Setting up and running model 4 cycle engines.

[brian Rupnow]

Having built about a dozen model internal combustion engines, I have decided to write down everything I know about building and running these engines as a guide to others who want to build one.
I’m not talking about high revving two cycle screamers here, not compression/ignition engines. Just plain old four cycle petrol fueled spark ignition engines, similar to the Webster or the Kerzel hit and miss engine.
It makes no difference if they are air cooled or water cooled, all of the following advice is relevant, with one caveat.—If the engine is air cooled and NOT running a propeller, it is probably going to overheat without some kind of auxiliary cooling fan after about 15 minutes running. A water cooled engine with a reservoir of water around the cylinder will run much, much longer without overheating.
These engines are built primarily from 6061 aluminum, with a cast iron cylinder liner (or the entire cylinder built from cast iron.) The crankshaft is generally made from mild steel, either machined from solid stock or “built up” from a number of pieces soldered or press fitted together. If machined from solid, 1144 stress-proof steel is highly recommended, because it doesn’t tend to deform from internal stresses during the machining operation.

[brian Rupnow]

Pistons can be made from cast iron, or from aluminum. Either will work equally well. High revving engines benefit from aluminum pistons, because they are lighter, and have less reciprocating mass. There is a school of thought out there that aluminum pistons cause problems because aluminum expands at a higher rate than cast iron, which in theory could make an aluminum piston “seize” in a cast iron cylinder, but I have never had that happen. For slower revving engines of the type I am dealing with here, 500 to 3500 rpm, a cast iron piston works fine as well.
Piston rings---I have never been very successful at machining and heat treating piston rings from cast iron and getting them to seal well enough to give good compression in an engine. I know that many people make their own rings with a high degree of success, but the secret has eluded me. Instead, I use a single Viton o-ring on my i.c. engines, and they work great every time. The only time I don’t recommend a Viton o-ring is in a hit and miss engine. The engine will still run fine with a Viton ring, but these rings put considerably more “drag” on the piston, which means that a hit and miss engine will get fewer “misses” between “hits” than an engine with cast iron rings, because of the difference in friction created by the rings “dragging” on the inside of the cylinder.

[brian Rupnow]

I am not going to get very deeply into ignition timing. There have been thousands of pages written about ignition timing, but the bottom line is that the spark should occur on the compression stroke, just as the piston reaches the top of the cylinder, or very slightly before. It is fairly unusual to see an engine running where the spark occurs after the piston has reached the top of it’s travel and started back down. Two important things to take notice of here—If you are using a “points" style ignition, the spark occurs as the points open, not as they close.---AND---if you are using one of the electronic ignition kits—NEVER lay the sparkplug out on the head and turn the engine over by hand to see when the spark is occurring. If the sparkplug is not really well grounded to the engine, it will almost instantly burn out the electronic “module”.
There is little difference in the cost of a simple, automotive ignition points and coil set-up and a complete magnet and sensor style set-up---IF you have access to a 12 volt car battery to run the 12 volt coil. If you have to buy a battery, then it ends up being cheaper and more portable to buy one of the electronic ignition “Kits”.
Sparkplugs---You can make your own. Just a word of caution though—If you do make your own, and then at a later date you can’t get the engine to run, there will always be an element of doubt as to what is causing this lack of performance. I think it is better to use a “store-bought” sparkplug and use it at least until your engine starts and runs.

[brian Rupnow]

Connecting rods—I don’t run any type of bearing on the connecting rods. I use 6061 aluminum for the rod material and finish the bore at each end to match the crankshaft and the wrist (or gudgeon) pin. It doesn’t “hurt” to run a bearing or bushing of some type, but as long as there is adequate lubrication supplied, no bearing or bushing is necessary.---I did design and build an engine where I machined the crankshaft end of the con-rod to take a ball bearing. This resulted in a terribly “out of balance” engine, that would “walk” across the floor if not solidly bolted down because of the massive out of balance condition caused by the large end of the connecting rod.
Crankshaft bearings---These can be ball bearing, solid bushing, or split bushing types. The bushings can be made from brass, bronze, or cast iron. I don’t recommend needle roller bearings because they really are supposed to have a hardened inner race and not just run on the surface of the crankshaft. Again, as long as the machining of the crankcase is done accurately, I don’t see one type of bearing as having an advantage over the other types. As in the connecting rod bearing surfaces, lubrication is the key to long lived crankshaft bearings.

