Can You Rebuild A Motorcycle Engine?

Motorcycle engine

The question of rebuilding a contemporary motorcycle engine will arise sometime during its lifetime. When, how and under what circumstances will depend on the physical condition of the internal and external components of the engine. 

Even the simplest of engines is made up of many moving components and each has its role to play in this complex assembly.It’s no secret that a motorcycle engine that has been looked after well will have a long and relatively trouble-free working career. 

When we mention ‘looked after well’, we mean serviced regularly with manufacturer recommended (preferably) oil and filter changes besides engine flushing at least once every 30,000km. An engine that has clean parts inside correctly lubricated will live longer. 

In contrast, a motorcycle that has been thrashed and ill-treated will have a short lifespan and will eventually involve costly repairs or in the worst case scenario scrapped.

So, back to the original question, can you rebuild a motorcycle engine and how does one determine when to act? Unlike tyres and batteries that are considered as distress purchases because most riders will not think about them until they completely fail and virtually immobilise the motorcycle. A used motorcycle will exhibit several signals from mild to distress. 

Most of the symptoms listed below can be observed by the rider. Besides normal wear and tear, even a new machine can be seriously impacted in case of flood damage or if it is an accident repair.

Besides the above, engines of vintage or classic motorcycles are rebuilt by restoration specialists who may have to incorporate some upgrades to make them more reliable and performance friendly.

Most period vintage and classic motorcycles that were designed and engineered to run on leaded fuel will need some upgrades to some of the engine parts to help them run on modern fuels that are available in the market now.

The rebuild could be partial or full from ground up. 

Here’s a list of common tell-tale symptoms that all is not so well with the engine.

Frequent or unexplained overheating

A motorcycle engine that seems to be running hot is not something that should be ignored. The heat that’s generated by combustion is so intense that it discolours the chrome exhaust and can warp cylinder heads or cause a complete seizure.

Usually happens due to faulty ignition or valve timing or thermostat failure in liquid-cooled engines. If the thermostat fails, it may not allow the coolant to circulate properly. 

What’s more, using an incorrect heat range spark plug can also cause overheating or in extreme cases auto ignition. A choked exhaust can also impact the motors’ back pressure and restrict exit of exhaust gases. 

Scale of repair: Medium to Major

Solution: This calls for an extensive investigation in all departments — inside out. Begin with spark plugs, ignition timing, thermostats, radiator or hose pipe leaks, fan and exhaust system. 

Although damaged radiators can sometimes be fixed, it’s usually a good idea to have them replaced to avoid future recurrences.

Seized or broken piston

Piston damage is usually the reason behind an engine’s catastrophic failure. Recognisable symptoms are loss in compression, increased emissions, gas leaks. 

And if there’s loss of lubrication due to oil pump failure, if the engine is pushed it will also damage piston rings.

During an extremely stressed engine operation, pistons (fatigue fracture especially in substandard ones) can collapse or break as a result of a forced fracture. 

A forced fracture happens when a foreign body enters the cylinder and has an impact with the piston while the engine is running. 

These foreign bodies could be anything — ranging from fragments of the connecting rod, crankshaft or valves. Two types of common piston cracks that appear at the top of the piston, on the crown, or down the side of the piston.

Scale of repair: Major

Solution: Whether you like it or not, a broken piston has to be replaced with a new one and will require re-boring the cylinder liner. Connecting rods will need to be checked along with big end bearings and gudgeon pin bushes. 

Crankshaft will also have to be balanced and bearings (roller pins in older design crankshafts) replaced and balanced. For multiple cylinder engines, irrespective of the number of pistons damaged, all of them have to be replaced.

Loose piston rings

The job of piston rings is to control oil pressure and regulate engine oil consumption. Worn or damaged rings if not attended at early stages will usually lead to more problems and complications that will require an engine repair.

Typical symptoms of loose piston rings will mean oil finding its way into the combustion chamber an d fouling the plugs frequently and bluish white smoke from exhausts. Bluish white smoke can also be due to other engine components failing.

