AOPA Pilot Magazine - February 2024 - 95

that lists virtually every Lycoming engine
model and which fuels-both leaded and
unleaded-they are approved to run on by
Lycoming. This service instruction shows
that the majority of Lycoming models are
approved for UL91 and UL94 fuels, and lots
are approved for premium unleaded mogas.
In contrast, Continental has been late to
the party. According to an outstanding investigative
report by AVweb's Paul Bertorelli,
Continental did not switch to improved
(hardened) exhaust valve seats until 2019,
which means a very large number of cylinders
with non-hardened seat inserts are
still in the field. As recently as 10 years ago,
Continental was actively warning that the
use of unleaded auto fuel could result in
unacceptable exhaust valve/seat deterioration
in as little as 10 hours-a little hard to
believe given the excellent results we've seen
with low-compression Continentals running
mogas in the field. Continental no longer
warns against the use of unleaded fuel but
does not approve it either.
To date, Continental has not to tested or
approved GAMI's G100UL despite the fact
that the FAA has issued an STC approving
its use in virtually all Continental and
Lycoming engines. Prior to obtaining the
STC, GAMI performed more than 700
hours of endurance testing involving both
Continental and Lycoming engines with
absolutely no evidence of exhaust valve/
seat recession.
Bottom line: We really shouldn't expect
any problem running unleaded avgas in any
engine with hardened exhaust valve seats-
this includes all Lycoming engines and any
Continental engine with cylinders manufactured
in 2019 or later. There's also plenty of
anecdotal evidence that low-compression
Continentals-such as the ubiquitous
O-470-R/S engines in legacy Cessna 182s-
do just fine on unleaded fuel despite their
older-style unhardened seat inserts.
Given all this background, what are we
to make of UND's decision to terminate its
use of Swift UL94 because of " significant "
exhaust valve/seat recession? To say this
is a real head scratcher would be a " significant "
understatement. Since UND
has elected not to make its data public-
they've shared it only with Lycoming, who
isn't talking-we can only speculate at this
point. Therefore, what follows is my own
Measuring valve and seat recession.
unvarnished speculation unsupported by
any hard data. (Your unvarnished speculation
may vary.)
Honestly, UNDs decision to withdraw
from the UL94 trial leaves me with
more questions than answers. How " significant "
was the recession? Was it causing
a substantial increase in the number of
burned exhaust valves, the number of cylinders
that had to be changed, and/or the
amount of maintenance-related downtime
in UND's fleet of Piper Archers? Or was it
simply a technical measurement of recession
with no real operational significance?
As I'm writing this, we don't know.
Dry tappet clearance
According to UND, the concerns about
exhaust valve/seat recession was derived
from their measurements of " dry tappet
clearance, " which is a measurement of
how much " lash " (slop) there is in a cylinder's
valve trains when the hydraulic valve
lifters ( " tappets " ) have been bled down, so
they no longer contain any oil ( " dry " ). This
strikes me as a rather peculiar method of
measuring valve/seat recession, and so
bears some scrutiny.
Normally, the only time mechanics
measure dry tappet clearance is when a
cylinder is replaced. The purpose of the
measurement is to make sure that the
amount of mechanical lash (slop) in the
valve train is within the range of what the
hydraulic valve lifter can eliminate when
it is pumped up with engine oil. If the
value is too large, then it's possible that
the valve might not open all the way; if it's
too small, it's possible that the valve might
not close completely.
Lycoming's spec for dry tappet clearance
is wide enough to drive an eighteen-wheeler
through: 0.028 inch minimum and 0.080
inch maximum. Anything within this very
broad range is acceptable. If a mechanic
replaces a cylinder and the dry tappet clearance
value is outside this range (which is
rare), then an approved undersize or oversize
pushrod needs to be installed to bring
the measurement within tolerance.
UND's decision to monitor dry tappet
clearance as a measure of valve/seat recession
strikes me as problematic in two ways.
First, it measures total valve train lash
(slop) that is the sum of numerous elements
of which valve/seat recession is only
one component. A decrease in dry tappet
clearance might indicate recession, but it
might also be caused by other things, notably
failure to ensure that the tappet body is
AOPA PILOT / February 2024
95
AOPA Pilot Magazine - February 2024

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