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SECTION VIII
CLEANING PARTS
8-1. MATERIALS AND PROCESSES.
8-2.
Equipment, processes and materials in general
use in aircraft engine overhaul shops will be entirely
satisfactory for cleaning the parts of these engines.
All light metal parts of these engines are aluminum
alloys, excepting the crankcase cover which is of
magnesium alloy.
8-3. Do not use any strong alkaline solution to clean
aluminum alloy castings or wrought aluminum alloy
parts, because all such solutions attack the bare
surfaces too rapidly to permit cleaning without de-
struction of the finish. For these parts use a forti-
fied mineral spirit solvent, sold under various trade
names, for degreasing.
If
rosin (oil varnish) or stub-
born carbon deposits must be removed from alum-
inum alloy parts, they may be imme rsed in an agitated
bath of an inhibited mild alkaline cleaning solution
marketed for that purpose. The bath should be main-
tained at a temperature of 180 o F. to 200
0
F., and
the parts should remain in it only long enough to
loosen the deposits. Immediately after such cleaning
flush away all traces of the alkaline material with
a jet of wet steam or by repeated brush application
of a mineral spirit solvent.
Any alkaline deposits remaining on engine
interior parts will react with acids formed in
the lubricating oil to form soap, which will
cause violent foam and may result in failure
of the lubricating system.
8-4. Trichlorethylene condensation plants provide
excellent degreasing action for steel, aluminum and
bronze parts.
Their disadvantages lie in the toxic
quality of the vapors, removal of enamel from painted
parts, and the drying and hardening effect on carbon
deposits.
8-5. No polishing compound or abrasive paste or
powder should be needed or employed for cleaning
engine parts.
Do not use wire brushes or wire
brush wheels, putty knives, or scrapers to remove
hard carbon deposits, since scratches resulting from
such methods allow a concentration of stress at the
scratch and may cause fatigue failure.
8-6. Various hot and cold working solutions have been
marketed for loosening carbon. Any of these may be
employed for that purpose
if
they do not attack the
32
metal; however, most such materials are ineffective
against hard carbon deposits, since they loosen by
dissolving adheSive rosins which cannot be dissolved
after they have been carbonized by heat.
8 -7. VariOUS blasting techniques can be employed to
remove hard carbon deposits
if
suitable equipment is
available.
The most suitable types of grit for dry
blasting are plastic pellets and processed natural
materials, such as wheat grains and crushed fruit
pits or shells.
Air pressure should be the lowest
that will produce the desired cleaning action. Small
holes and finished surfaces which do not require
cleaning should be protected from the blast by seals
and covers, particularly if the grit is sharp. Sand
and metal grit and shot used for blasting industrial
metals are too abrasive and too heavy for use on soft
metals such as aluminum.
The vapor grit process
employs abraSive grit, but of much smaller size and
carefully controUedgrades for various purposes. Car-
bon may be removed from piston heads by the vapor
grit blasting process, using No. 80 grit, which is also
suitable for cylinder head interiors, but much too
coarse for finished piston walls and ring grooves.
No. 50 vapor blast grit may be used on cylinder
heads for more rapid cleaning.
In any event, the
cylinder walls and valve guides must be shielded.
After any blasting process, blow off all dust with de-
humidified compressed air and make sure that no
grains have lodged in crevices.
8-8. SPECIFIC PARTS.
8-9. VALVES.
Hard carbon may be scraped from
valve heads with a smooth edge scraper, preferably
while the valve is rotated in a high speed polishing
head or lathe.
After removal of carbon, polish the
stems first with crocus cloth moistened in kerosene,
then with dry crocus cloth.
8-10. CYLINDERS.
Remove oil and loose material
with a solvent by spraying or brushing.
Remove
carbon from the combustion chambers by soft grit
or vapor grit blasting
if
equipment is available.
Mechanically driven wire brushes are not recom-
mended for this purpose, due to the difficulty of
avoiding abrasion
of
the top ends of the barrels.
8-11. PISTONS. Do not use wire brushes or scrapers
of any kind.
Soft and moderately hard carbon de-
posits may yield to solvent action, which should be
tried first in preference to harsher methods.
If
deposits remain, blast the heads with soft grit or
by the vapor grit method, first having installed
tight-fitting skirt protectors.
Ring grooves may
be cleaned by pulling through them lengths of binder
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