front of the CD-drive-tray. It's easy to remove with a sharp knife and easy to glue back on after you have installed the drive.
-Goetz"

(Other iMac owner said removing the bar wasn't necessary)

Before you remove the "little black plastic bar":
I found that if you remove the four screws that hold the DVD/CD drive to the carrier you can slide the drive back away from the opening in the case and easily lift out the carrier - without having to cut off the plastic bar mentioned above. Two of the screws are shown in the photo in section 2.1 of the article on the left side of the drive carrier, and again in the second photo in section 6, where they are highlighted. There are two on each side of the carrier.
(he also sent other comments, see bottom of page)
-Winston W.

(and another reader said)
Just to let you know, you don't need a sharp knife to take off the plastic bar that is glued onto the front of the CD-Drive-Tray. If you gently work out the Hard Drive assembly from the opposite side of the CD-Rom, ass first, if you will, it will remove quite easily.
I wish I had taken pictures!
-John M.

3. Now you got the drive carrier unit right in front of you. Disconnect the IDE-cable on the motherboard. Then remove the 2 screws from this silver thing (kind of faraday cage - on the top of the picture). Then remove the 4 screws holding the drive carrier in place (it's all Torx-10s). Now take the 2 cables out of the cable clip (marked blue on the picture). Just press on the left side of the clip and it opens.

4. You're ready to take out the whole drive carrier. To do this you must hold it on the bottom and pull it up and out.

(NOTE: In Dec. 2003 an iMac G4 owner posted a warning in a drive DB report.)

" ...In removing the drive chassis assembly, the video cable from the motherboard going up into the 'dome' was clipped to the assembly. I didn't catch that in time and proceeded to pull the video cable out of the plug on the motherboard, bending the pins in the plug. I tried to straighten, only to break two of the pins (which I recovered from the case)."

5. Remove the power cables from the harddisk and optical drive. Then remove this silver "faraday-cage-like" thing by removing the copper stripes from the drive (they're sort of glued on, but you can pull them off...but be careful, they are sharp as hell, I just cut my fingers).

Afterwards pull the IDE-connector off the optical drive. Now you can remove the four (Torx-10 again) screws from the drive carrier, so you can slide out the optical drive and exchange it with another.
(Note: If you're replacing the Optical drive with another model, check the size of the new drive's front tray edge bezel - if it's too large, remove it to avoid it hanging on the case when ejecting the tray. If in doubt, remove the tray bezel as most all OEM drives apple has shipped in systems since 2002 have no front tray edge bezel on them. Eject the tray (using a paperclip in the eject hole, etc.) and look under the tray front edge/bezel - usually there are retaining tabs that hold the bezel on the tray. Lifting the bezel edge outward and pressing up usually slides it off the tray.)

If you want to exchange the hd, just remove this white wrapper and the 4 screws from the HD-carrier and take it out of it's place.
When you install a new optical drive, put the copper foils earlier removed on your new drive. Now put everything together again.

6. Before putting the bottom housing to the rest you must remove the old thermal paste (red marked on photo below) and apply new paste. Remember to first scrape off all of the old paste from the surfaces with a soft tool. Next, simply apply the paste evenly to the area highlighted on the photo below, without getting any in vent hole itself. Remember, this is how your iMac keeps cool so be sure to do this carefully. (you do not need to go overboard with the thermal paste - it's just to fill in the surface imperfections in the mating surface - not build up a thick layer)

Notes on Optical Drive Lengths:
I wanted to exchange the stock superdrive in my iMac 800 with a Plextor 40x CD-writer. It didn't work because the superdrive is about 7.67 inches long, while the plextor is 8-9 inches long. So check out the length of the drive you want to install before buying it.

iMac G4 Big Drive Support: Just a FYI that in 2003 several owners have noted that the iMac G4 has native 48bit addressing support for "big/large" IDE drives (those over 128/137GB). Apple's kbase doc notes only the MDD G4 and Xserves have native support (without using a 3rd party driver or PCI IDE ATA/100-133 controller) - but as noted in the main site news page and drive upgrade database reports, several iMac G4 owners noted they are running 160 to 200GB IDE drives inside their iMac G4s with full capacity available. (The drives as usual format to slightly under the 'rated' capacity due to the usual binary vs decimal numbering system noted in the FAQ years ago - but without 48bit/big drive support, a large drive would be limited to appx 128GB even with multiple partitions.)

Update: I don't own an iMac G4 (and there may have been some changes from this original article which was based on the first iMac G4 model - some 2003 models now have FW800, etc.), but here's other iMac G4 owner comments in reply to Stephan's article above:

(mail dated May 5th, 2003)
" I followed Stefan Horne's excellent "iMac G4 - Exchanging Optical Drive, Hard Disk, RAM" (this article) this weekend, replacing the hard drive and optical drive with a 120GB Maxtor and a Pioneer 105. I'd like to suggest some additional wording for the section on the optical drive install. Stefan's wording implies that this is pretty simple.

