1983 380 SL
I only reamed one... the one that was sticking when I pressed the valve tightly against it's seat. Also, even though I used a 6 flute reamer, the general thought is that it doesn't make a perfectly smooth round hole. However, when you drive a ball broach of precise dimension through the guide it makes it smooth on the inside and more perfectly round (there's no such thing as perfect).Looks interesting, but if you already reamed to 9.00 wont it just pass right through. 9.00-8.99 = ~ 0.0004 inch, not much.
I've done a bunch of reading on this topic and from what I can tell, you first install the guides, then ream them to get to a precise size and then run a ball broach through them to make them more smooth and as close to perfectly round as possible. The ball broach also makes the interference fit tighter.
I test fit each of the new valve guides using the existing valve stems and the fit was perfect (as close as I could tell). The very act of pounding them in with an air hammer creates some distortion which is first corrected with a reamer and then perfected with a ball broach. Inasmuch as I don't have a 8.99mm reamer, and in consideration of the fact that all the valves still slide smoothly inside the new guides once they were installed, I'll skip the reamer part and go directly to the 8.99mm broach. If the broach slides through without resistance I'll at least know that the clearance is good. If I can't get the broach through the guide then I'll need to spring for a 8.99mm reamer, but the difference between a 8.99mm and a 9.00mm reamer is .01mm or .000393 so I don't expect that I'll have any difficulty getting the ball broach through the guides that weren't reamed.
There is a third step which I am omitting entirely... core drilling. Core drilling is used to make the hole in the head where the valve guide is pressed in and the valve seat circumference, concentric to each other. I have no way of doing it so I'll just have to accept any out of concentricity issues that may arise and hope that the hand lapping of the valve seat to the valve will be good enough. If it's out by more than a few tenths then hand lapping will not likely be able to correct it in which case the valve may not seal perfectly. I'll paint layout blueing on the valve seats when I hand lap them and that will enable me to determine if the valve is contacting the seat uniformly or if there is a concentricity issue. I'm not sure what to do if the concentricity is way off. I'll cross that bridge when I get there but I must assume that the valve guide holes in the heads were already core drilled at the factory to be concentric with the valve seats and given that I'm not replacing the seats it's a good bet that they are already concentric.