Friday, January 15, 2010
4-Stroke Engine Maintenance
Prepare your work area before beginning. I am a strong proponent of getting and staying organized while performing this type of work, and like to cover my work area with an old bath towel, or even an old bed sheet. A fabric surface helps prevent small parts from disappearing. Gather the necessary tools and supplies for the job—Allen keys, screwdrivers, pliers, a metal-marking scribe, cleaners, brushes, work tray, some shop towels and assembly oil. A heat source such as a propane torch or heat gun is also required. Keep organized and work somewhere you won’t mind making a little bit of a mess—not the good dining table!
STEP ONE - ASSESSMENT & DISASSEMBLY
When you have an old, worn-out engine, or one with crash damage - it is necessary to identify what needs replacing. Generally, if the engine was running fine before a crash, you won't have to worry about things like the piston or ring, valve leakages, etc. Identifying a bent crankshaft or cracked case might be more important! If you are trying to resurrect a completely unknown engine it is important to check every aspect of the engine.
I purchased the subject engine on eBay, in a box lot of engines and parts. I didn’t expect too much of the poor old Saito due to the photo, and when it arrived it looked to be in very poor condition. The front of the crankshaft was completely broken off in a crash. After I disassembled the engine and inspected the interior it looked very good, so I decided to rebuild it.
The first step in dismantling a 4 stroke engine is making sure you know how to re-time the cam gears. Several companied such as Saito include this information in the manual or on line, while others do not. On line forums are also a great resource if you can't find documentation for this critical step.
Cleaning the exterior dirt and grime is the next step. Knock off any crud and mud before you start to tear it down. Dirt packed into a machine screw heads can make it impossible to seat the necessary Allen wrench or tip - try cleaning the dirt out with a small awl or scriber, or even a jeweler’s screwdriver. Then you can start taking things apart.
I usually begin by removing the carburetor and back plate. If you have any trouble with screws, try heating them up briefly with the tip of a soldering iron or gun to break the bond of old oil or threadlock. The back plate is usually sealed with a paper gasket or o-ring. Carefully ease paper gaskets off with a knife to prevent tearing. Some four stroke engines do not use a seal on the back plate since crankcase leakage is not a concern for running performance, though a decent seal will prevent oil from leaking out and making a mess.
Next I like to remove the cylinder or head and liner, depending on the engine design. Once you remove the head, the push rods and cover tubes usually become loose as well. For now, place the head and other parts aside.
If you have had the engine running and the valves are not leaking, the overhaul will be easier. You can usually tell if the valves are leaking by turning the engine over with the rocker covers off. Listen and look for a tell tale hiss, or bubbles, or some sort of leakage up through the valve guides. Turn the engine over both clockwise and counter-clockwise and see how it holds compression at TDC. If you suspect a leaking valve, either the valves will need to be cleaned and lapped to the seat again, or new valves may be necessary. Lapping and replacing valves requires experience and is outside the scope of this tutorial. In general, the intake valve and seat rarely needs attention - the exhaust valve may see some carbon build up around it which can be cleaned off with a stiff toothbrush and cleaner. To extract the valves, you will need to compress the spring and remove the keeper or split collets without letting the parts fly away! Please take care and only remove the valves if absolutely necessary.
The piston/rod assembly can usually be slid off of the crank pin. If the engine you have has a small hole in the crankcase, you may need to extract the piston pin through this hole before the connecting rod can be slid off of the crank pin. Use a small piece of hard wire and form a small “L” on one end. Hook this through the pin and pull it out through the crank case access hole being careful not to score the piston or liner.
The camshaft and housing/cap (again depending upon brand) can now be removed. Again, take note how the timing mark is situated when the crank shaft is at TDC or BDC (makes no difference). On the Saito engines, the cam housing is attached with four cap head screws and sealed to the case with a paper gasket. The camshaft can be removed by loosening a small set screw in the housing top, and then extracting the camshaft spindle. On engines like the O.S. style, the housing is part of the crankcase casting. The end cap is removed, and the camshaft is extracted with a small pair of pliers with non serrated tips. Once the camshaft is out, the cam followers can be pushed out and set aside.
Removing the prop drive washer is easy if it is held with a key or similar anti rotation method. If the engine utilizes a tapered split collet, it is generally the easiest and safest method to use a small puller. You can fashion one from a small battery puller, or modify the tips of a small gear puller. Try and remove the drive washer without hammering on the end of the crankshaft - which will deform the threads and cause you grief later on! Now, the crankshaft generally slides out of the bearings by hand, or will come out easily with a gentle tap with a brass or plastic faced mallet or wooden block.
