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Rebuilding the Rotary Valve 2-Stroke Cycle Engine
 ...a cost-effective way to go real fast!

by Mark Bergfelt

This is the manuscript of the article published in the June 1995 issue of National Kart News.
There have been some additions to the original text.  These additions are in (parenthesis).
Contact National Kart News to purchase a copy of this publication complete with illustrations.

     When TKM first started importing engines to the United States, three of my old kart racing buddies were among the first to run them. Ed Hudock and Mitchell Matchett were the drivers and Russ Matchett was the owner. At the time there were still a lot of McCulloch 10ls running competitively in the open classes and I was among the racers still using them. To make a long story shorter, Hudock could not make it to some races due to a serious injury, my kart was not working right and Matchett asked me if I would like to use Hudock's kart for the day. I did, but maybe I shouldn't have. I was addicted immediately. 147+cc of raw horsepower:' Awesome! To­tally unexplainable. You have to try it.
     It's only natural for a new driver to want to go faster, and there are a variety of options available to that driver who wants to step into a quicker ride, all of which are possibilities for articles in this magazine: At the far end of the performance spectrum is the 150cc Open 2-cycle Class, as it is referred to by IKF, or the Unlimited 2-cycle, as WKA calls it, or Outlaws, as some non­sanctioned Midwestern tracks have referred to the division, or the Unlimited All-Stars, as it is known in my neck of the woods. On all but the tiniest of dirt bull rings, and a lot of those too, these are the quickest rides around.
The Unlimited Class isn't always the biggest class at a racing event, but it is always very fast and draws a lot of atten­tion. It is certainly not a cheap class, to get involved with (initially), but  after comparing initial purchase and maintenance prices to those of running an all-out Open Briggs, Unlimited 2-cycle is a very cost-effective way to go real fast. Intelligent upkeep is neces­sary to make that statement true. Mistakes can be costly.

     The axle clutch has also helped to make this class more affordable and less maintenance intensive. It has all but elimi­nated the expense created by making a tuning error with an engine clutch, which almost always resulted in destroying that costly component. Sure, axle clutches are expensive up front, but they're one of the best values in racing. The only thing I've had to buy for my 5-year-old Horstman axle clutch is oil! Prior to using an axle clutch, most of the race to race maintenance involved removal and inspection of the engine clutch. What drudgery, Engine clutches are also responsible for many of the "horror" stories associated with the "big block" 2-cycles. Not having to perform this time consuming chore has added to the fun factor of karting, which is most important.
(Since this article was published, it has become very popular to install a timing belt pulley on the engine.  The timing belt, usually 30mm wide, is very durable and is used to drive a jackshaft.  In many cases, the jackshaft is rotated at 1/3 the speed of the engine shaft.  This puts the rpm in the range of the clutches used for stock 4-cycle racing.  A clutch of this type is installed on the output end of the jackshaft.  This set-up has proven to be, reliable, relatively inexpensive, and very fast.)

     This article will outline the procedures for maintenance, and rebuilding of engines that started their existence as l35cc rotary valve engines, I say "started" because many of these wind up being 150+cc beasts. Most of the procedures will apply to 100cc rotaries, reed and piston port engines as well.

     The first step in rebuilding the engine is removing all of the external accessories, such as the drive sprocket or engine clutch, air filter, mounts and so on. Before taking any wrenches to the engine, though, a thorough cleaning of the, entire assembly is recommended. Toiling on a grimy, dirty motor is not pleasant, but working on a clean, exotic, foreign power plant can be much more enjoyable.

