Rebuilding a paramotor
by Had Robinson
updated February 23, 2020
This general article from Utah Motor Trails can help you decide when to rebuild and gives some additional tips on the basics.
The steps given here are not adequate for pilots who have never changed a sparkplug, pulled off a carburetor, or used a propane torch, a torque wrench or a digital caliper. There are many YouTube and Vimeo videos of how to accomplish these basic tasks and pilots will save a great amount of money by doing their own work if they master some basic skills.
It is *VERY EXPENSIVE* having us or other competent mechanics service an engine. The work may cost anywhere from $800 - $1,500 for the labor alone, depending on the motor and what is the problem. We routinely receive engines that, for some reason, no one is able to fix. We have extensive troubleshooting procedures that must be done to rule out the rare and bizarre problems. These steps can take hours. When we fix an engine, we bench test it. Pilots would not believe some of the problems we encounter, often no fault of the owner but, as often as not, a result of poor QC at the factory. The best places are usually chain saw, dirt bike, Jet-Ski, or go-kart shops. An established motorcycle shop can do it but make sure they know what a Walbro carburetor is and how it works. People would not believe how many lawnmower shops have wrecked paramotors. Do NOT use them.
Do NOT attempt a rebuild/overhaul unless you are willing to invest in the specialized tools needed to separate crankcase halves, reinstall the crankshaft, and remove flywheels and drive pulleys. Every engine requires unique tools to do some of these tasks. Using makeshift tools will most likely ruin the part unless you can weld and cut steel. Miniplane-USA has most of the unique tools: Minari tools Top80 clutch removal tool Polini has a special set of tools that must be used. If there were a way to avoid these expensive tools, I would post that on these pages! The steps given here are for most motors. Minor details may be different on each motor. For example, some engines have forced air cooling and some do not. If you find yourself struggling to remove or install a part, you are not doing it correctly. Stop and do some research. With heat and tools these engines come apart quickly and go back together easily.
TIPS: When disassembling an engine, it helps to put the small parts of each group e.g. ignition, carburetor, exhaust, etc. in a small container like a yogurt cup. It is easy to get things mixed up if they are in a big pile. You are taking photos as you go along to reference later, right? With these finicky engines, cleanliness is next to godliness. The photographs below are typical and are from various paramotors. If you are not that experienced in tearing an engine apart, mcbperformance.com has these tips to help prevent serious problems.
The essential tools, these bearing removal tools, a case splitter, crankshaft puller (some engines), and an infrared laser thermometer are needed. I love to make my own tools but there is no substitute for any of the last four of these tools. You *WILL* damage your engine if you do not use them. It is a lifetime investment that will pay for itself after a few years. Your particular engine may also need specialized tools to remove the clutch, flywheel, or drive pulley.
NEVER HIT ANY ENGINE PART WITH AN ORDINARY HAMMER. ALWAYS USE A DEAD BLOW HAMMER, PLASTIC HAMMER, OR PLACE SOME SOFT METAL (BRASS OR ALUMINUM) OR WOOD BETWEEN THE HAMMER AND THE PART.
1. Disconnect the spark plug wire and remove the spark plug. Remove the propeller and then the hub. Use these instructions for the Top 80 hub. Other hubs are much easier and simpler to remove. The Polini propeller hub bolt is a LEFT-HANDED bolt. That is, it is removed by turning it clockwise. The redrive can now be removed. The Top 80 redrive is held on by (2) nuts. Some engines have belt driven propellers which require the belt to be removed before the redrive can be detached from the engine. Other engines have wet clutches (Polini) which are better removed when beginning step #10 below.
2. Remove the engine from the frame. You will have to disconnect the throttle cable, the choke control (if present), all wiring, and the fuel line connection to the carburetor. The presence of engine monitoring equipment will have to be removed e.g. a tachometer and a cylinder head temperature gauge (CHT). It is ALWAYS easier to place the paramotor face down when removing it from the frame.
