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We know this is technically 'Thumper'Talk, but we also know a lot of you have 2-strokes in your garage. We want to share some tips we put together that will help you prevent and diagnose potential problems with your 2-stroke engine. Bookmark this maintenance guide to help keep your bike on the track or trail and off the bench! Article by Paul Olesen. Two-stroke engines have a storied history of being finicky beasts. If you’ve been around two-strokes for any length of time, you’ve probably heard stories that start and end to this effect: “It was running amazing...then the next thing you know - it blew”. We find these stories interesting, and empathize with the unfortunate owners or riders who tell them. At the same time, however, it is often wondered if there were any signs that could have predicted the fateful date with destruction. Photo: Steve Cox We're going to discuss and share a number of observations and diagnostic tests that can be performed to help identify whether or not your engine is going to leave you in the unfortunate role of the broken engine storyteller. While many operators are insistent that their engine gave up without warning, this is often not the case. We’ll start by going over observations that can be made with the engine running, and progress into diagnostic tests that can be routinely made to assess engine health. Recognizing Symptoms Startability Does the engine struggle to start when kicked, but is more prone to coming to life when the electric start is used or when the machine is bump started? Poor starting under normal conditions is not an inclusive sign that the engine is doomed to a spectacular failure, but it is a sign that something is amiss. Carburetion or injection issues are possible, but the bigger potential issue to be aware of resides within the cylinder. Worn piston rings can cause incomplete sealing, resulting in lower compression and more difficulty in starting. Worn piston rings or reed valves that are no longer sealing properly may be the cause of the poor startability characteristic. When the piston rings don’t seal properly, the engine doesn’t build good compression, so when kicked, the engine struggles to come to life. Similarly, if the reed petals are damaged or broken, less air will be trapped in the cylinder. Push the machine or use electric start, and the compression event is shortened via faster rotational speeds, which may be just enough to bring the engine to life. Damaged or worn out reed petals will allow air to leak out, creating less cylinder pressure and starting/running difficulties. Inconsistent Performance Does the engine struggle to hold a tune, or seem like the jetting constantly needs attention, despite relatively stable atmospheric conditions? Sporadic running is not always a death sentence, but should be investigated further. A dirty carburetor or worn spark plug can contribute to this behavior, but the problems that can lead to catastrophe are worn engine seals or gaskets. Stator side crank seals, leaking base gaskets, or intake manifold gaskets are all examples of seals that will result in air leaks which can lean out the air fuel ratio. Lean air/fuel ratios when running at full power can result in excessive combustion temperatures, which can melt a hole in the piston or seize them in the cylinder bore. Ignoring the possibility of a bad gasket or seal isn't worth the potential damage, especially with the affordability of OEM quality gasket kits from ProX. Gearbox Oil Consumption Loss of gearbox oil is abnormal, and in all cases should be able to be traced back to leaking seals or gaskets. In the unlikely event the bike tipped over or cartwheeled, gearbox oil can occasionally exit via the gearbox/crankcase breather. If the gearbox is losing oil but the leak path cannot be identified externally, there is a good chance the drive side crankshaft seal is leaking and allowing the gearbox oil to migrate into the crankcase. During the scavenging process the oil is transferred up into the combustion chamber and burned. Tracking down a leak like this and finding you need new crankshaft seals will commonly turn into a bottom end rebuild job. If you're going to tear into the bottom end to replace seals, that same amount of wear the seal experienced could be evident in other components as well, including your crankshaft and bearings. Not only does ProX make it easier to tackle a bottom end rebuild with our rebuild guide (10 Tips for a Dirt Bike Bottom End Rebuild), but one of the most recent additions to the OEM-quality replacements parts lineup are complete crankshafts. Dropping in a complete ProX crankshaft paired with a main bearing and seal kit is an affordable option for reliable performance. ProX crankshafts are assembled with double-forged, Japanese steel connecting rods, as well as Japanese big-end bearings, crank pins, and thrust washers, all manufactured by OEM suppliers. Excessive Smoke After Warm Up Since the engine is burning pre-mix oil we have to be careful here, because blueish-white smoke is a normal occurrence of two-stroke engine operation. However, excessive smoke after warm up can be an indicator of a couple