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Restoring a Mugen 125                                                                                                                             


Welcome all, I hope everyone who visits this site can find something useful and/or meaningful to take away from this.

Although this entire document references a Mugen 125, “anyone” building a 1979 Mugen 360 to a 1974 KX125 should be able to apply the majority of this information to their specific motorcycle project.

After collecting and restoring CR’s and Mugen’s for the past 20 years I’ve made more then my share of mistakes and errors. Learning from most of the mistakes (hopefully), I thought that it would be great to pass some of this information on to other’s just starting out with restoring dirt bikes; and perhaps save them some time and money.

Art of this world can come in several different forms such as: paintings, sculptures or drawings to name a few; but this document will give you detailed instructions on how to take your old, worn-out, dirty and abused Mugen ME125 motorcycle and restore it to a true work of art, inside and out. Good Luck!

Best Regards,
Robert D. Hawes Jr.
Gilbert, Arizona U.S.A.

First Step - Find your bike…

The process of finding the “right” bike could take as much time as one would spend actually restoring the bike; there are many things to consider, but the two most important are price and condition. Both should be directly related, the better the condition, chances are, the higher the price.

When considering the price, also consider what parts the bike comes with; such parts like aftermarket aluminum swingarms, aftermarket forks, high quality aftermarket shocks, Mugen engine/suspensions parts will dramatically affect the value of a bike.

Other major factors on the starting price of ME125 project will be the year of bike you chose and whether or not the its a “kit bike” or a “factory Mugen”. The main difference between the two would be the frame. The forks, swingarm and radiators (if air cooled) can also be factor in difference between the two types, but they can all be adapted to most CR chassis of like years. Because a factory Mugen frame is all TIG welded, it will be easily spotted when next to a Honda CR frame.

Second Step - Clean and Inspect

I would like to share with you some keen insight in restoring a ME125 to better then new condition,
while limiting the number of mistakes and time required in the process.

You finally found one, although dirty, oily and abused, the rare Mugen ME125 now sits in “your” garage waiting your attention and the only question is “where to start…the engine of course?”

When restoring this ME125 engine, you must first clean all the dirt and grime from the cases, cylinder and head. At this point, all fluids should be drained from the engine and is removed from the chassis. Before cleaning, take two rags and plug the engine’s intake and exhaust manifolds. Place plastic bags over the ports and attaché them with rubber bands. This will assist in keeping dirt and water out while cleaning. Check that all bolts and the spark-plug are tightly fastened. Remove the kick starter and gear shifter and set them aside for later solvent cleaning. I would highly recommend the use of protective eye ware. Use a small putty knife for removal of heavy dirt and grime.

Clean engine using soap-water, brushes, rags and degreaser, and then dry. Note the breather hole at the rear of the engine just below the swingarm pivot mount, in the center of the cases; avoid spaying water or cleaner into it. Remove all grease, dirt and grime from all surfaces, cracks and cranies. Between the cooling fins, under the exhaust port and around the counter shaft will be the heaviest soiled areas. After cleaning, lightly rinse the entire engine and dry with a towel or rag. Remove the right case cover which reveals the flywheel and stator assembly. If the area is excessively dirty, remove the stator and wipe out the area with a slightly damp soap-rag. Set the stator aside for later reassembly and place the bolts in a can. If you have compressed air, blow any remaining water from all possible areas. Remove the plastic bags from cylinder ports and let the engine dry in direct sun light or under a heater. If you don’t have compressed air, use a towel or dry cloth to wipe the water off, then let it dry in sunlight or use a hair dryer.

After the engine is completely dry, move it to a spacious and clean bench for disassembly. Loosen and remove the spark plug from the cylinder head. Pull off the cylinder head by removing the four nuts on the top of it. Avoid letting any foreign particles fall in the cylinder when removing the head. Inspect the cylinder, if the bore is damaged or worn, remove the four cylinder studs using a stud removal tool. Place the four cylinder studs and nuts in the bolt can. Remove the two bolts holding the exhaust flange on the cylinder, put the bolts in the can and place the flange with the other parts for solvent cleaning. Remove the four bolts which attach the intake and reed cage to the cylinder. Take the intake off and pull out the reed cage. Inspect the condition of both, replace if necessary then clean and remove all gasket material from, set aside for later assembly.

Now you may remove the cylinder from the bottom-end, this is done by removing the four nuts at the base of each corner of the cylinder. The cylinder may need to be lightly tapped with a rubber mallet on each side to break it free from the bottom-end. Tap the fins from straight on each side and not at an angle; if you hit them at an angle, they will bend or break. Work the cylinder from right to left and back to front in small amounts until it works free. After the cylinder has been removed, inspect it in detail for wear and/or damage. All Mugen ME125 cylinders originally came with hard-chrome bores, unless someone installed a steal liner or had it nikasiled. As with most abused 125’s, the cylinder will probably need some type of repair regardless of bore type.