Scale of repair: Medium

Solution: A pressure test done to the engine will highlight the exact problem area that needs attention. Removing the cylinder head is another way to inspect the piston, rings and condition of the liner. 

If the cylinder is smooth and doesn’t have scratches, and the tolerances between piston and liner within acceptable limits, then a fresh set of rings will do. 

Of course, things like gaskets and consumables like lubricants have to be changed. 

Blown head gasket

If you notice continuous white smoke coming from the exhaust pipe then there are several possibilities for trouble spots. A blown head gasket is one of them. 

However, there are a few more symptoms that can exactly pinpoint a gasket leak. In water-cooled engines there will be unexplained coolant loss as no external leaks are visible and symptoms of frequent engine overheating. In extreme cases of overheating, one may notice water bubbling in the radiator and coolant reservoir.

Scale of of repair: Medium to major

Solution: The engine head has to be removed for inspection first. If the water in the jackets appears milky white or beige in colour, that’s a surest sign of a blown gasket. 

This may happen due to uneven torque of the head studs/bolts or excessive heating leading to warping of the aluminium head. 

If warping has happened then it has to be sent to a specialist lathe shop that has milling machines. Repairing of a warped cylinder head isn’t recommended if the problem persists. A new replacement is then required for a fool-proof solution. 

Engine oil leaks

You park your motorcycle after a ride and notice signs of oil drops near the centre stand (mostly). It may be a few drops but if you are not cautious, oil levels could gradually dip to lower levels in the sump and damage expensive components like crankshaft bearings, oil seals, pistons and even transmission pinions.

Oil leaks can happen due to several reasons: a loose drain nut is the most common fault, followed by kick shaft, starter motor, alternator oil seal failure. Sometimes, excessive motor vibrations can cause loose crankcase screws (very rare chances) that will cause the binding gaskets to fail.  

One area often overlooked is the breather valve set-up. If it is clogged, it will impact the exit flow of blow-by gases. It’s absolutely vital in the interest of the engine’s health to have them out asap from the system. 

Hydrocarbons in the blow-by gases causes premature degradation of engine oil and excess pressure hinders the piston’s downward movement. Such  pressure could eventually even blow out seals and gaskets, causing oil leakage and result in catastrophic engine failure if neglected.Most modern motorcycles have built-in positive crankcase ventilation valves or PCV systems that check and control the gas build up and its effective management.

Scale of of repair: Medium to major

Solution: The engine must be cleaned of debris and dirt by washing with degreasing solutions. Place the motorcycle on clean ground and start the engine and leave it on idle for a few minutes. 

Switch off the engine and wait for half an hour. Observe source spots for possible oil leaks and take appropriate action. 

Also give the PCV systems a thorough check and clean-up.

Drop in oil pressure

Oil is very important for the engine as it lubricates moving parts. It also becomes an area of great concern if the oil pressure drops below the manufacturer recommended specifications. Drop in oil pressure could be a result of a worn out oil pump or clogged oil line. 

Very few motorcycles actually offer an oil pressure indicator or gauge. In vintage/classic oil-in-frame models it was possible to check the oil flow with the engine running. 

Modern high end motorcycles will have an oil pressure lamp that glows when ignition is on and goes out when the motor is running.

Rule of the thumb is that a good oil pump in prime condition should will produce between 30-60 pounds per square inch (or psi) during cold starts. As the engine warms up, the oil thins out and there will be a marginal drop in oil pressure which is quite normal.

For motorcycles that don’t have any manufacturer supplied kit to check oil pressure, then it’s back to old school. 

Scale of of repair: Medium to major

Solution: First start by checking the level of oil in the engine.  Remember to use the right viscosity oil in the engine. Secondly, remove the cylinder head and take out the spark plugs (to prevent engine starting/running). Wipe clean the parts like rockers, chain and holding brackets. Turn the engine on a few times using the self or kick starter. 

If the pump is working properly, oil should ooze out from the oil line (usually one of the dowels). If not, then there’s a potential problem in this department.

