Rev: I have TFS TW's too and have had no problems with valve guide issues. From what I understand, TFS corrected this valve guide problem a few years ago (like right when I bought mine). The heads they offer now should not have this problem.
This problem was really bad with the early castings, but now you really don't hear much about it since they have made the changes.
Now go get those heads and start whompin' on some cars!!
E
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1991 5.0 LX Coupe - 38,000 miles
13.17 @ 106.14 mph w/ 2.138 60'