No amount of adjusting and fiddling will help an engine that is damaged, worn, or just plain "cooked"". While external damage
resulting from a crash is easy to spot, internal problems can sometimes be difficult to diagnose, especially for those who
are not "engine people". Even a relatively new engine with little running time may have suffered internal damage if it was
not operated correctly. Following are some simple procedures you can perform to help determine the general condition of your
NOTE: Certain procedures may require engine disassembly. If you are not comfortable doing this, try to enlist the help
of someone in your club who is. Most clubs will have at least one engine "expert".
Over the years Fox has produced
an unusually large number of engine models and variants. Early designs used steel liners exclusively, and pistons made from
either mehanite or aluminum. Some later engines employed ABC technology, and more recently, even some AAC versions were being
produced for the U-control fraternity. Because diagnosis techniques can vary depending upon the particular piston/liner combination
used, be certain you know which you have. Following is a summary of the Fox engine models most commonly seen:
Liner with Lapped Mehanite Piston:
Related engines: All .15-.25, Small frame .40 bushing and .40BB standard, Quickie .40 sport.
Steel Liner with Ringed Aluminum Piston:
Related engines: .45, .50, Eagle .60 and .74, older .40BB and .78.
ABC (Aluminum piston, chrome plated brass liner)
Related engines: Quickie .40, .40 Deluxe, .46 ABC, Eagle .60 and .74 ABC.
Varnish Formation and Removal
We rely on the oil added to
our fuel to prevent the friction that would otherwise result in quick destruction. Unfortunately, the heat of combustion can
cause chemical changes in the oil resulting in the formation of a varnish-like substance on the piston and cylinder. This
is particularly true of castor oil. The varnish can act as an insulator between the piston and liner impeding proper heat
transfer to the crankcase. A small formation is normal in most engines and does not significantly affect operation, but if
the buildup becomes too great the engine will run hot and performance, and general handling qualities, can degrade significantly.
Engines with EZ series carburetors (particularly the .60 and .74), are very susceptible because an overly lean midrange often
caused them to run too hot.
To check for varnish buildup, inspect the piston through the exhaust port while slowly turning the crankshaft. Since the
exhaust side of the piston runs the hottest it will have the greatest buildup. A light brown formation is typical in an engine
that has some time on it and is of little concern, but if the formation is very dark, even black, the buildup is too great.
In ABC engines the piston should be "scuffed" free of varnish near the top indicating a good piston/liner fit. (See section
below on ABC fit). A dark buildup that covers the piston to the top may indicate excessive wear in "ABC" engines, but not
necessarily in other types!
In most cases, engine performance can be restored by simply removing the varnish buildup. This will require disassembling
the engine to remove the piston and liner. (Unless it appears damaged, it is not necessary to remove the connecting rod from
You will find the greatest varnish buildup on the exhaust side of the piston, while the top of the piston will likely be
covered with a dark layer of carbon. ABC engines tend to form varnish mostly on the piston, while engines with steel liners
can form varnish on both piston and liner. A few fine "apparent" scratches are also quite common, and often go no further
than the varnish layer. If deep score marks are present, and/or the cylinder liner has areas that are blue in color, (indicating
extreme overheating), the piston, liner and ring (if used) should be replaced.
Varnish and carbon formations can be scrubbed off with a "non-metallic" scouring pad and hot soapy water. We recommend
3M ScotchbriteŽ pads as they will not cause damage even with vigorous scrubbing. Steel wool is not recommended, and never
use a wire brush, emery cloth or any type of sandpaper! If the piston contains a ring (some older models had two) remove it
first. In severe cases you may even find that the ring has seized in the groove. Heating in an oven or with a heat gun will
often free the ring. Do not use a torch for this operation! You will find that lots of "elbow grease" is required, but keep
scrubbing until all the varnish has been removed from both piston and liner.
Non-ABC engines may be re-assembled at this point but always install a new ring (where used)! The old ring cannot be counted
on to seat properly again and you will have a nice clean engine that runs worse than before!
If the engine is an ABC type, before assembly check the piston-liner fit, as described below. If the fit is poor do not
continue to assemble the engine, it will not run properly! Obtain a new piston and liner set.
Once the engine has been re-assembled check for head leaks (as shown below) then treat it as you would a new engine requiring
some break in time.
Determining Fit in ABC Engines
If the piston-liner fit is
poor, ABC engines can exhibit a number of frustrating problems including the inability to maintain a good needle setting,
sagging and even quitting suddenly in flight. Even if the compression seems fine when flipping the engine over, this does
not always indicate a good piston-liner fit! We have also seen seriously worn ABC engines that started easily and would
idle indefinitely, so these qualities do not necessarily indicate that the engine is in good condition either. There are,
however, some things you can do that will help determine engine condition. First, check for varnish buildup on the piston
as described above. The pattern of the varnish buildup can give an indication of ABC engine condition.
Varnish formation in "ABC" engines can be an indicator of wear
If the fit is good, the piston will be scuffed free of varnish near the top due to the fit becoming increasingly tighter
as the piston travels up the tapered liner. Over time, the fit will degrade allowing varnish to form all the way to the top
of the piston.
