| I've never fitted a set to a 916/996/998 etc but I don't believe they are any different from anything else. You will need a long drift about 9-12" long, a length of 3/8" or 1/2 rod is ok. Lay the wheel on it's side and start by knocking out the bearing at the bottom. There is a tubular spacer between the bearings that will make it difficult for the drift to get a hold but by angling the drift so that it leans diagonally across the space you should be able to get a notch for it to bear on. Use a fairly heavy hammer, say 2lb and tap gently until the bearing just begins to move. Then turn the wheel or move the drift so that the drift is now contacting a patch 180 deg opposite and tap again. keep this up, rotating the contact point every mm or so until the bearing drops out the other side. Once the bearing starts to move you will find it much easier to get an edge for the drift to sit on.
When the first bearing comes out the spacer will come out behind it. Then turn the wheel over and knock the other bearing out. This should be quite straightforward but remember to keep rotating the point of contct for the drift so that the bearing comes out squarely.
Fitting the new ones is easy. Use a socket as a drift that is approximately the same diameter as the outer race of the bearings. Knock in one bearing until it is fully seated. Turn the wheel over and insert the spacer then knock in the second bearing until it is fully seated - job done.
Always make sure when removing the old bearings or fitting the new ones that they remain square in the housing.  | 
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