Frank---For the daily or occasional driver who doesn't spend much time
over 5,000 rpm, not much valve seat recession should be expected. I,
too, once followed the line of thought that 'no-lead meant the valves
were sure to recede into the seats, so I added the lead substitutes for
a few tankfuls. I mean, some of our faithful parts suppliers warned us
of this, right?! Of course they wanted us to invest in their hardened
valve inserts, stainless valves, etc. It dawned on me that if recession
is going to take place, it will show up in REDUCED clearances, over the
miles, between the stems and rocker pads. In short, if you set the valve
lash at a certain clearance, and this measurement either stays the same
or 'grows', the valves are NOT receding. Actual inspections of the seats
over the last 100,000 miles of 'no-lead' confirmed that no undue wear
has taken place. I would just use the fuel available that satisfies the
octane appetite of your engine. The compression ratio of a stock '70 TR6
would call for about a 90 octane, using the combined rating that we use
in the States. As always, ignition timing and compression ratio plays a
big part here.
Dick Taylor
'73
California
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