[brian Rupnow]

Valve timing.—Many of these small engines will have an “atmospeheric” intake valve. That means that there is no cam nor lifter nor rocker arm associated with that intake valve, only a very light compression spring to hold it closed. When the piston travels down in the cylinder, it creates a partial vacuum in the top portion of the cylinder, and as a consequence atmospheric pressure will push the valve open against the light compression spring and cause an air/fuel mix to flow into the cylinder. When the piston reaches bottom dead center and starts back up the cylinder, the light spring will pull the intake valve closed and the pressure created in the cylinder will hold it closed. **** There are many more “scientific” ways to set the point at which the exhaust valve begins to open, but a good rule of thumb is as follows.—Turn the engine slowly by hand in the direction you want it to run, with the set-screws which hold the cam gear in place loosened off. When the piston gets about 1/8” from bottom dead center turn the cam gear (in the opposite direction) until the cam is just up against the stem of the valve (or until all the “slack” is removed from the valve train), but not yet opening the valve, and lock the cam gear in that position. As the engine continues thru bottom dead center and then starts to travel upward in the cylinder, this will be the exhaust stroke.

[brian Rupnow]

Four cycle engines always follow this order of operation---Intake (piston moving from top dead center to bottom dead center, intake valve open exhaust valve closed, Compression—piston is moving from bottom dead center to top dead center, with both valves closed.—Power, in which the fuel mixture is burning and expanding and the piston moving to bottom dead center with both valves closed, and lastly Exhaust, where the piston is moving from bottom dead center back up to top dead center and the exhaust valve is open with the intake valve closed. The valve opening and closing does not occur exactly in the same time span as the piston travel. Both the exhaust valve and the intake valve begin to open slightly BEFORE the piston goes past top or bottom dead center, and both valves close slightly AFTER the piston has reached top or bottom dead center. If the plans for your engine have a valve timing diagram, then use it. Timing diagrams specify when the valves should begin to open in terms of “degree of crankshaft rotation”. –when the valves begin to close is a function of cam shape and configuration. In instances where the intake is actually controlled by a cam, then you will have to refer to the instructions which hopefully come with the plan you are working from, which should tell you the number of degrees of cam lobe separation between the intake and the exhaust cams. Unless your engine has individually adjustable cam timing (which is very unlikely) then by setting the intake and exhaust cams the proper number of degrees apart and establishing when the exhaust cam begins to open, then no farther setting is required for the intake valve. (Although the intake valve SHOULD begin to open 10 to 20 degrees before the piston reaches top dead center.)

[brian Rupnow]

