I was looking through some heads trying to find a good candidate for rebuilding for my next project (or maybe put on my 68) and I came across a 71 or later head (it has the rolled pins for locating the rocker boxes) that has what looks like shallow threads (.002"-.003" deep) on the inside of the valve guides. The groove appears uniform in depth and runs from top to bottom. All four guides bores look identical. The valves fit nice and tight in the guides. I'm wondering if it was an attempt to save the guides or if it was ever a factory install. I've heard of people knurling the inside of the guides to displace metal and then reaming them to fit and suspect that this might be the case but the rest of the head looks nearly new. The seats don't appear to have ever been recut (they are not recessed at all) and there was no carbon in the ports. The valves have very little tappet wear on the stem and the seat faces are in great shape. Any thoughts? Do you think it will smoke if used in this state. I hate to change them if they're ok. Haven't had much luck with machinist.