3. Remove the exhaust system. For the Polini, Vittorazi, and some others, use a loop of PG line to stretch and remove the springs that hold the exhaust pipe to the cylinder port. DO NOT USE PLIERS! If the engine case has rubber mounts with studs, remove them. Use HD pliers to grab the steel plate at the end of stud, just below the rubber.
REINSTALLATION Top 80 and similar: Go to the exhaust port springs page for reinstalling these springs. They must not be torqued down but set to the correct compression. All others: The reverse of removing them. Use PG line (best) or aircraft wire to secure the springs in case they break. They will mangle your propeller if they get loose. That is, EVERYTHING that gets loose on a paramotor goes into the propeller. When reinstalling the rubber mounts on the case half, do not torque them down but use blue threadlock and HD pliers to snug them up. The threadlock holds them. Anyway, you cannot get a torque wrench on these mounts in the first place.
4. Remove the air box and carburetor.
5. Remove the reed valve assembly
6. Remove the air duct(s) surrounding the cylinder, if applicable
7. Remove the cooling box. Some motors e.g. the Minari do not have this. The Top 80 and Polini have one. In the Top 80, the starter pawl assembly must be removed first. The cooling fan will float inside the cooling box. The cooling box must be taken apart in order to remove the fan from the box. If the (4) holes are widened, an ordinary hex bit can be used to remove the cap screws rather than an extended hex bit (illustrated in the photo below).
8. Remove the ignition coil with the primary and secondary wire. If there are any washers beneath the coil, note their orientation and position.
REINSTALLATION: Follow the instructions on the ignition coil page.
9. Remove the flywheel The Polini, Vittorazi, among others, have a custom manufactured puller used to remove the flywheel. It must be used. Use a chain wrench or oil filter tool to hold the clutch on the other end of the flywheel (if there is a clutch) in order to loosen the flywheel nut. If there is no clutch to grab, you may have to hold a pulley with a big adjustable wrench. It is not a good idea to push on the center of the crankshaft (unless the puller screw has a point). If the puller has a flat headed screw/bolt, thread the flywheel nut back on just a bit higher than the shaft so that the puller does not push against the threads (they are easily damaged).
REINSTALLATION: Installation of the flywheel on most engines is a simple job, but not on the Top 80 (and some others) which must have the ignition timed. Other engines have a keyed flywheel so retiming is not necessary. In any case, it is never a bad idea to check the timing, anyway.
Sometimes, a chain wrench cannot be used to hold a flywheel while the flywheel nut is being removed, as here with the Minari. In this case, use nylon webbing.
An ordinary wheel puller may be used to remove the Minari flywheel. The Minari special tool kit, however, also contains a puller to do this and is easier to use.
The Top 80 flywheel is easy to remove but you must have long 6mm bolts. Finger-screws work very well.
10. Remove the clutch/drive pulley from the crankshaft. This task varies from engine to engine. The Top 80, Polini, and Minari require special tools. It is impossible to remove the clutch shaft nut on the Minari without their custom made tool. For aluminum pulleys, 60 seconds with a propane torch makes them come off much more easily, even just fall off.
REINSTALLATION: Make sure the clutch/drive pulley nut is torqued to specifications. Polini: see below.
Removing the clutch nut on a Top 80. An oil filter wrench, nylon webbing, or a chain wrench (best) can be used to hold the clutch. Miniplane also makes a special tool that bolts to the clutch to hold it but using an oil filter wrench is much faster.
Holding a Minari clutch with a chain wrench, the simplest and best way to hold pulleys, flywheels, and clutches. The grip on the object is immense but the chain wrench will not harm the surfaces because of the large surface area of the chain. The shaft nut on the Minari requires a custom manufactured tool to remove it because it is recessed in the clutch and an ordinary shaft nut spanner cannot be used.
Ordinarily, the Minari clutch/drive pulley will slide right off the shaft after the circlip has been removed. In the engine below, the engine shaft was frozen/corroded to the (2) bearings inside the clutch/drive pulley. Sometimes, 60 seconds of heat to the exterior of the pulley will help things come off. It helped here. The pulley has no threaded hole with which to attach a wheel puller so this kind of puller (rarely used) had to be used instead.