So---we have touched on materials, ignition, valve timing and operation and a small bit about cooling.
A brief word about carburation. In general, conventional four cycle engines require a carburetor, which serves to add the correct amount of fuel to the stream of air flowing into the cylinder to be burned. This mixture of fuel and air is generally controlled by a needle valve. A fuel heavy mixture is said to be “rich” and a mixture with only a small amount of fuel is said to be “lean”. A rich mixture is desirable for starting a cold engine. After the engine warms up for about a minute, the fuel mixture can be “leaned out” by screwing the needle valve in “about” ¼ to ½ turn. If the mixture is left “rich”, the engine may flood out and stall, and the high amount of carbon produced by an overly rich mixture can foul the sparkplug and prevent it from firing. The needle valve is not intended as a device to control the rpm of an engine. For that, you need a carburetor which has a “throttle” built into it. The throttle can be in the form of a butterfly valve plate, or a portion of the air intake that revolves to partially close the main air passage through the carburetor. The placement of this throttle is very particular, as it needs to not only control the air passing through the carburetor, but must not interfere with the flow of fuel through the needle valve and “spray nozzle”. If the throttle plate is placed too far upstream from the “spray nozzle”, it will act as a very effective “choke”, but very poorly as a throttle. Small 4 cycle engines don’t need a throttle, but if you have any desire to make major adjustments to the rpm of the engine, then yes, you will need a carburetor with a throttle built into it.—Now---this is very important!!! The carburetors designed for use with these engines create a slight amount of vacuum right at the point where the “spray nozzle” enters into the main intake airstream. This vacuum is what pulls the droplets of fuel from the spray nozzle into the airstream and mixes it with the air to form a true “gas” as opposed to a stream of liquid fuel being pulled into the carburetor. This same vacuum has the ability to pull fuel up from a tank mounted in such a position that when full to the top the fuel level is 1/2 to 3/4” BELOW the point where the spray nozzle empties into the carburetor air stream. These carburetors do not have a float and shut-off valve like the older automobile engines to stop the flow of fuel. Consequently, having the tank higher than what I have specified leads to uncontrolled gravity flow of fuel into the carburetor, which in turn causes flooding, poor running, and in a “worst case” scenario a total hydro-lock of the engine or even worse, fuel pouring out through the inlet of the carburetor and creating a fire hazard. Now, if I haven’t confused you already with this depth of wisdom about carburetors, Hit and miss engines do NOT require a carburetor with a throttle. On hit and miss engines, the carburetor is adjusted by means of the needle valve for “optimal performance”. The governor, which operates from centrifugal force holds the exhaust valve open when desired rpm is reached, and this in turn prevents the engine from firing until the engine slows down enough that the governor return spring returns the governor to it’s previous position, allowing the exhaust valve to close and the engine to fire once more. The strength of the governor return spring is what controls the speed of the engine, not the carburetor.

[brian Rupnow]

Gaskets—I have read numerous accounts of people building model engines, and using no gaskets at all---maybe just a fine amount of Permatex “instant gasket” goo, that is all. That does not work for me. Never has worked for me. I gasket everything that bolts together, where-ever there is the faintest possibility of compression loss from the cylinder, or air leaks around the carburetor. If the intake manifold block is made from 3 separate pieces as in the Webster engine I put a thin gasket between all of the pieces. I use a head gasket, even if it is just a round ring of treated gasket material. If the intake manifold bolts to the cylinder I gasket that. If it screws into the cylinder, then I use a sealant on the threads. I even put a gasket between the cylinder and the exhaust manifold. I don’t use any kind of special “high heat resistant” gasket material, just the garden variety treated gasket paper/cardboard that they sell in automotive supply shops. This ranges from 0.020” thick up to 0.035” thick---I try to find the thinnest gasket material whenever I have a choice. The displacement on these engines is so small that you simply can not allow any leaks at any of the bolted joints and expect them to run properly. Another caveat that I have found with the “permanent gasket in a tube” type of thing, is that there is no way to control where it goes when the mating surfaces are bolted together. A blob of that stuff squeezed inadvertently into an intake valve passage is almost impossible to detect and can prevent an engine from starting or running.
Valve Guides and Seats---I prefer to keep my valve guide and seats all in the same piece of material, (preferably brass) where the bore of the valve guide and the outer diameter of the guide can all be machined in one set-up in the lathe.—Notice that we have not yet machined the valve seats. These “valve cages” as they are called are a very light press fit into the cylinder head or intake manifold body (as in the case of the Webster), and should be liberally coated with Loctite 620 before being lightly pressed into place. The hole through the side of these cages should not be drilled until they have been Loctited in place and allowed to set up for 24 hours. The inlet or outlet cross hole should then be drilled through the cylinder head and the valve cage at the same time. Only then should we machine the valve seats. To machine the valve seat, I have made up a special tool which was originally designed by George Britnell, which has four cutting facets and a guide the same diameter as the valve stem. This tool is inserted by hand into the guide and spun lightly be hand, to give a very shallow valve seat, about .015” to .020” maximum x 45 degrees. By making up “valve cages” in this manner, as opposed to machining the valve guide and seat right into the cylinder head, if you should manage to “mess one up”, then you have only to press out the “valve cage” and remake a small simple part rather than remaking an entire cylinder head.