The original bearings had suffered from poor maintenance
and storage. I ordered replacements from Boca Bearings. The next step is to remove the bearings. In my case, the bearings were rough and slightly rusted - definitely candidates for replacement! I ordered a set of high quality bearings from Boca Bearing. In the last overhaul article I showed how a heat gun could be used to warm up the crank case in order to remove the bearings. This time, I used a small propane torch - just keep the flame moderate, and always moving around the case exterior where the bearing is seated. Once the bearing is loose, tap the bearing out with a wooden dowel. Use a heavy work glove or oven mitt to hold the case while hot!
At this point, all parts should be apart, laid out in front of you and ready for the next step.
For cleaning the parts of the engine, I like to use non-solvent based cleaners if possible. Although chemicals like lacquer thinner and even mineral spirits can be used for cleaning parts, solvents are flammable, give off harmful fumes, and are not good for your skin. I like using a water-based citrus degreaser and an alkaline detergent for cleaning my engines. I buy my detergent in bulk from an industrial supplier, but it is basically the same product sold in hobby shops for cleaning burnt-on oil and gunk off of your engines. Taking the parts one at a time, I pour some citrus cleaner into a plastic container and use a toothbrush and small stainless steel wire brush to clean the respective part. I only use the wire brush on external surfaces with the toothbrush used to clean all internal parts - the nylon bristles won’t hurt anything but are stuff enough to clean well.
Then I dilute some of the detergent in a 50% mix with water and wash the part again before rinsing the part in clean warm water. After rinsing, set the part aside on a clean shop towel to dry.
For steel parts such as the crankshaft and any fasteners, you can spray a light coating of a rust preventative such as WD-40 on the piece after the rinse. This will ensure no rusting will occur. If you reassemble the engine soon after cleaning, the assembly oil will take care of that issue.
Mild cleaning in a citrus-based solution is the order of the day. I use a toothbrush to avoid scaring the metal components. Stubborn burnt-on oil can usually be removed with the fine stainless brush. I also lightly scrape off the heavy stuff with an X-acto blade. Heavy carbon build up on the piston top can also be removed with a knife blade but be VERY careful not to nick the edges of the piston. Sometimes, a small piece of a medium grit 3M Scotchbrite pad can also be used to clean the stubborn stuff.
Some engines may have so much burnt on oil that they need a much more extensive cleaning by other means - ultrasonic cleaners, caustic detergent baths, and a warm antifreeze bath are all methods that can be used. For the majority of engines though, a simple hand cleaning as I describe here is more than satisfactory. Once all of the individual parts are clean and dry, we can move on!
Note: some of the steel components such as the prop washer & nut, and the steel screws are finished with a rust preventative (and attractive) black oxide finish. This can, in time, wear away exposing the bare steel which can then easily rust. Here is a method I use to reblacken these parts - fill a quart size can with regular motor oil. Using an old pair of needle nose pliers, grip the part to be retreated in the jaws, and heat to a dull red with a propane torch. Immediately quench the part in the can of oil for a few seconds, and then place on a heat resistant surface to cool. This will restore the black oxidized finish. Just be very careful, and only perform this outdoors with nothing flammable nearby!
In most cases, you will probably just be doing a cleaning and maybe a bearing replacement. From time to time, engines which have been run for extended periods of time (and perhaps in less than ideal conditions) will need some other parts replaced. Looking at the piston and liner, both should have a smooth, shiny appearance and few vertical lines (some light scratches are normal and in most of our four strokes which use a ringed piston, are generally not an issue). Deep scratches or noticeable nicks in the ring are a sign that some foreign material has passed through the engine and if compression is suspect, a new ring may be required.
Connecting rods should have no noticeable play on either end - generally if the crank pin end is over 0.003" it is considered worn. Piston pin fit both into the upper end of the connecting rod and piston bosses should not exceed 0.002" - if it is not possible for you to measure these accurately, when assembled you can usually “feel” excessive play. If you are not sure, send it in for service or seek help from another member of your club who knows more about engines. Gaskets and seals should be unbroken and flat. Damaged gaskets can be re ordered from suppliers, or you can purchase gasket sheet material and cut your own like I prefer to do.
