You can run your designed for leaded fuel engine on unleaded for quite a
while, depending on the way you drive the car. High cylinder head
temperatures caused by say towing a caravan, or really hard driving will
tend to accelerate valve seat recession. The lead in petrol was originally
added to help prevent knocking, (pinking or detonation depending where you
come from) i.e.to uprate the octane number. However it also had the dual
secondary effect of providing a very thin lead coating on the exhaust valves
and their seats, thus largely preventing the heavy wear on the cast iron
exhaust seat and also the thin lead coating will help to lubricate the
exhaust valve stem.
If your car has run for some years on leaded there will have occurred a
build up of lead on the valve seats and stems which will take many miles of
running on unleaded to dissipate, in other words your cylinder head has
built up a "memory" of lead.
The conversion to unleaded involves replacing the exhaust valves with valves
made of a nimonic material very much more heat resistant, and machining a
recess in the cylinder head into which you shrink a new valve seat made of
stellite material. The new valves and seats are much harder and more heat
resistant than the old ones so the lead lubricant/heat conductor/anti cast
iron recession properties are no longer needed.
IMHO you can run on unleaded until you suffer valve seat recession which can
no longer be corrected by tappet adjustment without a performance loss. Then
you can fit the new valves and have your head fitted with new exhaust valve
inserts.
There is no need to do anything with the inlet side as it runs so much
cooler.
FWIW, there are documented cases of people driving their six cylinder Rovers
more than 25k miles on unleaded without any adverse effects, and other
people who tow caravans up the Pirynees (sp!) mountains suffering valve
seat recession in a very short time.
HTH
David Brister
1967 TR4A
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