[brian Rupnow]

Valves---I make my valves from plain cold rolled steel. That is strong enough, easy to machine, and I have never “burned a valve “ in any of my engines. There are a couple of tricks that I employ, and I will outline them here. The valve stems are generally only 1/8” in diameter, and if you try and turn down the full length of about 1 3/8” in one operation, you will get massive deflection of the valve stem and find it very hard to end up with a good result. Instead, I chuck up my material with the full length of the valve plus about 3/8” protruding from the chuck jaws. Then starting at the outermost end, I will reduce the diameter for a length of about 3/8” with successive passes until I get down to about 0.128” diameter. Then I do the next 3/8” of length, then the next, until I have turned down the full length of valve stem required, leaving it about .002 to .003” oversize. I then use #220 grit emery paper with the lathe turning at about 200 rpm to bring the valve stem down to finished size. It helps a great deal if you have previously prepared a short piece of steel or brass with a reamed .125” diameter hole in it so that you can manually try it for fit on the valve stem. Without removing the valve material from the chuck, I set my lathe topslide over so that when I cut what will become the valve face it will give an “included angle” on the valve face of 92 degrees. Note that when swung over the way I have discussed, the dial and handle of the topslide will actually be on the far side of the lathes central axis, and you will be cutting outward from the near side of the lathe stem towards yourself to form the valve face.---Do not run the tool into the spinning chuck jaws!!! Do not part off the valve to length. Leave a “handle’ about 2” long between where the valve face ends and the cut to part off the stock. You will need this material to use as a handle when you manually lap the valve into the seat to ensure an air tight seal.
The head of the valve should be of sufficient diameter so that the angled “face” of the valve lays fully on the “seat” area in the “valve cage” and is at least equal in diameter to the outside of the valve cage.
Apply a bit of 600 grit carborundum paste to the face of the valve (don’t get it on the stem) and lower the valve stem into it’s guide until the valve is closed. Now grip the “handle” which you left on the valve between thumb and finger and spin it back and forth with a slight amount of pressure against the seat. Spin it back and forth 10 times, the lift it up slightly and rotate it 90 degrees, then repeat that 10 times spin. Repeat this four times. (Never try to take a short-cut here and spin it with an electric drill---You will ruin the valve and the seat.) You should end up with a dull ring of material around the contact area on the face of the valve and on the face of the seat. This will be the area that seals the valve against engine compression. Now take the valve over to the lathe, chuck up the handle, and part off the valve from the handle. This sounds relatively simple, but it is the most important part of the entire engine building process. More model engines refuse to start because of compression leaking at the valves than for any other reason.

[brian Rupnow]