problems. Blue smoke exiting the exhaust pipe after the engine has warmed may be a sign that gearbox oil is burning in the combustion chamber. While I would never encouraging sniffing your exhaust, combusted gearbox oil will have a different odor than the normal pre-mix oil the engine is using. White smoke exiting the exhaust pipe after the engine has warmed may be a sign that coolant is burning in the combustion chamber. The root of this problem is typically a leaking cylinder head gasket or o-rings. Excessive Coolant Exiting the Overflow Tube While it is common for coolant to exit the overflow tube when the bike has been tipped over or when it has overheated, it should not occur regularly. Coolant blowing out the overflow tube is another good indicator of a leaking head gasket. Note where your coolant overflow line runs, so you can keep an eye out for overheating issues. Coolant Weepage Dribbles of coolant exiting the engine around the coolant pump are indicative of a faulty water pump seal. If left unattended, the entire cooling system will eventually empty, causing overheating and an incredible amount of damage. Excessive Top End Noise Isolating top end noise in a two-stroke is easy since the only moving component is the piston assembly. Discerning what is normal takes a trained ear and familiarity with the particular engine in question. However, audible cues often present themselves when components wear or clearances loosen up. The most common noise associated with a two-stroke top end is a “metallic slap”. This is commonly referred to as piston slap, and is a result of the piston rocking back and forth in the cylinder bore as it reciprocates. This phenomena is normal, but the intensity of the slap will increase as the piston skirt and cylinder bore wear. Left unattended, excessive piston slap can result in failure of the piston skirt. Check out our complete 2-stroke top end rebuild guide here. Excessive piston slap can cause damage to the piston and weaken the skirts. It's important to check piston-to-wall clearance when installing a new piston to ensure a long operating life. 2-Stroke pistons fitted with skirt coatings also help reduce friction and operating noise. Diagnostic Checks & Tests Engine Coolant Coolant contaminated with black specks can often be traced back to a leaking head gasket or o-rings. Combustion byproducts are forced into the coolant system due to the high pressures in the combustion chamber during the combustion event. These black specks will often float and show themselves as soon as the radiator cap is removed. Gearbox Oil The composition of the gearbox oil can provide a lot of clues as to what is happening within the engine. For starters, what color is it and what is in it? Oil that appears milky is a good indicator that moisture is finding its way into the gearbox oil. The most common culprit is a faulty oil side water pump seal. A keen eye can spot various metallic particles within the oil itself. Aluminum will appear silvery gray. Bronze particles will have a gold shine. Ferrous particles will be dull and are often more discernable by dragging a magnet through the oil. Accumulation of all of these aforementioned particles will be normal in small quantities, but excessive amounts of any of them could be cause for concern. Fortunately, since gearbox and power cylinder lubrication are separate the number of causes for problems is limited and more easily pinpointed. Other than changing your gearbox oil regularly, keep an eye out for metallic particles, as those can be a sign of accelerated wear on internal parts. Cylinder Leak Down Testing While less commonly prescribed on two-stroke engines, performing a cylinder leak down test is by far one of the most definitive diagnostic procedures that can be performed to determine the health of the piston rings, cylinder bore, and cylinder head seal, whether gasket or o-rings. If any of the previously mentioned symptoms are observed, a leak down test is almost always a great next step. A leak down test pressurizes the engine’s combustion chamber and compares the amount of pressure going into the combustion chamber to the pressure that is retained. Pressurized air is administered via the spark plug hole and two pressure gauges are used to make the comparison. The piston is positioned at top dead center. Air exiting the combustion chamber can then be traced back to the piston rings or cylinder head seal. Compression Testing A compression test can be an tell-tale indicator of the health of your top end components. Be sure to compare your reading the manufacturer's recommended compression measurment. A compression test aims to quantify how much pressure builds during the compression event. A compression tester which is connected to the spark plug hole consists of a pressure gauge and a one way check valve. The engine is kicked repeatedly or turned over a number of times using the starter. The resulting pressure that is recorded can then be used to assess the health of the cylinder bore. Low pressure readings can then be attributed to problematic piston rings or leaking cylinder head seals. Crankcase Leak Down Testing A crankcase leak down test is utilized in order to assess the sealing