Sorry, realized I forgot to include one article from the EAA
Dave
Engine Overhaul - Major or "Top":
FIELD INFORMATION
NO.
308
Use of Unleaded Automobile Gasoline in Freshly Overhauled Engines
The purpose of this bulletin is to explain the procedure for the use of
unleaded automobile gasoline in new or freshly overhauled engines where
the valves have been refaced. There have been reports from the field of
valve sticking or actual valve/valve seat damage when unleaded automobile
gasoline was used in some freshly overhauled engines that have previously
used automobile gasoline successfully for long periods. Subsequent valve
problems occurred in as little as 10 hours of engine operation.
Small engines from the 1940s were designed for fuels with octane ratings
of 63 to 73 octane. These fuels contained no lead at that time. There
were
valve sticking problems in some engines which were resolved by the
manufacturer issuing service instructions to change the valve seat angle
and to replace soft valve seats with hard seats. This had been done so
many years ago that there is little likelihood of an engine still in
existence
which has the original soft seats and old valve seat angles. Thus aviation
gasoline, with its very high lead content (approximately 2.0 grams/gallon)
compared to the need for these engines, had caused many problems as
outlined in AC 91.33. Note that grade 80 aviation gasoline for which
these
engines were initially approved, has a maximum allowable lead content of
approximately 0.5 grams/gallon and may be supplied with zero lead
content.
Engine manufacturers, in order to minimize valve problems caused by the
excessive lead content of 100 low lead aviation gasoline, have increased
valve stem diameters and added hardened inserts for the valve seats. Both
of these changes favored the use of a gasoline with a very low lead
content
also, and as pointed out above, showed normal wear characteristics in the
EAA's 500-hour flight test program.
As indicated in the June 1985 issue of SPORT AVIATION, a Continental
Motors Special Service Bulletin M46-2, dated November 25, 1946,
addressed this situation. An investigation by Continental Motors revealed
that the "absence of lead from the fuel has resulted in the 'picking up'
or
'welding' of the valve seat to the valve." (Remember, at that time, high
quality gasoline was "white gasoline" - with no lead content.)
The special bulletin goes on to say that "there is nothing wrong with
either
the gasoline or valve materials, but that the two will not work together
except after a protective coating is deposited on the valve and seat
contact
surfaces by use of leaded fuel for the first two or three hours of life of
the
new engine or an engine in which the valves have been refaced, after the
initial 'lead treatment,' however, no valve trouble should be encountered
and operation should be better with the 73 or 80 octane clear gasoline
than with the war and prewar leaded fuels." Since that time, TCM has
changed valve and seat angles for further improvement.
308-1
Revised 5/20/96
Although this procedure was not used during our flight test program and
none of the above mentioned valve problems were experienced, we
recommend operating on leaded fuel immediately after engine service
involving the valves.
EAA members report that their experience indicates that 10 hours of
operation on leaded fuel (100LL aviation gasoline, not "leaded" regular
automobile gasoline, which now has no lead content) is a more
conservative program.
Experience has shown greater extended life, and more time between
overhauls, with the use of unleaded automobile gasoline. EAA had more
than 500 hours of flight test time on each of the following aircraft when
EAA's STC's were issued: Cessna 150 with a Continental engine, Cessna
182 with a Continental engine, Piper Cherokee PA-28-140 with a Lycoming
engine, and a Cessna 172 with a Lycoming engine. A Cessna 172 in
pipeline patrol satisfactorily flew a supervised 1,200 hours observed by
the
FAA. All aircraft performed very satisfactorily in actual flight tests for
EAA
and the FAA. Engine teardown inspection by an independent engine shop
and witnessed by FAA representatives indicated no adverse wear
problems with the use of unleaded automobile gasoline.
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