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In my 1935 Hudson 8 I have four exhaust valves that are 3/8". They were in the car when I bought it with about 113,000 miles. The car now has 156,000 miles. I have not had any trouble with wear of the valve guides, either the 3/8" or all the rest which fit thinner valve stems.They are not bronze guides. I did a valve job on the car at about 120,000 miles, when I replaced all the valves. I did another valve job at 150,000 miles. I found that only the four 3/8" exhaust valves needed replacement. I attribute this to the thick stems being heaver, and not quite closing when driving at 50 to 55 miles per hour, which I did a lot. Therefore I recommend 11/16", not 3/8".