     The starter nut can be removed at this time. I prefer to use an air or electric impact wrench to remove this item, Using this method eliminates the need to lock the crankshaft to remove the nut. Many methods of locking the crankshaft can cause it to be twisted out of alignment. Remove the third bearing support cover. A 5mm hex key /wrench is usually what will get the screws out that hold the cover on. Next, remove the extension shaft. You will need a motoplat wrench or a large adjustable wrench to hold the rotor while using another adjustable wrench or open end wrench to unscrew the extension shaft. These are usually very tight, so watch your knuckles. Use a motoplat rotor puller to get the igni­tion rotor off. These are available from most kart shops that deal with 2-cycle engines. Before removing the ignition stator, use a center punch to mark the stator and crankcase so that you can get the ignition timing set where it was. That's mainly a precaution to take if you don't know what the ignition timing dimension is. It also gives you a baseline for a setting that works if you want to experiment with  ignition timing. We'll cover the topic of ignition timing when we put the engine back together.


     Before opening up any racing 2-stroke cycle engine, it is very wise to pressure check it, especially if it is doing weird things like not shutting off, even when you choke it violently, or it won't idle down when you get off of the throttle, or it gets real hot and the carburetor screws are turned out so far they are ready to fall out and there is little pop off pressure. Engines are literally air pumps that also burn fuel, so the more efficiently they can pump air, the better they are going to work. If there is an air leak any­where in the engine, the suction pulses created by the piston mov­ing upward will pull air in through the leakage point, causing the problems mentioned above and worse. It is so important that 2­stroke crankcases hold pressure and vacuum, that regular pressure testing should be part of the maintenance program on all racing engines of that type. To perform the test, you will need to have some type of hand air pump, hooked up to a pressure gauge. Azusa Engineering makes a unit that checks both pressure and vacuum. Walbro and Tillotson both make units for testing carburetors that can be used. I prefer my homymade version of the tool that has served me well for at least the past 18  years (28 as of the date this was posted to my web site).  I was lucky enough to have an acetylene gauge, an old bicycle tire pump, a brass brake line fitting and a fuel fitting fuel petcock and some fuel line laying around in various corners of the garage at a time when I had no cash. The tool was originally intended for tempo­rary use until I could get a better one. However, I could never find what I thought was a better one. The most critical thing to be concerned with when putting such a device together is making sure that it does not have any leaks around the fittings. The tool needs to be checked by plugging up the outlet tube and pumping it up. Shut off the inlet valve and stick it in water. If there are no bubbles, you're in good shape. If there are any leaks, fix them now.

     Now that you have a pressure checking tool, you will have to block of the exhaust port and intake port where the carburetor bolts on. A,II you need is an old inner tube. Cut out some pieces of the rubber just big enough to replace the exhaust header and carburetor gaskets, but don't cut out the holes in the middle. Remove the carburetor and header, and install these rubber inner tube gaskets. Reinstall the carbu­retor and header. If you want to get fancy, you can cut out and drill metal block off plates for pressure checking your engine. You will now need a fitting to pump air into the engine. You could install a fuel hose fitting on one of your block off plates. Azusa Engineering makes a nice fitting for hooking up your air hose to the spark plug hole. You can also make your own. by using an old spark plug shell. You have to break out the porcelain and thread the hole in the center of the plug. It's easier to buy one, but a lot of guys have more time than money, so it's nice to know that this is another item the do-it-yourselfer can make. I had to weld the hole shut and redrill it to get the size hole I wanted.

     Install your fitting, hook up your hose and you're ready to go. Pump air into the engine until the gauge reads 8 to 10 pounds. Shut off the Petcock. The gauge pressure should hold indefinitely. If the pressure drops, you have a leak. If it is a bad leak, you may be able to hear and even feel it. Brush some water and detergent solution on the suspected area to verify the situation. Slow leaks are tougher to find. Brush the leak detector solution all over the engine and look for bubbles. Sometimes it may be necessary to dunk the entire engine in a stationary laundry tub filled with water to find the leak. Re­move the ignition components before doing this. It would be wise to use a block off plate instead of the carburetor before giving the engine the dunk treatment. Make a note of any leaks that are found, and make sure that repairing them is part of the rebuild process. All types of things can cause air leaks. Remove everything you put on to pressure check the engine and it's time to move on.
It is often advisable to check the engine for vacuum leaks as well. Azusa's deluxe tool does this, as well as the vacuum pumps available in auto parts stores. I have been able to solve all of the leak-related problems that I have encoun­tered by simply using pressure.