Gas Tanks—I don’t have a lot to say about gas tanks. In my opinion, they should be made of metal, not glass. With glass you will always know whether or not you are out of fuel---but glass will shatter and there is always the possibility of catastrophic failure and the resulting fire, so I don’t recommend glass. I do recommend the use of a transparent fuel line, because this will give you a good visual of what is going on in the tank. Gas tanks should always have at least a 1 mm (0.040”) diameter vent hole in the cap. Without a vent hole, these small tanks will air-lock very easily, causing reduced or intermittent fuel flow, and you will go crazy trying to diagnose what is wrong with your engine. Another device I strongly recommend is a small, one way ball-valve in the gas line, arranged so that it sets in a vertical section of the line. This is simply a 5/32” steel ball setting in a brass cone. The vacuum created by Venturi effect in the carburetor will pull fuel up from the tank and lift the ball off it’s seat, allowing the fuel to flow to the carburetor. When the engine is on some other cycle than “intake” and no air is being pulled through the carburetor to create the Venturi effect, gravity pulls the ball back into the cone and prevents the fuel from back-flowing into the tank. This is especially important on slow running four cycle engines, and almost imperative on hit and miss style engines. I have no experience with the type of gas tank made popular by Jan from the Netherlands. This is a fuel tank and carburetor combined, where air is pulled across a reservoir of fuel in the tank, thereby picking up enough fumes to make the engine run. I know that they work. I offer no opinion on them because I haven’t used or made one.

[brian Rupnow]

Fuel----I use Coleman fuel, which is also known as Naptha gas in some parts of the world, and white gas in some parts of the world. My main reason for using it is that the exhaust is almost odorless, compared to the stink of burning automotive pump gas. (Although automotive pump gas will run these small engines just fine.) Since I use a Viton o-ring on my piston, rather than a cast iron ring, I add enough synthetic 2 cycle oil to my fuel to give a bit of lubrication to the ring and piston. A 40 parts fuel to 1 part oil seems to be a sufficient mix ratio.

[brian Rupnow]

Flywheels---All of these engines require a flywheel to get the crankshaft over the “dead spot” that occurs when the piston is at top dead center and at bottom dead center. The larger in diameter a flywheel is, the more effective it is. The “web” of the flywheel serves no real purpose, other than to locate the outer rim in relationship to the hub which attaches to the crankshaft. The actual “working part” of the flywheel is the heavy outer rim. As a consequence, the web can be made thinner, or spoked, or drilled with cosmetic holes without having any detrimental effect on the way the flywheel does its job. There are any number of ways to attach the flywheel hub to the crankshaft. I have seen everything from a cross drilled hole through the flywheel hub and the crankshaft, to a single keyway with a set-screw above the key, up to tapered “squeeze lock hubs”. I find that a square key with one set screw above it to lock it in place and a second set-screw at 90 degrees to be quite adequate for holding the flywheel in place, yet still be removable if the need arises.
.....Lubrication---As said before, these engines absolutely must have lubrication. The most important areas to be lubricated are the main bearings of the crankshaft and cam-shaft, the connecting rod big end, and the connecting rod small end. The crankshaft and cam shaft bushings are generally easy to provide lubrication for, by the simple expedient of drilling a small hole straight down from above through the engine frame or the bushing/bearing caps, though which oil can be applied by a squirt can of oil. The big end of the con-rod, particularly on “open crankshaft” style engines like the Webster or Kerzel can get a squirt of oil from the same squirt can, and that will suffice. The small end of the con rod, up at the wrist-pin can be easily accessed by rotating the crankshaft until the piston is at bottom dead center, and at that point it becomes quite easy to apply a liberal squirt of oil to the inside of the piston to lubricate the small end of the con-rod. Many of the open crankcase engines also had a “cup-oiler” mounted near the open end of the cylinder which was filled with oil and had a needle valve adjustment to supply an auxiliary supply of oil to the cylinder and rings. Closed crankcase engines have a reservoir of oil in the bottom of the crankcase, and depend on the splash effect of the con rod big end constantly splashing the oil all around inside the enclosed crank-case to lubricate everything.

[brian Rupnow]

Casting kit or Bar-stock?—Casting kits are great, if made by a reputable supplier. They cost a considerable amount of money, and I don’t recommend them for a novice machinist/engine builder. It is far too easy to miscalculate and ruin one of the castings, and then what do you do? Can you even buy “one of” pieces, or can you only buy another complete kit?---My vote is for a totally bar-stock engine, until you have at least three running engines under your belt, built totally from bar-stock. Then you will have hopefully reached a skill level that will allow you to work from a purchased casting kit. With a “bar-stock” engine, if you totally screw up a part, you are only out your time and another piece of round or flat-bar.