This Minari has no clutch so the drive pulley requires a custom manufactured puller to remove the pulley. Ordinary wheel pullers do not have the inner holes close enough to mate with the holes in the pulley.
The Polini must have the propeller hub removed in order to remove the redrive/clutch assembly. The hub bolt has LEFT-HANDED threads and is removed in a clockwise direction.
The Polini has a wet clutch. Remove the (7) cap screws, tap the outer housing a few times with a dead-blow hammer, and slide off the outer half of the clutch/redrive assembly. If the exterior rim of a pulley or hub is accessible, a quality oil filter wrench is the quickest way to secure it so nuts/bolts can be removed or installed. A chain wrench will also hold pulleys and hubs without damaging them but takes a few minutes to install it properly around the pulley/hub.
There has to be some way to hold the clutch from turning when removing the center nut. This tool is homemade. A chain wrench or oil filter tool will not work. Polini has a similar specialized tool in their kit.
The Polini clutch puller is shown below but an ordinary wheel puller may be used, as well.
The Woodruff key must be removed for everything to slide off. Grab it with a quality set of pliers and gently wiggle it loose.
If the clutch shaft and bearing must be removed, it will have to wait until the case halves are separated. At that time, the shaft and bearing may be removed following the IMPORTANT INSTRUCTIONS on the bearing replacement page. Only a tool-monkey would even think of prying the shaft and bearing out. Our part of the world has a lot them...
For reinstallation: If the old gasket is still intact (not torn) it may be reused. The redrive casing bolts must be torqued in a cross pattern and incrementally.
11. Remove the head and cylinder. There are just (4) nuts on the top of the cylinder that hold the top end of the engine to the crankcase. Note the orientation of the gaskets! Some engines are specific as to which side of the gasket contacts a particular surface. If the cylinder is going to reused, be certain that the decompression port (if it has one) is clear. A clogged port will greatly shorten the life of the starter systems and may damage the top end of the engine due to increased compression.
REINSTALLATION: Install the piston ring(s) on the piston. Install the cylinder gasket. Note whether your engine requires the squish to be measured and the correct sized gasket to be used. Use the small piston ring compressor to just squeeze the ring(s) enough to clear the cylinder. Note that there are special pins in the lands to ensure that the rings are in the correct position. If the rings are not oriented to the pins, they will not compress and the piston will not go in to the cylinder. Oil the sides of the piston and use your finger to smear oil around the inside of the cylinder. Slide the cylinder down on the studs and push the piston into the cylinder. Use a new cylinder head gasket and install the head. Torque to specifications in a cross pattern.
12. Remove the piston. Use a small screwdriver, pick, or pliers to pry out or grab, respectively, one of the circlips that hold the wrist pin in place. The Polini circlip has ears which make it easy to remove with a pair of needle-nosed pliers. Use a hot air gun to heat the piston just hotter than you can touch. The pin should slide nicely out. Some pistons may need to be heated with a torch if the clearances are particularly tight. The piston is hot enough if a drop of water on it turns to steam. (This engine still has the clutch and redrive installed.)
REINSTALLATION: There is a small arrow on the top of the piston that *MUST* point to the exhaust port. If the circlip has ears (as in the photo below), use a pair of needle nosed pliers to make the circlip just the right amount smaller to fit in the piston. The Top 80 does not have ears so a small screwdriver must be used to work the circlip into the piston. Orient the piston properly and then install a circlip in the side of the piston that faces the front of the engine. Heat the piston with an air gun, as above, and slide in the wrist pin. Install the other circlip. Turn the engine upside down and use 2 stroke oil to lubricate the connecting rod bearing.
After the circlip is removed, a brass drift, a finger, or a wooden dowel can be used to push the wrist pin out. Heating the piston is the secret to getting it out easily.