     At this point I like to check the crankshaft end play. It's a good idea to measure it now because if there is too much we can shim the crankshaft bearings when we change them, but we need to know now, while the engine is together. To check the end play, attach a dial indicator to one end of the crank­shaft so that the plunger contacts the side of the crankcase. There are a number of ways of attaching the indicator to the end of the shaft. I made a device from an old starter nut that allows the indicator to be bolted in place. The next step is to pull on the crankshaft length wise. While holding the shaft, zero the dial indicator. Next, push the shaft firmly the other way. Check the dial indicator. The reading will be the end-play in thousandths of an inch. Record your measurement for future reference. Remove your dial indicator and proceed with the teardown.

     At this point you may remove the cylinder head or the rotary valve cover. We'll proceed with the rotary valve cover, or intake manifold, as some call it. Your 5mm hex key will probably get the screws out. Gently tap the cover with a plastic tipped hammer to jar it loose and remove the cover. Push out or pry the oil seal from the cover and remove the gasket. Hopefully, the rotary valve did not fall on the floor when you took the cover off. Use some nail polish or automotive brush-on touch-up paint to mark the rotary valve and crankshaft to make sure you get it back on in the right position. On many engines it can go on in six possible positions, but only run right in one of them. It is very satisfying to get it right the first time, but very frustrating the other five ways.

     The cylinder head comes off next. There are various sizes of stud/nut combinations used to hold the head on. Ones that require a l3mm or 16mm wrench are the ones most familiar to me. A socket wrench with a ratchet and extension will work here. TKM engines have two additional screws that come off with a 6mm hex key. After removing all nuts and screws, pull the head off. On the majority of engines, you can pull the cylinder off now. On others, a few more nuts need to come off first.

     Some  people (actually most engine builders) refer to the cylinder as the "jug." You may need to provide some gentle persuasion with a plastic tipped hammer to jar the gasket seal loose. Measure the thickness of the cylinder base gasket with a dial caliper or micrometer. You will need to obtain a new one of the same thickness before you put the engine back together.
The piston is the next candidate for removal. It would be a good idea to get a pair of safety glasses for this next operation. Removal of the piston pin clips ranges from simple to difficult, depending on what type you have. The "rabbit ear" type are easiest. Needle nose pliers make short work of those, but they still have a habit of slipping off of the pliers and sailing off to some hidden corner of the shop. Spiral lock clips are tougher. Start fishing them out with a scratch awl or other thin tool. Good luck. There's also clips that have no ears to grab with pliers.

     It is best to use a piston pin removal tool to get the piston pin (wrist pin)  out. MISI makes one that works real well. The tool reduces the risk of bending the connecting rod while removing the pin. The special tool is not absolutely necessary. If the piston is adequately supported, a large flat punch will do the job, but it is important not to stress the rod from the side. When you remove the pin, regardless of the tool, lay a rag on the bench to catch the wrist pin bearings. That step is important if you have loose rollers or don't know what's in there. I recommend the use of the caged type top end bearing that is used in K-30 Komets in all of the big 2-strokes. Once the pin is out, the piston falls right out. Be gentle with it if there's a chance you might use it again, even though you probably shouldn't.

     ­Get your 5mm hex key wrench and remove the screws that hold the crankcase halves together. Gently tap on the case halves with your plastic hammer. Once they're apart the crankshaft/connecting rod assembly can be removed.


     Crankshaft rebuilding requires special tools that most people don't have and prob­ably won't use very often, so it is advis­able to have the crankshaft rebuilt by a reputable kart or even motorcycle shop. Truing a built-up crankshaft is an art and requires practice and patience. The proce­dure wil1 be explained next. The reader may want to give it a try (If you screw it up, send it to BRE to get it straightened out.  After all, fixing engines is a big part of how we earn our living.), or should at least know what he is paying for.