integrity of the crankcase and cylinder. Personally, this is one of my favorite tests to perform because of its ability to isolate a number of potentially problematic seals and gaskets all at once. Components such as crank seals, base gasket, and power valve seals can all be checked to determine if they’re leaking. In summary, a crankcase leak down test is performed by sealing the intake manifold, exhaust outlet, and any power valve breathers. Then the crankcase is pressurized under low pressure. Typically, the goal is to retain the pressure in the crankcase over a set length of time. Loss of pressure is indicative of leaks, which can then be traced to their cause. Preemptively replacing components before the engine suffers a major failure is both safer and more affordable than dealing with the problem after the engine has stopped working entirely. Most problems that can occur within the two-stroke engine can be mitigated by servicing components such as pistons, rods, rings, bearings, seals, and crankshafts. Many riders dread the thought of having to service these items due to the excessively high costs associated with OEM or premium aftermarket parts. Fortunately, brands such as ProX offer a comprehensive lineup of OEM-quality components at reasonable prices, many of which are produced by OEM suppliers. Depending on what you need to service, components such as piston kits, connecting rods, crankshafts, bearings, gaskets, and seals can all be found in the ProX catalog. Replacing components as part of preventative maintenance can save time and money, especially with the availability of affordable, OEM quality parts. Find ProX parts for your machine here. Discussing specific time intervals in regards to when things should be replaced is futile. The reason is simple: different engines, maintenance practices, and applications will all have different intervals. Installing an hour meter on your engine so that you can log the number of hours the engine has run can be one of the most insightful ways to establish maintenance and replacement intervals specific to your engine, riding, and maintenance habits.

We have a used 2006 YZ450F that we're rebuilding step-by-step, and documenting along the way. In this part 1 feature, we'll go over how to replace a 4-stroke piston. Click here to watch the quick tip video to go along with it! The top end in a four-stroke can be split up into two major sections: the head, and the cylinder and piston. They both require specific attention and critical steps to ensure proper opertation once everything is back together. We replaced the worn stock piston with an OEM quality forged ProX piston kit. It includes the rings, wrist pin, circlips, and installation instructions. The pistons are available in A, B, and C sizes, to accomodate for the size of your cylinder as it wears. Our new ProX forged piston compared to the stock, used piston. Carbon deposits on the crown are common after running hours, but can decrease power and efficiency. Disassembly To prepare to disassemble your head and cylinder, you'll need to remove the seat, gas tank, exhaust system, and carburetor (or throttle body). While not always required, removing the sub-frame, shock, and air boot make accessibility to the engine a lot easier in most cases. Once those major components are removed, you'll need to remove any other components attached to the head or cylinder, such as clutch cable guides, spark plug boots, and electrical connections. Removing the subframe, airboot, and shock, in addition to the other components, provides much better access to all sides of the motor. Don't forget to remove any cable guides or other items bolted to the head/cylinder. Next, remove the cam cover, loosening the bolts incrementally until they are all loose. With that off, it is best to make sure your camshafts are not fully compressing any of the valve springs before you loosen the cam caps. You can do this by slowly rotating the crankshaft via the kickstarter. With the cam caps removed, loosen and remove the cam chain tensioner next. This will give you the slack to remove the timing chain completely. You can now lift the camshafts completely out, handling carefully. Now you can loosen the head bolts in incrementally in a crossing pattern. Remove the head and place it aside, handling it carefully. Next, do the same for the cylinder bolts, and carefully remove the cylinder. As you remove the cylinder, the piston is going to stay on the connecting rod, so it helps to hold the connecting rod steady as you wiggle the cylinder off the piston. It is always a good idea to fill the opening of the cases with a lint free rag to prevent debris or loose parts from falling in. Remove the cam cover and head bolts incrementally until loose. This prevents the chance of warping. Finally, you can remove one wire lock from the stock piston using a pick or small screwdriver. Slide the wrist pin out, and remove the piston from the small end of the connecting rod. Be very careful no to drop anything into the cases during this step, and throughout the entire process. Cleaning With everyting removed, you'll need to clean any old gasket material and other residue off your sealing surfaces. This