13. Remove all of the case bolts then split the case halves using the special tool. Carefully follow the directions that come with the tool. Always thread a nut on the end of the shaft that the center bolt of the tool pushes against or the bolt may damage the end of the crankshaft. Carefully watch the halves separate when force is applied by the tool to be sure the halves separate evenly. Gently tapping things with a dead blow hammer helps.
REINSTALLATION: Put the crankshaft in a freezer for at least 12 hours. It can be a tight fit on the clutch/drive pulley side of the engine and the cold will help. CRANKSHAFTS MUST NOT BE PRESSED INTO THE BEARINGS, THEY MUST BE *PULLED*. Use the crankshaft puller tool to pull the crankshaft through the bearing on the drive pulley/clutch side of the engine. Some of the older engines may have a drive shaft that has no threads or the shaft is to big to fit the puller. In these rare cases, use a crank bearing/gear puller to get underneath whatever can be fixed on the crankshaft, as in this photo of a Minari. You will have to think about how to do it and discover the means of how to protect the soft aluminum case from damage. The threaded holes in the tool can take bolts which can push against the case halve and pull the crankshaft home.
Only the thinnest coating of sealant should be used on one of the case halves. It is best applied with nitrile gloves. Put old screws, toothpicks, cotton swabs or anything that will keep the sealant *OUT OF ALL HOLES* while applying the sealant. Any sealant on threads will lock the screw and it will be very difficult, even with heat, to remove it in the future. I occasionally run into this problem due to incompetent mechanics and DIYers, including the manufacturer!
Splitting a Minari crankcase.
Once the crankcase is split just a little, it should be easy to use a dead-blow hammer to tap the shaft the rest of the way out. This motor had one of the main bearings frozen on the crankshaft (red arrow). Normally, the bearing stays in the bore of the crankcase.
An ordinary wheel puller may be used to push the crankshaft out from the other crankcase half.
Alternately, the special crankcase splitter tool can be used to push out the crankshaft. Always be sure to put a nut on the end of any shafts being pressed out in order to protect the threads.
The crankshaft completely removed from the halves. Bearings frozen on a shaft should be removed with a crank bearing/gear puller. *DO NOT REUSE BEARINGS REMOVED WITH THIS TOOL!* The puller exerts extreme forces on the bearing which may damage the inner race. Pullers should not push directly on the ends of threaded shafts. Thread a nut on the shaft so that the puller contacts it instead of the threads.
The inner race of the bearing was heated to a blue color with the torch and finally was able to be pulled off. The bearing was ruined and had to be replaced.
Another engine was badly overheated. It got so hot that the lower rod bearing and the steel face of the counterweight was scorched and the bronze bushing had begun to melt. The pilot had misadjusted the carburetor. It was an expensive mistake.
When an engine is completely disassembled, it is common to find parts frozen together. This is where the use of heat is critical in order to prevent damage.
14. Bearings may now be removed by following the instructions on the bearing replacement page. NEVER BANG OR PRESS A BEARING IN/OUT OF AN ALUMINUM CASING REINSTALLATION: Follow the installation instructions on the bearing replacement page following seal installation.
15. Oil seals are best removed with a press and the correct sized pipe/socket/drift. The drift must just clear the opening in the case. REINSTALLATION: Follow the installation instructions on the bearing replacement page. IT IS BEST TO INSTALL THE SEALS AFTER THE BEARINGS ARE INSTALLED.
16. Once the engine is completely disassembled in must be thoroughly examined and cleaned with mineral spirits and compressed air. When an engine is disassembled to this extent, it is always a good idea to replace the main and clutch bearings and all seals. In any case, follow the maintenance intervals given for your particular engine. In general, 300-400 hours is the time when a major overhaul should be performed. If you wait until the engine fails, you will have a MUCH MORE expensive repair.
17. Reassembly is the reverse. The various links may contain special information regarding reassembly. Note: The only parts that are pressed into the crankcase are the seals. Bearings must always be installed per this link. It is also good to record the default compression of the engine when it is rebuilt/new. With this information, it is easier to know when a top end rebuild is necessary.