    Before rebuilding the crankshaft, mea­sure the width of the assembly at the crankpin. The width of the crankshaft affects the end­play. If you arbitrarily change the width somewhat, you will need to compensate when installing the main bearings. To re­build a built up type crankshaft, the crank­pin must be removed. This is accomplished by using a crankshaft splitting plate and splitting cylinder, a push pin and a hefty hydraulic press. Set up the splitting cylin­der and plate so that the crankpin is lo­cated at the center of the splitting plate. Be sure to locate the connecting rod so that it protrudes from the slot in the splitting cyl­nder. Set up the splitting cylinder in the press so that you can use the push pin to push the crankpin out of one-half of the crankshaft. After doing that, remove the thrust washers, connecting rod and big end bearing from the crankpin. Flip the side of the crankshaft with the pin remaining over and set it up on a plate in the press. Using the pushing pin, press out the crankpin.
Carefully inspect the connecting rod for any imperfections. You don't want to replace this part unless you really have to because it can be very expensive, but if it isn't in perfectly serviceable condition, don't risk trashing an engine with a marginal rod. It should also be checked for straightness by using a surface plate, parallel bars and pins that have been machined to a close fit of the bores of the big end (bottom) and little end (piston pin end) of the connect­ing. Insert the pins in the rod and set it on the parallel bars. It should sit perfectly flat. Then lay it flat on the plate to see if it lies flat. A twisted or bent rod can be straight­ened using specially made tooling. It is highly advisable to take the rod to an auto­motive machine shop that has magna flux testing equipment and get the rod checked. While you're at it, take the crankshaft halves and get them checked too.
Before pressing the crankshaft back together, obtain a new crankpin, bearing assembly, and thrust washers. Lay one crank­shaft half on a plate on the press and care­fully press the pin into the hole from the inside out. Apply some good quality grease on a thrust washer and put it on the pin. Grease up the new bearing assembly and place it on the pin and install the connect­ing rod over the pin. Install the second thrust washer. Carefully place the other crankshaft half on the pin, align them up close by eye and place the parts in position in the press. Press the crankshaft on the pin just enough that they will stay together. Remove the as­sembly at this point and check to see that they are closely aligned. A square laid across the crankwheels will help with the check. Use a brass hammer to smack the crank­shaft halves into alignment. Place the crank­shafcassembly back in the press and press it all the way together. If you have the right pin, the rod will rotate free and smooth on the pin but it will not be sloppy.
Now comes the tedious part - aligning the crankshaft. You will need two v-blocks and a surface plate or the centers of a lathe or a truing fixture. The main criteria of these tools is that they are perfectly rigid and sta­tionary. I prefer to use myoid faithful South Bend lathe. You will also need two dial indicators and the necessary fixtures to mount them in place. Position the indicators so that they contact the main bearing journals of the crankshaft. This setup will allow you to check the run-out of the journals. To check the run-out, rotate the crankshaft using the connecting rod. Check to see the highest reading on the dial indicators. Zero the indicators at that point. Rotate the crank again and check your indicators. If the one needle rises while the other falls, and the crank halves are twisted about the crankpin, they are not concen­tric. This is called vertical misalignment.  If this is the case, remove the crankshaft assembly from the centers and smack the crankwheel where it aligns with the high reading dial indicator. Use a large brass hammer and hit it hard. If the needles rise and fall together, the flywheels are not parallel. This is called horizontal misalignment. If the high point is on the crankpin side, the flywheels will need to be spread apart. I use a large modified cold chisel wedge and my brass hammer for this operation. Insert the wedge between the flywheels and tap. Re­move the wedge and recheck the alignment. If the high point is opposite the crankpin, the flywheels are spread apart. Use the brass hammer and smack the outside of one flywheel toward the other and recheck the alignment. As you can probably imagine, this can become a tedious production, especially when you realize that the goal is zero run-out.  Those needles on the dial indicators should sit perfectly still when you are done. Don't be surprised if, after a long time, you get very close but can't get it right on the money. If the center lines of the crankpin holes and the center of the halves are not exactly the same distance apart on both crankshaft halves, perfect alignment may not be obtainable. This is most likely to occur when a broken half is replaced. Remember, the closer to perfect you can get on alignment, the more RPMs and power the engine will make.

