If you’re going to ride, you’re also going to wrench now and then. Most of the time that means nothing more than changing the oil, cleaning an air filter, or simple adjustments. Every so often though your mechanical buddy will need a major rebuild, the most common of which is replacing piston rings. On a two stroke this was no problem; pop the cylinder off, hone the bore, install new rings, and get back to riding, often times in the same afternoon. Today’s high-performance four-stroke motors are an entirely different animal however but there’s no reason to be afraid of the job. We’ll show you how to rebuild a modern four stroke engine and we’ll give you a few tips to make the job easier.
Piston rings are designed to fit snugly against the cylinder bore, and to capture the full power of the combustion stroke by preventing superheated combustion gasses from escaping past the piston. Any leaks past the rings reduce compression, which means power loss. We recently changed rings and cylinders on our Can-Am Renegade 800 but changing the rings on any modern four stroke will follow the same procedures. Here is how to get your engine back in top shape.
1: WASH YOUR MACHINE. It makes it so much easier to work on your engine without dirt constantly trying to fall into any open orifice.
2: OWN AN AIR GUN, SAFETY GLASSES, and GLOVES. Safety glasses and gloves are a must, especially when working with springs, rings, or grit that can fly in practically any direction.
Compressed air is a very useful tool. A good air gun nozzle is super handy for blowing away grit you missed with the power washer. Be sure to blow out spark plug holes before taking out plugs.
3: TOOL TIME. You’ll need a decent tool kit. No need to buy everything, but a set with some good sockets, (preferably 6 point), a couple different sized ratchets, Allen wrenches, torx bits, and box end wrenches will be needed. An electric impact is a HUGE bonus and one of the best investments you’ll ever make.
5: COMPRESSION TEST. A compression test can tell you what’s happening with the engine before you ever open it up. Test both cylinders and record your findings.
COMPRESSION TEST TIPS. Install the tester into the spark plug hole. Turn the key on, and crank over the motor with the throttle held WIDE OPEN until the dial shows maximum pressure and holds it. You must hold the throttle fully open for maximum air flow. Record your finding; this is the DRY TEST. Now pour a small amount of oil (a small spoonful) into the cylinder and repeat the test, again with the throttle fully open. This is the WET test. If the WET TEST maximum number is significantly higher than the DRY TEST number it means exhaust gasses are leaking past the rings and the rings need replacing. If the WET TEST number is about the same as the DRY TEST, it means the valves are leaking and they need adjustment or replacement. Now you know where the problems likely are.
6: DRAIN THE OIL and RADIATOR COOLANT. We drained the coolant by removing the radiator hose where it entered near the bottom of the engine.
7: TAKE GOOD PHOTOS OF BOTH SIDE OF THE ENGINE. A couple good detail shots of each side can really help out during reassembly.
8: REMOVE EXHAUST. You’ll need to remove the exhaust to remove the cylinders. On our Renegade, that meant pulling the rear storage box for better access to a couple nuts that hold the muffler on. To remove stiff exhaust springs, tightly clamp one end of the spring with a VISE grip and use it to remove the springs. Don’t forget your safety glasses and gloves for this step!
9: REMOVE ELECTRICAL CONNECTIONS, FUEL LINES, and EFI THROTTLE BODY. Remove EFI throttle body fuel lines by releasing the spring loaded connectors. Some connecters require you pry them open, and some require you to compress them enough they release internally. On our Renegade you must press the black portion tightly to release the pressure. Once you remove the fuel line you will spill a little fuel. Cap the open line so no dirt enters. (We taped ours up with electrical tape.) Remove the EFI Throttle body from the bottom of air box. You can leave it attached to the throttle cable however.
Remove any electrical connections in the work area carefully. Look for the little release tab on the connectors, and always pull while holding the connectors. Never try to separate connections by pulling on the wires.
10: REMOVE COOLANT LINES IN WORK AREA. For crimped on connections (one-use-wonders) you’ll need to cut the steel band and replace with new hose clamps during reassembly.
11: REMOVE HEAD COVERS.
12: REMOVE CAM CHAINS. We did this by loosening the cam chain adjuster, and by removing the center bolt on the cam gear. When removing head and cylinders DO NOT let cam chains fall into bottom of motor. A small wire or zip tie around cam chain can help prevent this.