Physical symptoms will be burning oil smell, unusual, grinding noises from engine, engine overheating, diminishing performance. A clogged oil filter can also be responsible for not letting oil pass to the engine. 

If these are the symptoms, then begin the job with an engine flush. Use new oil and repeat the process described above. Even with clean oil lines there’s no oil coming, then zoom in on the pump. 

It could be as simple as a leaking gasket or an expensive replacement of a worn out pump.Lack of powerMany modern motorcycles are equipped with an array of warning lights, including a check engine light. 

When the check engine light comes on, it means a diagnostic health check is required to identify the problem. If the light continues to flash this indicates a more serious issue that should be attended promptly.

Engine knocking, loss of power

A four-stroke motor involves a four stroke cycle – intake stroke, compression stroke, combustion stroke, and exhaust stroke. Failure during any one of these moves will result in a drop of power to the engine resulting in compromised engine performance. Engine knocking, hissing, popping or backfiring are typical symptoms.

Scale of of repair: Medium to major

Solution: Check the fuel and electrical system first. A clogged fuel line, stale petrol, worn out spark plugs, gummed up fuel injector or carburettor could be the source of the problem and are easy fixes.If none of the above, then turn your attention to the cylinder head and have the valves, valve guides and seats examined. Fixing any problem out here can be expensive as it will require specialist services.

Unusual Mechanical noises

A poorly maintained engine will definitely not sound normal like other motorcycles. Mechanical noises can come from worn out rockers, tappets, connecting rod, rings, big end of connecting rod, timing chain among others.  

Scale of of repair: Medium to major

Solution: A total rebuild is in order and it can be an expensive affair. New valves, guides and seats, new piston and rings, crankshaft pin, bearing, oil seal replacement are among the major components besides machine work, service charges and consumables.  Recommend to use OEM parts only for trouble-free service.

White smoke

Again this happens when components like valves, valve seats, piston rings, timing chain etc are beginning to wear out. Engine oil seal failure is quite common in two-stroke engines and it will result in large plumes of smoke. 

Smoke is a product of engine oil burning that results in depletion of engine oil levels which in turn wears out the moving components through friction.

Scale of of repair: Medium to major

Solution: Take a step by step approach using the laws of elimination before moving further. Begin from the top of the engine. The cylinder head has to be examined thoroughly for valve leaks. The condition of the cylinder and piston have to be factored in. 

If there are signs of oil seal failure, then the engine has to be disassembled for access and replacement. 

After-effects of modifications 

A lot of young enthusiasts are keen to accessorise their motorcycles with aftermarket products. There’s no harm in that as long as they are cosmetic and add aesthetic value. The problem arises when there’s a mismatch of aftermarket parts like silencers, fuel delivery systems, sub standard air filters or mods made to chassis, wheel set-ups or other performance related modifications.A free flow aftermarket silencer may make the motor sound throaty and entertaining, but the flip side is that it may impact the back pressure values. Likewise an improper air filter can restrict air flow or all excess air to go inside the intake manifold. Stock fuel-to-air settings can get altered leading to lean or rich running motors that can overheat or burn out the motor prematurely.

Scale of of repair: Medium to major

Solution: First find model-specific recommended performance parts from specialists or the manufacturer if planning for modifications. 

Substandard parts will prove costly eventually. Avoid it if possible. A burnt out engine is quite expensive to repair or to refurbish. 

Misfiring/plug shorting

Misfiring usually happens when there’s a problem with the ignition timing. This can also be attributed to faulty valve settings. Generator problems are rare and can impact the CDI unit’s performance if the rate of current produced is not what is required. In cases, where the regulator rectifier (the finned part that converts AC to DC and checks over voltage) fails, over voltage can also cause a host of electrical problems from plug shorting, HT coil failure to blown bulbs. 

Scale of of repair: Medium to major

Solution: If the generator is at fault, do not attempt to have it repaired. Replace with factory supplied OEM parts for perfect results. Regulator rectifier is a quick fix and is not an expensive part to replace.

Transmission issues

Since modern motorcycles are of unitary construction that is engine and transmission are housed in a common shroud, these days work on these are included in engine rebuilds or refurbishing estimates.