      LAD makes a fixture for pressing crankshafts together that is supposed to make this task easier. I have never used one so I can't comment on its effectiveness. The method I use is found in every motorcycle mechanics textbook I've ever seen, and it works.


     Once the crankshaft is aligned, put it away somewhere where it won't get knocked around. It does not take much to knock a crank­shaft out of alignment. Once it is supported by the main bearings between the crankcase halves, it will be considerably more durable. Before you put it aside, though, get your calipers or micrometer and measure the width of the crankshaft and compare it to your measurement before rebuilding the crankshaft.  ­If your crankpins were the same length, then the chances are the measurement will be the same.

     A hot plate can be used to remove the main bearings from the crankcase halves. Lay one case half on the hot plate with the bearing opening facing down. Heat the case half up until the bear­ing either falls out or can be removed by light tapping with a flat punch or similar tool. For the next step, get a large pair of
pliers or thick welding gloves because the case half is hot enough to bum you badly. Remove the case half from the hot plate and lay it on the bench with the bearing opening facing up. Drop a new, cool, bear­ing into the opening. Make sure it goes in straight.  (I am assuming that the case half is squeeky clean to begin with.  If it isn't, make sure that it is before installing the new bearings.  The inside of the engine can't be clean enough.)  Repeat the procedure for the other side. If the rebuilt crankshaft turned out to be narrower, or if you want to cut down on the amount of crankshaft end play, drop a bearing shim that is one-half the amount of space that you want to take up under each main bearing before it is inserted. Allow the case halves to cool until they are cool enough to touch with your bare hands.

     Apply gasket sealer to the gasket sur­face of the case halves and let it get tacky. I prefer Yamahabond No.4. (I now use Mega O.E.M. Grey Import Gasket Maker that I buy at a local Auto Zone store.)  Slide the mag­neto end of the crankshaft into the magneto half of the crankcase. Slide the rotary valve case half over the rotary valve end of the crankcase. Gently push the two case halves together, being careful not to apply any force that could cause accidental crankshaft misalignment. Insert the appropriate screws into their holes and tighten them in a criss­cross fashion. Torque them to 85 inch pounds. If everything is right, the crank­shaft will revolve smoothly and easiIy.

     Now is as good a time as any to recon­dition the cylinder. If this is a simple "fresh­ening-up," the cylinder is probably not dam­aged and is only glazed. "Glazed" is a term that refers to the shiny look that the cylin­der gets from the rings rubbing on it. This glaze must be removed if the new piston rings are to break in properly. A hone is used to remove the glaze and replace it with a crosshatch finish. Crosshatch is extremely fine, uniform, crisscross scratches that are usually about 45 degrees to each other. These scratches give the 2-cycle oil a place to sit, so that it can lubricate and help the rings seal things up. They also help the rings to seat in and match the contour of the cylinder better.
In order to ensure that the cylinder is round when the engine is totally assembled, the cylinder should be sandwiched in a torque plate for the honing operation. This is most important if a precision ridged hone is to be used, and it isn't a bad idea even if you are using a ball-type hone. Spring-type glaze breaker hones are not recommended for use on racing engines. Follow the directions for the hone that is being used for best results. Only a few strokes of the hone will be nec­essary to restore the crosshatch finish if the cylinder is in good shape. A cylinder with vertical scratches should be honed until they are gone. (especially in the area above the exhaust ports.) At that point the cylinder diam­eter should be carefully measured and a piston obtained that corresponds with the result­ing oversize. The cylinder should be finish honed to obtain the clearance recommended by the piston manufacturer.

