On the valve guides, I generally go by
Rowe's recommendation, which is:
"1: Clean carbon from old guide O.D. before removal.
2: Guide to head press fit .0015" / .002" (head smaller than guide).
3: Heat head to approximately 200° - 250° F.
4: Install the guide at room temperature (Head hot & Guide at room temperature).
5: Let head cool to room temperature before final sizing of the bore.
6: Determine desired clearance valve to guide. Recommended clearance Shovel Head with cast iron guides
Intake .001" - .0015" Exhaust .0015" - .002" (Ampco-45 add .001")
7: Select the correct plug gage by adding the valve stem diameter and clearance desired for the finish diameter.
8: Select a plug gage .0005" under finish size for the other end of the handle.
9: Finish hone the bore to size. Check the size as you hone with the -.0005" under size pin gage, finish hone
to size with the finish plug gage pin.
10: Cut or Grind the seats, lap the valves to seats and check valve seat concentricity before assembling valve springs.
11: Use a good assembly lube.
12: Use valve stem seals. Factory OE seals will stay tight with guide top size +.005" to -.015" from published size.
13: After the head is assembled and still on the bench check for any leakage valve to seat. "
I have been using those specs on my Bonnie (.0025" intake and exhaust ) with Ampco 45 guides successfully. Just for the record, Bernie Nicholson used to recommend replacing guides that were worn .003" or more.
I had problems in the past with sticking valves until I began using plug gauges and reaming the ID after installation to remove any high spots, so I wonder if the original factory figures used to specify a bit of extra clearance to allow for the fact that guides tended to contract bit under the interference fit, and very few mechanics used to ream or hone guides after installation back then. Certainly the tighter fits are less forgiving of any high spots, but they sure do last longer, and the tighter fits seem to lead to longer valve face life as well, with less burning.
I guess, like everything, this is all just my opinion, but, I figure Rowe is very experienced, and he essentially agrees with Kibblewhite, our guide supplier, who recommend .001- .0015" on intake and .002" on exhaust as a good minimum safe clearance, and the .0025" compromise has worked well for me.
Just for interest sake, Triumph service bulletin 3-80 of February 1980 stated that "valve guide bore diameter specification is now reduced from .3127/.3137" to .3120/.3130" before installation," and that "average closing -in of a valveguide bore after assembly into a cylinder head is .0005"