Plans—There are many engine plans free on the internet, such as the Webster and the Kerzel. There is an endless number of engine plans available “for sale” from the internet, from private individuals, and from hobby stores.---A word to the wise though.---Do a little research!!! Google that particular plan you are thinking of building and see if anyone else has built it. See if there are any remarks about how good or bad the plans are. Try and find a forum that shows that particular engine running. Listen to what the builder has to say about difficulties encountered while building the engine. I know for a fact that there are some very bad and incomplete engine plans “for sale” out there of engines which have never successfully ran. And plans that contain errors that you won’t find until you have expended a great deal of your time and effort into a dead end project.

[brian Rupnow]

Let’s see---What else haven’t I talked about? Forget about starting your new engine with a “flip of the wrist”.—No doubt this can be done, eventually, after the engine has been started, ran, adjusted, fine-tuned, and warmed up. I would be so bold as to say that less than one in one thousand engines have started right up, first time ever, with a ‘flip of the wrist”. Make yourself a starter similar to the one in the attached link and use your variable speed drill as a starter.http://www.homemodelenginemachinist....ad.php?t=22559
Kill Switch---something that is often overlooked during the excitement of wanting to see a new engine run, is a switch which will cut off all power to the sparkplug. TRUST ME---there is nothing worse than having an engine start and immediately rev up to a point where it literally explodes before you can pull a wire off of the battery or pull off the spark-plug wire. Don’t ask me how I know. Just trust me on this one!!!
Why won’t my engine start??---Okay. You’ve built your engine according to the plans, you have fuel in the tank, there is spark at the plug, coming at the correct time in the cycle.---But---your engine won’t start. Assuming that you have the cams made correctly, and the valve timing adjusted to spec, and the engine has at least SOME compression, and the needle valve on the carburetor is open at least two full turns, then you should at least hear it fire while being turned over, or see puffs of smoke. If you don’t hear or see at least some effort to fire, hold the tip of your finger over the carburetor air inlet to choke the engine while turning it over. This will pull an extra charge of fuel into the cylinder. If it doesn’t begin to fire immediately, then don’t choke it anymore, because too much fuel will bridge across the sparkplug electrode and there won’t be any spark to light the fuel charge. If you flood the engine, then you will have to remove the sparkplug and dry it off.
If your engine starts but runs rough, then try opening or closing the needle valve on the carburetor until the engine smooths out.

[brian Rupnow]

If after choking the engine you still have no hint of firing or see puffs of smoke---Are you turning the engine the right direction. Although a two cycle engine may very well run in either direction, a four cycle engine will not. Shut off the ignition, pull out the sparkplug, and while turning the engine over by hand, watch the operation of the exhaust valve (and if it is cam driven watch the intake valve)*** Well okay, you aren’t going to be able to see the head of the valve, but you can see the operation of the valve lifter or rocker arm. Make sure that everything is working in the correct sequence—Intake, compression, power, and exhaust.—Don’t feel silly if you are turning the engine the wrong way---it’s happened to the rest of us too.
If you have choked the engine and even seen some wet fuel coming out of the exhaust pipe, and still no firing, then pull out the sparkplug and ground it solidly to the engine block.—And I do mean SOLIDLY. Its not such a big deal with points ignition, but with electronic ignition if the plug isn’t solidly grounded this will burn out the electronic module. Dry the engine off with a bit of compressed air.—We don’t want a fire here. Turn the engine over by hand with the ignition on and make sure that at some point during the rotation you get a nice blue spark and a “snap” showing that the plug has fired. If you can’t see the piston clearly, straighten out a paper clip and shove it down the sparkplug hole until it rests against the top of the piston. Turn the engine over by hand and make sure the spark is occurring when the piston is at or just before top dead center. (as indicated by the position of the paper clip).