includes the base for the cylinder on the cases, top and bottom surfaces of the cylinder itself, and the bottom surface of the head that seals to the cylinder. For large or difficult pieces of material, it is common to use a razor blade for removal. However, be gentle and careful not to put deep grooves or scratches in the surfaces. Also, don't cut your finger open, or off. Scrape old gasket material off carefully, being cautious of any grooves or scratches in sealing surfaces and personal injury. Final cleaning commonly consists of using carb cleaner, or a similar chemical cleaner, and a rag to achieve completely clean and flat surfaces. Cylinder Prep Before you go and put that cylinder back in with your new piston, you'll want to inspect it for signs of wear, and measure it to make sure it's within spec (refer to your owner's manual for proper specifications). If there is minimal glazing on the cylinder, no grooves worn in, and it's within spec, you should be ready to reinstall after a good honing. Always use a diamond tipped honing brush for resurfacing work. If you're unsure about performing any cylinder prep work yourself, talk to your local dealer about cylinder shops, where any prep work required can be performed. ProX pistons are available in multiple sizes to accomodate for cylinder wear, so be sure your bore measurements correlate with the size of piston you're installing. Make sure your cylinder is the correct bore size for your piston, and properly cleaned and honed, as pictured here. Reassembly When you have your cylinder prepped and ready, now is a good time to double check your piston-to-wall clearance and ring end gap. For piston-to-wall, measure the size of your ProX piston using a micrometer only. Measure the piston on the skirt, 90 degrees from the wrist pin bore, at the point on the skirt that is 1/4 of height of the piston from the bottom. Refer to your manual for acceptable piston-to-wall clearance range. When measuring ring end gap, install the top ring and second ring (seperately, and if applicable) approximately 1/4" into the bore. Use a feeler gauge to be sure ring end gap is within the dimensions specified in your piston kit instructions. ProX rings are pre-gapped, but it is always good practice to double check. While ProX rings are pre-gapped, it's still a good idea to double check your ring end gap. Install the rings in the proper order and location on your pistons. Refer to the instructions that come with ProX piston kits to be sure you are installing the rings in the correct fashion and location. After this, install one wire lock into your piston, being sure it is properly seated. Click here for our tips on installing wire locks. Use your finger to put a layer of motor oil on the cylinder wall. Next, put a layer of oil on the outside of your new piston (on the outside of the rings, on the ring belt, and on the skirts). You don't want your new piston and rings breaking in under dry conditions. Use the normal motor oil you use in your 4-stroke. Piston installation can be done via more than one method, but in our case, we installed the piston in the cylinder before attaching it to the connecting rod. Either way, be sure your piston is facing the correct direction, meaning the exhaust valve reliefs line up with the exhaust side of the head. There will be markings on the crown of ProX pistons to indiciate which side is the exhaust side. Also, make sure your rings remain in the proper location as you slide the piston into the cylinder. The arrow shows the marking on the piston crown that indicates that is the side of piston that needs to face the exhaust. Before installing the new base gasket, piston and re-installing the cylinder, make sure the surface is clean and the crankcase is free of debris. While the top end is off, this could also be a good time to make sure your crankshaft is in spec. Next, lay your new base gasket on the cases, lining it up properly. Install the piston (which should remain in the cylinder) onto the connecting rod by lining up the pin bore with the small end bore, and sliding your new wrist pin (put a layer of oil on this before installing) completely through, until it stops against the one wire lock previosuly installed. With the piston secured to the connecting rod via the wrist pin, install your remaining wire lock, and make sure it is properly seated. You can now slide the cylinder all the way down to meet the cases. Note: Make sure you take any rags out of the cases before reassembling! You're now at the point in reassembly where you will install your rebuilt head (details in part 2 of this top end rebuild soon to come) with the proper head gasket, and re-install all the items previously removed. Be sure you are following all proper torque specs specified in your manual. Head back for part 2 of the the top end rebuild, where we'll show you some great tips on assembling a four-stroke head with new valves and valve springs, re-installing camshaft(s) and timing chain, and checking and adjusting valve clearance. Our new ProX piston and freshened up clyinder successfully installed. Note the dot on the piston crown, indicating that is the exhaust side. Stay tuned, more rebuild tips to come!