When you install a new optical drive, put the copper foils earlier removed on your new drive. Now put everything back together again.

It isn't that simple, though. My suggestion:

When you install a new optical drive, you'll want to set it up as "Master" if it's a Pioneer 105 or other iDVD-compatible drive, and the hard drive as slave. (Setting the drives to "Cable select" will result in the same setup.) (Note: generally with Macs that have both HD and Optical drive on the same cable (only one IDE bus), they have the HD set to Master. I'd Suggest NOT changing the Hard Drive Jumper setting - look at the old Optical drive you're removing and use the same jumper setting on the new drive that replaces it. (Jumper settings usually marked MA/Master, SL/Slave, CS/Cable select - normally iMac G4 optical drives are Slaves IIRC.)
A reader also mentioned that when using Firewire Target Disk Mode to transfer data to his new iMac G5 didn't work if the iMac G4's HD was set to slave, referring to an Apple kbase doc FireWire Target Disk Mode: Target Computer Shuts Down at Startup (no longer online) which said a Hard Drive must be on "Bus 0", but the iMac G4 has only one IDE bus (not two as many other Macs have). "Bus 0" isn't the same as Drive "ID 0" (master). (Drive -ID's- are 0 for Master, 1 for Slave.) OS X 10.2.4 had a problem not recognizing Superdrives if they were set to slave (noted back then in the news page) but OS X 10.2.5 fixed that issue.
In the past I have seen FW target disk mode *not* mount a Hard Drive set to slave on a system where I had 2 Hard Drives (one master, one slave) on the primary bus, although that may no longer be a problem for later macs, personally I'd set the new Optical drive to the same setting as the drive you're removing.
Starting with the MDD G4 towers, Apple suggests cable select for all drives, HD or optical. This automatically makes the end drive on the cable a master and the drive on the 2nd connector a slave.)

You'll notice that the bezel on the optical drive in the machine, along with the appearance panel at the leading edge of the tray, was removed by Apple in order to make the drives fit into the minimum available space. (The front tray edge of optical drives are also removed on the MDD G4 towers to clear the opening in the case.) You need to remove the bezel and the appearance panel before continuing. If you're careful to avoid damage, and you store the plastic parts in a safe place, you may be able to keep the drive warranty in force.

First, use a paper clip to extend the tray. Drive trays vary in design, but the usual arrangement is that there are a couple of clips on the lower edge and a channel on the upper edge attaching the appearance panel to the tray itself. Flex the plastic so that the clips on the appearance panel clear the edge of the tray, and slide it up and off.

The bezel is typically attached at four points around the perimeter of the metal case. All you have to do is depress the plastic tabs so that they clear the metal, and pull the bezel clear.

After all is installed and running, extend the tray in the normal way, and reattach the black plastic trim piece that you removed earlier to the leading edge of the tray.

Finally, the copper foils that you removed earlier can be placed on your new drive. Not sure what they add to the equation (heat dissipation?), but you'll feel better knowing that they're there.

Now put everything back together again.
-Robert C. "

Other iMac G4 owner notes:

" Differences from the photos (above) and description I found in my 1Ghz G4 iMac:

The fixed connector that makes it tricky to remove the bottom housing, shown in section 2, is not present in my 1 Ghz iMac G4. This makes removing the bottom housing (which contains the motherboard) much easier than the article describes, although it still needs to be done gently. The fixed connector has been replaced with a small bundle of wires that connect to the motherboard.

The IDE power cable is now behind the drive connection cables (on the outer edge of the motherboard) instead of in front of them. The thick black cable (video?) toward the bottom of the motherboard in sec. 2.1 now connects to the left (below in sec. 2.1) of the IDE data cables, in the same area as the other cables. Only the power cable now stretches across the motherboard.

There are now two silver heatsink bars instead of one, creating two spots that need to have the thermal paste replaced. The second spot is at the end of the memory card (at the top of the memory card as shown in the sec. 2.1 photo). The second silver heatsink bar is parallel to the first one, and is also under the silver "strap" that runs diagonally across the heatsink bar.

Some of the connectors are hard to get off, especially the IDE power cables. I could not figure out how to remove the thick black (video?) cable from the motherboard, nor how to detach the power cable from the lower housing. Both of these, plus one very small black wire that has an inline plug, need to be removed to fully separate the lower housing from the upper housing.
Sorry I don't have a photo to send. Will do so if I can get one.
-Winston W. "