Besides routine maintenance or replacement of cables, adjusters etc, transmission gears and its hardware and clutch assemblies can be impacted seriously if there are instances of running the motor on low engine oil levels or use poor quality oils, incorrect viscosity oils. 

In the worst case scenario you are looking at fried clutch plates or worn out gears, bushes etc.Symptoms generally noted are clutch slipping, engine overheating or missing gears. Performance motorcycles these days come with slipper clutch assemblies.

Scale of of repair: Medium to major

Solution: Among consumables are the clutch and friction plates that usually wear out and have to be replaced. Cost depends on the brand and motorcycle model.If gearbox work is involved, depending on the problem observed, then you are looking at refurbishing or replacing parts like gears, shafts, linkages, bushes and guides besides a few trips to the machine shop.


Monsoon season can be devastating for motorcycles that get submerged during flooding. Dirty water can enter the engine via the air filter or silencer or fuel tank. 

This occurrence will hydrolock the engine and cause expensive damage to the engine parts. Mind you, if excessive water gets into the cylinders, the piston or related components like the piston rod can break as it tries to compress it. Water doesn’t compress that easily. 

Scale of of repair: Medium to major 

Solution: Time is of essence about here so try to attend to the machine problems as quickly as possible. Swift action will avoid rust build up and lubricating oil losing its protective properties. 

However, the engine will require thorough flushing from top to bottom. If required, open crankcase covers and check for water inside the generator area, pick-up coil etc. Have them removed, cleaned and dried before refitting. 

Same for the clutch and oil pump assembly and fuel tap and fuel tank. Change all consumables with new ones— oil, fuel, air filters. If you are lucky, there will be minimal damage to working parts. The worst hit are the engine bearings. 

Even after a clean-up, there’s a minuscule probability that they could be impacted if the oil lines are not 100% clean.How to know? Just start the engine after the clean-up job is done. If there are unusual and continuous mechanical noises, then a re-inspected is required that might involve completely dismantling the engine find the root cause.

Cracked piston 

Symptoms include white or blue exhaust smoke, excessive oil consumption, a decrease in engine performance and engine power, and a reduction in engine compression for the affected cylinder.

Scale of of repair: Medium to major 

Solution: This is a no brainier and it’s obvious that the broken components have to be replaced with new ones. Costs will go up if the broken part has damaged other parts inside the engine. Full disassembly is recommended for peace of mind. 

Four-stroke vs two-stroke engine

Two-stroke engine rebuild

A typical two-stroke motorcycle engine will have less working components than its two-stroke counterpart. Lesser the number of parts means lesser complications and cheaper repairs.An engine rebuild could be partial or complete depending on the job requirements. Parts that are changed in a complete engine rebuild would include piston, piston rings in case of a rebore. New gudgeon pin bushes and crankshaft work if required. Transmission parts usually replaced are the primary drive sprockets and chain. The only valve is a reed valve, that’s found in the intake manifold around the carburettor.

Four-stroke engine rebuild

Rebuilding a 4-stroke engine is a bit more complicated as it involves many parts. Parts that are changed in a complete engine rebuild would include intake/exhaust valve assemblies (valve, springs, locks, valve seat, valve guide), timing chain, timing chain sprockets (top and bottom) piston, rings in case of a rebore, new jug (if cylinder being replaced). New gudgeon pin bushes and crankshaft work if required. Times that with piston assembly for motors with more than one cylinder.

Precautions to take during rebuild

A typical engine will have an assortment of nuts, bolts, screws, lock/shim washers. The threads can be of different types and in general most motorcycle engines will have employed three standard thread series in the unified screw thread system that are highly important for fasteners: UNC (coarse), UNF (fine), and 8-UN (8 thread).

Always check for cross threading to avoid damage to the aluminium crankcase. After an engine rebuild, let the engine run-in for the first 1,000km. 

Avoid overloading during this period and do not push the machine beyond its limit. After the run-in period, have the motor checked and replace the oil filter and lubes.