Check the bore at the exhaust port bridge for scuff’s, scratches or flaking with chrome bores; if any damage is found, it should be repaired. Which repair method, depends on how the engine will be maintained when reassembled. Sleeved cylinders will need to be bored or re-sleeved (depending on the damage). If you don't normally have engine seizure problems and take good care of your bike(s) (i.e. keep the air filter very clean, run good pre-mix oil, perform proper maintenance and etc.), then you should have the cylinder re-plated with nikasil if possible. The heat transfer is much better then that of a sleeved cylinder, plus it keeps the total weight down. If you're concerned about costly repairs and possibly see yourself having top-end issues due to poor maintenance, then you “might” get more life out of your engine with a sleeve.


Now the bottom end must be masked for bead blasting. Use duct tape to mask off the cylinder base area. Put a small rag on top of the crank assembly to fill any empty space between it and the duct tape. Check that there are no missing case screws, seals or parts that would allow sand to enter the engine cases. Be sure to leave as much of the base studs revealed as possible and check that they are only finger tight. Mask the rear breather vent and the second vent located on the right side of the bottom-end, to the right of the kick start shaft seal. Bead blast the bottom-end removing all the paint, rust and corrosion. After the studs have been bead blasted, remove them and blast in each of the threaded holes. The swingarm pivot hole should be blasted as well. With an air compressor, blow off all the excess sand and wipe with a rag or brush if needed. Tilt the bottom-end up side down. Carefully remove the duct tape and rag so that any sand falls away from the crank area. Wipe the crank area free of any remaining particles. Remove the tape from both vent holes and wipe away any sand left behind. The bottom-end is now ready to be disassembled according to the maintenance manual.

Before the disassembly of the bottom-end, place some newspaper under the bottom-end to soak up any oil still left in the cases. Remove the flywheel from the crank. Take off the left cover, the clutch assembly, crank drive bolt with gear and collar, the shift shaft and the shift drum-stopper assembly. Remove the clutch pull arm, drain plug and split the cases, note the order of assembly for the transmission. Set the left case half aside to let the excess oil drain. Remove the kick-starter assembly from the right case half. Pull all the seals from the right case; then remove the right crank bearing, counter shaft bearing and right main shaft bearing from the right case half and set aside. Remove the transmission assembly, shifting mechanism and crank assembly from the left case half. Then remove those accocitated bearings and seals. With a gasket scraper, lightly scrap off all gasket material from all mating surfaces before cleaning the parts. Clean with solvent or gasoline, clean all critical parts first to make better use of the solvent. For example: clean the transmission, clutch, and crank assemblies first, these parts should only dirty to solvent slightly. Inspect all parts for damage or excess wear as you clean them. Note all parts which need replacing and/or repairing. With the remaining solvent, clean the kick starter, shifter and case screws. Use carb cleaner to remove any oily residue from the parts. After all parts are clean, group them in baggies for later assembly.

The cases and covers are now ready to be masked for powder coating, using the special hi-heat tape and a hobby knife. Apply tape to all mating surfaces of the center cases and covers. Trim off excess tape overlapping the outside edges. Clear the tape away from the holes matching both cases and the cover so they can all be bolted together. Use 8 or 9 screws for the cases and 4 or 5 for the left cover. For example: 4 screws around the crank, 1 screw at the front of the engine, 1 at the rear behind the counter shaft, 1 screw under the clutch pull arm and 1 at top center for the cases. The right cover will be coated as a separate part. All shaft/seal holes should be masked off. Bolt the cases together and the left cover on the left case half. Mask the entire cylinder base area and trim off the excess tape around the edges. Don’t for get to mask off the drain plug, SN# tag and the swingarm pivot area, the more attention to detail.


Once the cylinder has returned from being repaired, it and the head must be masked for powder coating. Mask the spark plug hole and four stud bolts holes in the cylinder head. Mask the cylinder base, exhaust port, intake port and cylinder head mating surface areas. Now the cases, right cover, cylinder and head can be powder coated.

After the cases, cylinder, head and right cover are returned from being powder coated and all engine parts have now been replaced, restored and or cleaned. Disassemble the cases and remove masking from all parts. Install new bearings, seals and misc. parts then reassemble the engine according the manual. You now have a ’79 ME125 Mugen engine restored with the greatest of detail, a true work of art.