Steven,
what is your plan, tidying up the valve guide bores ? Sorry for you, that will be a quite lengthy setup , when really a reamer with pilots would do it in minutes. Maybe you could find a GOOD reamer , best with uneven spaced blades for avoiding chatter effects. That might be upgraded with a sort of pilot replacing one nut on it. Once again the Hunger company in Munich has all sorts of handy tools that don´t require a machine. -- Like on big ship engines, you cannot possibly use a machine for head and valve jobs. So they came up with manual tools for same jobs - for perfect results in a fraction of time . In my photo is one of these piloted reamers that locates with its taper face in the 45 degree seat ring and you can ream the bore from the spring side down. When you set up the head on a machine you will be challenged to stay concentric from seat ring to lower guide to upper guide. No question I did that only with Hunger tools , no mill near . Same idea for line reaming bushes in girder forks, no other ways to do that in reasonable time . So maybe you´d like to find a GOOD adjustable reamer , best with uneven spaced blades to avoid chatter effects, and add a sort of pilot on it by replacing one nut with an extension. While trying to remember, I think I used the upper guide as datum for locating the taper of the reamer and have another taper sitting in the seat ring. Between them I reamed the guide bore - or so . . .
Vic
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some more types of Hunger reamers for engine jobs etc .
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