13: REMOVE THE HEAD.
15: REMOVE CYLINDER BOLTS and CYLINDER. If cylinder is stuck to base gasket do not pry between cylinder and mating base surfaces. You don’t want to scar the mating surfaces and create a possible leak. GENTLY tap the cylinder with a rubber mallet, a piece of wood, or the wooden/rubber end of a hammer handle while lifting cylinder. Now remove all remaining gasket material from all mating surfaces on engine cases, cylinder, and head.
16: INSPECT PISTON & RINGS. When checking piston, check for deep grooves & wear on skirt and sides of piston. Look for wear on rings, or a ring that is stuck in groove on piston. Also check top of piston to see if anything has contacted it. When done, cover open holes into motor with a CLEAN rag. You don’t need anything extra falling into motor.
17: INSPECT THE CYLINDERS. Ordinarily a cylinder should still display a dull crosshatch from the last honing. Ring areas may be worn completely smooth however. Worn or damaged cylinders will often show a gouge or groove from the ring, or you can feel and see a slight groove where the ring reached maximum height near the top of the stroke. If you can feel or see any grooves / wear areas your cylinder has problems and exhaust gas is flowing past the rings.
18: BOTTOM END BEARING CHECK. When the cylinders are removed you can give your lower end rod bearings and crank main bearings a quick check by grabbing the connecting rod and giving it a tug or two. You should not be able to feel any up and down movement, and only a little side to side movement from the rod and main bearings.
TIP: A sure sign of bad crank or rod bearings is excessive noise from the engine when it’s running. If you hear lots of clatter from the lower end of the motor, chances are you’ve got a lower end bearing problem
19: CHECK RING GAP and INSTALL RINGS. This step used to be mandatory but rarely do we need to adjust the gap on new rings anymore. To check for proper ring gap, slip a ring into the new cylinder, then using the piston as a tool, slide the ring up until it is square in the cylinder bore. Now you can measure the end gap (The gap between the ends of each ring) and compare it to proper specs as called for in the Clymer manual. After checking gap, gently install new rings onto the pistons while being careful not to bend or break them. A bent or broken ring means that you have to buy a whole new set.
20: INSTALL NEW PARTS. Once you’ve determined what parts you need (rings, cylinders, pistons, or all of the above) it’s time to start putting everything back together. Don’t forget the new gaskets. We usually put the gaskets on just as they come from the factory, but some riders will coat them with a thin film of oil first. During installation of new parts be sure to leave a clean rag covering the hole into bottom of engine. You don’t need anything falling down into the motor. This is especially true when installing the circlips that hold the wrist pins. Lose your grip on a circlip and they can shoot anywhere and they’re hard to find.
TIP: Remember to keep cam chain from falling into bottom of engine
21: INSTALL THE PISTON INTO CYLINDER. Coat the inside of the cylinder and the rings with a thin coating of engine oil. Now insert the piston into the cylinder by compressing the rings with either your fingers or a ring compression tool. GENLTY slide the piston and rings into cylinder. DO NOT force anything. If you feel anything catch, stop until you can correct the problem. You don’t want to break a ring in the assembly process. When cylinder is seated on base begin tightening cylinder bolts in a cross pattern. By “cross pattern” we mean by tightening one bolt part way, then a bolt on opposite side of cylinder part way, repeating the process by continually jumping across cylinder to a bolt on opposite side. This applies even clamping force to the cylinder. Snug all bolts down evenly. Now, using your torque wrench and the correct torque specs in the Clymers manual, tighten bolts to correct torque spec, again using a cross pattern. Now slip cam chain sliders (the large black plastic pieces) and tensioner into correct position while fishing cam chain up through cylinder
22: INSTALL HEAD. Install the new head gasket and thread the cam chain up through the cylinder and into the head. Now install head bolts. Again snug down evenly using a cross pattern to tighten bolts. Next grab your torque wrench again and torque bolts to correct spec using cross pattern method for tightening. Set cam chain onto cam gear, and slide gear onto cam shaft in correct position. When seated correctly, tighten cam bolt.
