We have been modifying TR-4s for unleaded here in California for going
on to 20 years now. The guides that we use are a molybdenum bronze material
but we have also use phosphor bronze and aluminum-silicone bronze. We
recently did a 1956 AC ACE also. The ACE was set up with the clearances
that you mention in your posting and had valve stem seals also. We have
problems with "squeaking" from the inlet valves and guides until we removed
the seals! The coefficient of expansion of the bronze alloys is so much
higher than that of the cast iron head that as the temperature goes up the
valve stem to guide clearance goes down. We have actually had exhaust valve
seizure problems on a hot engine when the valve stem clearance is less than
.002". What we normally use and have had good success with is .0015"
clearance on the inlet valve stem and .002 to .0025" on the exhaust valve
stem. These are run without valve stem seals on TR-3s and TR-4s that have
had the rear camshaft journal modified to increase the amount of oil
supplied to the cylinder head and valve gear. With these clearances and
using stainless valves with hardchromed stem we have had neither oil
consumption nor seizure problems and have had great durability even under
extreme racing conditions.
Santa Cruz, Ca.