     After the cylinder is honed, the edges of the transfer and exhaust ports should be broken (rounded off very slightly) with a piece of fine emery cloth. This helps the rings to hold up longer.


     The cylinder head is a relatively simple part but should not be neglected. Carbon buildup can be removed with steel wool. The steel wool can also be used to polish the head to a high gloss. It is harder for carbon to stick to a shiny, slick surface. A cylinder head turning tool that screws into the spark plug hole of the head (installed in the chuck of an engine lathe) makes this task easy. If the head has been dam­bged by a bearing or some other item coming apart, the head can be turned in a lathe to restore the surface. Care must be taken to retain the original contours and head vol­ume. This article assumes that the head has not been damaged or the compression ratio is not to be altered.

     Many of the foreign rotary valve en­gines do not use a cylinder head gasket. You can't blow a gasket that isn't there and this arrangement ensures good heat transfer between the parts. In order to en­sure a good seal between the cylinder head and the cylinder, the head can be matched to the cylinder by lapping the parts. Apply valve grinding compound to the top of the cylinder and place the head on top of it. Rotate the head on the cylinder like you were lapping a poppet valve. (4-cycle guys should be really good at this step.) Wipe off all traces of the valve grinding compound. In­spect the mating surfaces of both parts. They should both have a uniform dull gray ap­pearance
     Proper cleaning of the cylinder is a simple, but most important step in putting together an engine that will give maximum service. This is also true of any part that is put into a precision engine, especially those that have had machine work done on them. First, parts need to be cleaned in a typical parts cleaning solvent and blown off with compressed air. Second, (AND MOST IMPORTANT) the parts need to be scrubbed in a solution of hot water and laundry detergent. They then need to be rinsed in hot water and then dried immediately. Lightly oil all ferrous metal parts (iron or steel) so that they do not rust. It is important not to skip the detergent bath. This step strips all oily residue and very fine metal shavings from machined parts, especially cylinders. Most solvents can't completely strip all resi­due and particles away. These particles are abrasive and will cause premature wear of expensive vital parts if left in the engine. Everything going into the engine should (must)be squeaky clean.

     Before installing the rings on the pis­ton, they must be checked in the cylinder for proper end gap. Quite often, the en­gine, ring, or piston manufacturer will specify the best gap for his or her product. It is wise to follow their instructions.
To check the gap, place the ring into the cylinder and even it up with a piston. Measure the gap with a feeler gauge. If it is too small, the end of the ring will have to be filed carefully until the desired gap is achieved. If the gap is excessively big, get a larger oversize ring and file it to the de­sired gap. Make certain, when filing end gaps, that the ring fits around the pins prop­erly and without binding. It is better to have a gap that is a few thousandths too big than one that is too small, but right on the money is always best.

     At this point in the rebuild process, I prefer to install the piston. Carefully in­spect the top end needles, cage (if so equipped), wrist pin (piston pin), and thrust washers. If any of these parts are not in perfect condition, replace them as needed. (Actually, you should always replace these parts when freshening up an engine.  Sure it costs more, but that's a whole lot better than loosing that $1,000 first place money because a bearing came apart on the last lap of that big race.  When I rebuild an engine, especially for a customer, I would rather hear complaints about the price tag for the rebuild, than the complaints that come with an engine that came apart at a crutial time.) If you are using a loose needle-type bear­ing, use white grease (Now I use Sta-Lube Engine Assembly Lube with moly-graphite, from CRC that is available in auto parts stores.) to glue the rollers in place in the connecting rod. The grease can also be used to hold the thrust washers to the side of the rod while installing the piston. Whenever possible, I prefer to use the caged type bearing that is used on Komet K-30 engines. With this type of bearing the grease won't hurt, but a good motor oil is all that is needed as an assembly lu­bricant. They're easier to install, and much easier to deal with if, in case of an emer­gency in the pits, piston service becomes necessary.