TIP: When reinstalling head on the rear cylinder of the Rotax engine it will be impossible to install long head bolts under fame rail after head is in place on cylinder. Slide the long head bolts into their holes before setting head on cylinder to save yourself the aggravation later.
23: CHECK TIMING. Open the two crank positioning holes on the lower right side of the engine. The big hole at the center is for turning the crank, and the second hole above it is for marking position. Don’t lose the O-rings to either cover! Install the cam chain and with the spark plug holes open roll over motor BY HAND to check timing. DO NOT use the electric starter for this step. If you have the timing off the piston may hit open valves. When checking by hand you’ll feel this if you’ve got a problem. The electric starter, however, has enough torque that it will likely bend the valve. Be sure the engine goes through a complete revolution with zero problems.
TIP… If you don’t have an Allen wrench large enough to fit the crank bolt head you can easily make an ordinary bolt do the trick. Find a bolt that fits the head and clamp it with a Vise Grip. This will work as your makeshift Allen wrench, and it will easily have enough torque to turn over the motor with the spark plugs removed.
Look into the crank position observation hole, and with a large Allen wrench slowly rotate the crank until you can read either T1 or T2 on the fingers going by. T1 means cylinder one for the forward cylinder, while T2 is for the second or rear cylinder. Stop on T1 in the EXACT CENTER of the observation hole. When you are stopped in the correct position, the two lines on the side of the cam gear should be parallel to the top of the head gasket mating surface.
24: VALVE ADJUSTMENT. Now is the perfect time to check the valves. Try to move the valve adjusters with your fingers. You should feel a slight amount of play. Take the feeler gauge and measure the gap between the valve top and the adjuster base. On our Renegade the INTAKE valves should have .004” (4 thousandths) gap, while the EXHAUST valves should be set for .006” (6 thousandths) gap. Slide the feeler gauge in and out. It should go in with just a slight bit of resistance, and have a slight bit of drag when coming out if used properly, almost like you were pulling it between two of your fingers as you lightly squeezed it.
If the proper feeler gauge won’t go in, or there is extra play, your valves are out of spec! Loosen the 10mm jam nut on the top of the adjuster, turn the adjuster as needed, and retighten the jamb nut. Firmly tighten the jamb nut (a little more than snug) but don’t go overboard like it was an axle nut. Recheck valve for correct feel with feeler gauge.
Repeat this process for each adjuster. Be sure to get correct gap for each adjuster. .004” gap (4 thousandths) for INTAKE VALVES, while the EXHAUST valves should be .006” (6 thousandths) gap. Since the Renegade has two heads, with 4 valves each, you should have 8 adjusters total.
After FRONT (T1) cylinder is done, turn crank to position T2 for REAR cylinder. Repeat adjustment process.
25: REINSTALL VALVE & TIMING COVERS. With all valves now properly adjusted, check valve cover gaskets, and reinstall covers.
26: REINSTALL HOSES, LINES, & ELECTRICAL. Reattach all connectors, fuel lines, hoses, etc. Make sure no dirt enters fuel lines, and use new hose clamps where coolant lines attach to head. Route wires neatly using plenty of zip ties.
27: REINSTALL MOTOR MOUNTS & EXHAUST. Reinstall motor mounts, exhaust, and all shields.
28: FLUIDS & FILTERS. Install new air and oil filter, and refill all fluids. We recommend the OEM fully synthetic oil, or Mobil 1 fully synthetic.
29: REINSTALL BODYWORK
30: BREAK IN. When everything is reassembled you should be ready to fire the engine. As always, let it warm up sufficiently before heading out for a ride. Break in your new motor as you would a new machine as called for in your manual. That’s usually 30 minutes of normal riding without pulling heavy loads, major hill climbs, or anything that requires extended full throttle operation or lugging the motor. Basically, vary the RPMs and take a nice, 30 minute casual trail ride that wouldn’t scare your grandmother. Your motor will thank you for it.
Back on Track
Regular maintenance is part of the fun of riding. Changing the rings on a high-performance engine sounds more complicated than it actually is, and regardless of the model, the required tools and techniques will be very similar. Usually the hardest task is getting all the other parts, plastic, and hoses out of the way! Take your time, get the correct specs for your machine from a manual, and dive in. Your engine will be running like new in no time!