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 the piston.)

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 Scotchbrite® 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.

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:

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.

Compression
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.

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

Bearings
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
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.

Ball Bearings
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!