The "absolutely minimal valve movement" and the ribbed valve guides tells me they may have been replaced recently, which is good. The genuine Porsche guides are smooth all the way up and down the outside, save for a very fine grooved pattern encircling the outside, left from turning them on a lathe, or however they machine them. The aftermarket Canyon brand you can buy anywhere else has 3 or 4 larger ribs on the end to help grip the seal.
I used a deep socket (13mm or close) to push them on. Make sure all of the old seal material is scrubbed off the guide or it can tear away the rubber on the new onewhen installed. The vitton rubber will come apart fairly easily. The socket edge should make contact only on the outer metal ring. Smear some engine assembly lube or moly-graphite grease on the end of the guide and inside the seal to help it slide on and make sure you push them on as squarely as possible. Just push firmly and steadily, rocking the socket around the seal in small circles helps to work it on there if it fits really tightly. Just go until the guide makes contact with the little moulded step inside the seal where it begins to converge down to the smaller valve stem openning, but not squished against it so hard it starts buldging the rubber out the top of the metal ring, or the seal lip won't sit correctly on the valve stem.
You know how this "good" thead ended, right?