In my case, with this Saito FA80, a new crank shaft was definitely needed, and obtained from Horizon Hobby. The bearings were also in trouble, feeling rough and pitted - a new set was obtained from Boca Bearing. Although the exterior of the engine looked bad, it was really only the rocker covers (which originally were chromed) that had suffered poor storage and peeling chrome. I carefully bead blasted them back to a natural aluminum finish to match the crank case. Inside, the parts looked great and the engine still had excellent compression.
The first step in getting everything back together is to install the new bearings. Again, heating the crankcase with the torch (or heat gun, or in the oven set at 250 F to 275 F) will allow the new bearings to slip into their cavities. Wear a glove to prevent burns and when the case is warm enough, press the bearing in and tap it home with a wooden block, or a plastic hammer. Do not force the bearings in, and do not use a steel hammer to try and pound them in. If they are too tight, warm the case further until they slip in easily or with a gentle tap. The crankshaft should be able to slip back into the bearings now, and a gentle tap on the rear end will seat it up against the rear bearing. The bearings should come pre greased. However, I like to use oil on all parts during reassembly. I use my bottle of after run oil for an assembly lube. Making sure all parts are well lubricated will prevent any chance of damage when you run the engine for the first time after its back together.
Saved from the scrap bin, this Saito is now restored and ready to go, promising many
years of dependable service. After the crankshaft is in, slip the rod and piston back onto the crank pin in the same order they came off. Slide down the liner onto the piston and into the case. Some engines have a liner that is also a slight interference fit in the case and the case will have to be warmed up in order to get the liner back in. Ease the liner over the ring while compressed with your fingers. Four stroke engines do not use pinned rings so the ring orientation is unimportant.
The camshaft, housing or cover, and cam followers can be reassembled. Set the timing precisely and check it twice! A little grease can prevent the cam shaft from moving when trying to mesh it with the crankshaft gearing. The push rods and covers can be placed into the rubber boots, and then the head and head gasket can be placed on the cylinder and tightened down.
When the head is seated on the cylinder (or in the case of the Saito’s, when the cylinder itself is positioned onto the crankcase) care must be taken in order to line the push rod covers up into the rubber seals, and also to get the push rod ends into the cup seats on the rockers.
Snug each head screw evenly and not too tightly. Without a glow plug installed, you should be able to easily turn over the engine when gripping the crankshaft nose. Any unevenness in the head bolts may cause a distortion in the liner which you will be able to feel when turning over the engine.
One important thing to take care of with a four stroke engine each time it is disassembled, is the valve clearances. Too little clearance in between the rockers and the valves will not allow the valves to seat properly and they will leak. Excess clearance will allow the cams, followers and push rod ends to get “hammered” and worn. With the majority of our model four stroke engines, a minimum clearance of 0.0015" to 0.002" and a maximum of 0.004" is where you should try and get. With a set of steel feeler gauges (some engines are supplied with them, if not, you can get these at any decent tool or machine supplier) you can loosen the locking nut and screw in/out the rocker adjustment until you can just get the thinner feeler to slip in between the push rod and the rocker, yet not the thicker one. Tighten the lock nuts while holding the adjustment screws in place and you are done!
Put the rear cover back on, and then the carb and muffler. Assemble the collet and/or key, drive washer and put a prop onto the shaft and tighten with the washer and nut. A few extra drops of oil down the head before installing the plug and muffler will again insure adequate oil to all parts.
That’s it! Flip the prop over - check for compression, leaks from the head or backplate, around the carb, etc. The engine should be now ready to mount and test run.
TEST RUNNING & SET UP
Once the engine has been reassembled and everything checked over for tightness, you can install a new glow plug and try a test run. With my old Saito, I chose to install a new O.S. F four stroke plug, and an APC 13x6 propeller. Fueled up with some Wildcat 10% Premium Xtra glow fuel on my test rig, It started up easily and within a short time I had the needles set up again (I had removed them for cleaning of the carburetor) and the engine purring away.
The best part, and most satisfying to me, is taking an engine many would consider junk - rebuilding it and placing it back into service powering another model through the air! This Saito had definitely seen better days, but it will continue to run for many seasons with its new crankshaft and bearings.
Working on your engines doesn’t have to be intimidating. With some time, patience and experience you can do it. Send any comments you may have regarding this and other articles to myself or the editors here at Fly RC.
more detail on www.rcecho.com