     Put a few drops of engine oil on the piston pin bores of the piston. Slide the piston in position over the connecting rod. Carefully slide the wrist pin through the piston and connecting rod. This operation is much easier if you use a piston pin tool, such as the one available from MISI.


     Be sure that the piston is not put on backward. The location of the piston ring pins is most important. A mistake here could result in a damaged piston or cylinder. Se­cure the wrist pin by installing new piston pin clips. Never re-use old clips and watch your eyes while dealing with those little devils. The piston pin clips should be in­stalled with the gaps toward the top of the piston or the bottom, but never to the side. This precaution will eliminate the piston motion from causing the clips to come out.
Find your new cylinder base gasket. It needs to be the same thickness as the old one unless you wish to alter the perfor­mance of the engine. Changing the thick­ness of this gasket will alter the port tim­ing and the compression ratio. I like to coat the gasket with motor oil before in­stalling it. It helps it seal by making the gasket swell slightly and also makes it easier to remove later.
Before installing the cylinder, oil the bore with motor oil (I keep a bottle of chainsaw/lawn mower 2-stroke oil in a squeeze bottle for that purpose). This will ensure ini­tial lubrication and keep the cylinder from rusting if the. engine is not goingt to be used right away. Slide the cylinder on to it's studs and carefully lower it onto the pis­ton with one hand. Use your other hand to squeeze the rings together and slide the piston into the bottom of the cylinder.
I have found that two three-fourth inch wide strips of plastic cut from an old number plate make a good tool for hold­ing the piston steady while sliding the cyl­inder over the piston. The strips should be four inches long or longer and be placed under the piston, parallel to and on either side of the rod. After the piston is started into the cylinder, the strips can be easily slid out.  (I don't actually do this any more.  I've done it so much that I personally find it easier to leave the strips out, but I remember well when it seemed like I just had to use them.)  Carefully slide the cylinder on all the way. Make sure everything is together right by turning the ends of the crank­shaft. Everything should slide or rotate smoothly.

     Crankshaft seals are a very important part of a 2-stroke cycle engine. A leaking seal can (will) cause disastrous results. Seal re­placement is a mandatory part of a 2-stroke rebuild, and periodic replacement between rebuilds may be necessary to prevent ex­pensive failures. Apply motor oil to the lip of the seal that will contact the crankshaft.  (Now I stuff the inside of the seal with the moly-graphite engine assembly lube.) Apply a thin film of silicone sealer to the outer edge of the seal. This acts as a lubri­cant while installing the seal and ensures that there will be no leaks around the out­side of the seal when it cures. On most for­eign engines the seal can be pressed in with your fingers. One seal will be installed on the motoplat side of the crankcase, and one will be installed in the rotary valve cover.

     Install your motoplat ignition stator and rotor at this time. Use the same wrenches that you took it off with. Do not tighten the stator screws yet. Attach a dial indicator to one of the cylinder studs. Zero it at top dead center. Turn the crankshaft backwards until the indicator is between. 0.110 inch and 0.085 inch. These dimensions are for l35cc engines. Longer stroke engines, like many Komets, will want timing closer to 0.110 and shorter stroke engines like TKMs will be happier closer to the 0.085 dimension. Ex­periment to get the best results for your engine.  (More advance, ie., 0.110 will give crisp throttle response, good bottom, but may heat up more.  Less advance, ie., closer to 0.085 will give better top-end or sustained full throttle operation.  Much more can be said on the topic.)  With the piston set at the dimension that you desire, line up the marks on the stator with the rotor, then tighten the stator screws.

     The cylinder head can go on at this point. If your cylinder head was modified to use an o-ring seal, install a new o-ring at this time. Some engine builders prefer to wipe a very thin film of copper-type sili­cone sealer on the sealing surface of the cylinder before installing the head. I have done this, and it works, but it is far better to use none at all than risk using too much. Install the cylinder nut flat washers. They are easier to get on if you place them on a small punch or screwdriver and place the tool tip on top of the stud and allow the washer to slide down the tool onto the stud. Install the nuts finger tight, then torque them in a crisscross manner, first to one-third the final torque, then to two-thirds, then to their final spec. Torque the cylinder stud screws to 200 to 250 inch pounds (thicker studs use the higher torque). Of course, you'll need to use a torque wrench.