If the engine has not seen much use (which doesn't mean it is in good condition), or you have been using fuel that contains
no castor oil (synthetic), there may be little varnish buildup indicated. In this case, remove the glow plug, then turn the
engine over slowly. There should be a noticeable "tightness" as the piston nears the top of its stroke. The engine
must be cold! If there is considerable varnish buildup to the top of the piston, or the fit does not become tighter
at the top of the stroke, disassemble the engine and clean the piston and liner thoroughly. Next, apply a very thin
coating of oil to the piston and liner with your finger, and check the fit as indicated below:
Spot at which the piston becomes tight in liner - Fox ABC Engines
When inserted from the bottom of the liner, the piston should begin to become tight as displayed in A, indicating a good
fit. If the fit is more like that in B, you will find the high-speed needle becoming more finicky to adjust. It will be difficult
to find that "just right" setting that is neither too rich or too lean. The engine may also develop an appetite for glow plugs.
The fit displayed in C will result in a very unfriendly engine. While it may still start easily and idle, finding a steady
high-speed needle setting will be almost impossible, and frequent flameout's may also occur, particularly when operating in
the midrange. The only remedy for a poor fit is replacement of both piston and liner, which come in matched sets.
Check for head leaks, particularly if the engine
has just been re-assembled. This can be easily done with a small container of water, as shown below. Make sure the glow plug
is tight, and do not allow water to enter the exhaust port or carburetor. Only the head and top few cooling fins should be
immersed! Slowly turn the crankshaft through a complete revolution and watch carefully for a small flow of air bubbles around
the head. If a leak is indicated, first try tightening the head screws then test again. Older engines, with single piece heads,
use an aluminum gasket, which should be replaced if a good seal cannot be achieved. Note that current Fox engines utilize
a two piece head with a separate head "button", and do not use a gasket.
Checking for Head Leaks
Engines employing a separate head button sometimes do not seal perfectly, even though there is no apparent damage to the
head button or the top of the liner. This can often be cured by "lapping" the head button into the top of the liner. Apply
a mild abrasive, such as metal polish, or automotive polishing compound (used to restore paint finishes), between the head
button and mating surface on the liner "top flange". Press the head button firmly into the liner and rotate back and forth
a number of times. Be sure to thoroughly clean all remnants of lapping compound from both head button and liner before reassembly!
Occasionally, even new liners require "facing" to achieve a good seal. This is easily accomplished by placing a sheet of #600
sandpaper (wet) on a smooth glass surface, then carefully facing the top of the flange using a swirling motion. Be sure to
hold the liner square to the glass/sandpaper. Only moderate pressure is required.
If a proper seal cannot be achieved it is likely the top of the cylinder liner and/or the mating surface on the head button
has been damaged. This may happen when poor engine service practices have been employed. A small gouge from a pair of pliers
or scratch from a screwdriver is all it takes!
When you are satisfied the head seals properly, place a few drops of light oil in the carburetor and exhaust port, install
a propeller, and flip the engine over a number of times. Do not connect power to the glow plug, we are not trying to start
the engine! If there is "significant" resistance as the piston nears the top of the stroke, and it "snaps" smartly over the
top, the compression is likely acceptable in non-ABC engines. If this is not the case, it is likely time for a new ring, and/or
a piston-liner set.
Occasionally, just replacing the ring will not restore compression even though the piston and liner "seem" to be in good
condition. If the engine has accumulated considerable running time, liner wear and/or wear on the lower edge of the ring groove
can prevent a proper compression seal. In this case, the piston and liner must be replaced as well
Typically, the crankshaft runs in bearings that
may consist of either a bronze bushing or dual ball bearings, depending upon engine model. An exception was the .29, .36 and
.40 "Compact" series which employed the unusual combination of a single rear ball bearing and
aluminum front bushing (no
bronze). This series suffered from durability problems and was produced for only a short time.
Bronze bushings are generally employed where price, and/or weight are the main factors. The
bushing is a permanent part of the crankcase and is not replaceable. Some end-play (in and out) is normal as is a "small"
amount of side play. If the crankshaft can be moved sideways significantly, the bushing is excessively worn. The only remedy
is to replace the crankcase, however, bronze bushings are generally quite durable and usually outlast the rest of the engine.
For maximum bushing life avoid the use of heavy spinner nuts that are intended for the purpose of adding nose weight. Note:
Bushed engines often have a "rattling" sound when at idle. This is the normal result of end-play, and does not mean the engine,
or bushing, is worn out.
Upper engine models employ two ball bearings to support the crankshaft. There should be very little
end-play and virtually no side to side crankshaft movement. With the engine still warm, the crankshaft should rotate very
freely with little sound. Swishing, scraping or clicking sounds and rough spots, indicate worn or damaged bearings. If the
engine has been run on fuel containing only synthetic oil, or it has been stored without after run oil, the bearings should
be inspected for rust formation. The front bearing may be inspected by just removing the thrust washer while the rear crankcase
cover must be removed to view the rear bearing. If rust is evident, replace the bearings.
If it has been stored for some time and the crankshaft does not rotate freely, inspect the bearings before running the
engine. All of the ball bearings should rotate freely in the bearing races. If they do not, it is often due to thickened castor
oil. The bearings should be flushed with solvent until they are free. (It is not usually necessary to remove the bearings
from the crankcase) Do not expect them to free up by just running the engine. Stuck ball bearings will often "skate" in the
bearing race and wear a flat spot before becoming free! Hardened castor on bearings can be very difficult to remove, and may
require strong solvents such as automotive carburetor cleaner etc.
It is particularly important that the rear bearing be in good condition. A bad bearing can come apart, sending metal
bits through the engine. When this happens, the engine is usually ruined instantly!
Fayetteville RC Club
AMA Silver Leader