     Apply a coating of motor oil to the surface of the crankcase that the rotary valve will slide over. Install the rotary valve, lin­ing up the marks made earlier on the valve and crankshaft. If you dropped the valve while removing the cover, and could not figure out where to mark it, install it so that it is closing as the exhaust port is opening. The events probably won't correspond per­fectly, but they will approximately, usually within 10 to 20 degrees of crankshaft rota­tion.


     Apply a coating of motor oil to a new rotary valve cover gasket. Place the gasket on the cover and be sure that the carbure­tor pulse holes are not blocked. Poke a hole through the gasket if any of the holes are blocked (This should have said, carefully cut out a small hole with an exacto knife or some similar tool.). Install the rotary valve cover and screws. Tighten the screws in a crisscross manner.

     Screw on the starter nut as tight as you can by hand. The electric starter will take care of torquing it down (You could do that, but torque it down now, just in case). At this point, it is most important to pressure check the en­gine as described earlier. Do whatever is necessary to repair any leaks that may be found. Install a freshly re-built carburetor. Car­buretor rebuilding and tuning is another article in itself. Set the carburetor slightly rich for initial start-up and break in.

     Many engine builders have different recommendations concerning what spark plugs to use. I have developed a prefer­ence for NGK B9EGV and BIOEGV plugs.  (Since this article was written I have switched to ND W27ES-ZU (warm), W29ES-ZU, W31ES-ZU, and W34ES-ZU (coldest)  but I still like the NGK.  It is just a little easier for me to get the ND through our supply channels.) The B9EGV is the warmer of the two and I use it the most. I prefer the B10OEGV for very hot days. These are fine wire, gold­palladium type racing plugs. They offer good performance and resist fouling. Set the gap to .040 inch. I have also used the equivalent heat range Split Fire plugs and these also work well. (I have since developed the opinion that these are more of a gimmick but they do work ok).

     Install all of the engine accessories and bolt the engine assembly to the kart. I can't count the number of times that I have bro­ken in engines under racing conditions, but it is advisable to run the engine under a changing load with a rich carburetor set­ting for about 15 to 30 minutes before leaning it down and running it at full tilt. Running a kart on a stand will not do much to break in an engine. It will simply ensure that the engine does run. An engine must be under load to exert the right pressure on the rings so that they break in properly to make full power.

     Most of the engines that I build are used for Saturday night dirt oval type rac­ing. Under that type of condition I have found that a bottom end can be used safely
for about 10 shows (After using an hour meter on a variety of karts, the average engine running time for a Saturday night dirt show is about 20 minutes.). After the fifth show, I like to pressure check the engine before each race as a precautionary measure. I do the same with the carburetor (check pop-off). By all means, correct any problems as they are encoun­tered. For good performance, I like to change the rings after the fifth race. (For IAME engines, Komet, Parilla, etc, you can't get rings separately so you would have to change the entire piston assy.)

     Although I have pushed engines some­what beyond 10 races before a total re­build, this experience taught me that 10 is the practical absolute limit. Many engine builders feel that's too many.  (The newer Parillas and similar engines produced now can safely go a few more shows).  After that, don't be surprised when the main or bot­tom end rod bearing comes apart, destroy­ing other parts in the process. That's one of the causes of the horror stories that are associated with the big rotary valve 2-strokes.  Don't misunderstand me here. These engines will take a beating and perform well, but don't expect the bearings to last forever. They won't

     A 135cc Open rotary valve engine is certainly not cheap (to buy brand new, initially), but when properly pre­pared and maintained, it can provide a cost effective­way to go real fast. (It could even wind up costing no more, or possibly even less to maintian than a top level "stock" class flat head 4-stroker.)

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