Do-It-Yourself: Rebuild a Motorcycle Engine

Old engine
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Rebuilding a motorcycle engine can be very easy whether you have a single cylinder (2-stroke) or a multi-cylinder (4-stroke) engine. The same basic rules and procedures apply, regardless of the type or size.

Engines have to be rebuilt for a variety of reasons. Some are redone to replace worn or damaged parts, others are part of planned maintenance, and others simply need to be tuned or upgraded. Carrying out a planned engine rebuild is not beyond the experienced owner/mechanic with good quality tools, a workshop and a manual.

As with many jobs on a classic motorcycle, preparation is the key to a successful outcome. This preparation must include having the workshop and motorcycle thoroughly clean (especially the external engine components).

The sequencing for doing an engine rebuild is very important to the ultimate success of the project. The following is typical of the order a professional mechanic would perform the task. It should be noted that removing the engine from the frame as soon as possible is a typical amateur mistake and must be avoided.

  • Prepare the workshop (motorcycle lift, containers for parts, receptacles for all associated fluids, etc.)
  • Locate the motorcycle on lift allowing for weight shift as the engine is removed
  • Drain all associated fluids into appropriate containers
  • Disconnect the battery (ground lead first)
  • Turn off the fuel supply and remove the fuel tank
  • Remove muffler and header pipe/s
  • Remove the airbox and carburetors
  • Remove drive chain or disconnect the drive shaft as appropriate
  • Remove all items necessary to gain access to the engine and auxiliaries
  • Wrap the frame rails where the engine is likely to come into contact with it during removal
  • Remove external cases (clutch cover, alternator cover, etc.). It may be helpful to remove the oil filter canister on some engines
  • Using special tools as necessary, remove the clutch, crankshaft drive gear, alternator, stator, ignition plate, gear change lever, and associated components, as appropriate
  • Disconnect all oil lines
  • Disconnect all electrical connections
  • Loosen all engine mounting bolts
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Secure the Bike

Some of the component parts fixed to a motorcycle with bolts and nuts require a lot of torque to loosen or undo them; it is very important, therefore, to secure the bike before attempting to undo items like this.

If the mechanic is working on a lift the bike’s front wheel must be secured in a wheel clamp and ratchet clamps should be used to stop the bike moving laterally.

Note: The mechanic must allow for the considerable weight change when the engine is removed.

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Drain Fluids

Using suitable containers, the engine, gearbox and radiator fluids (as applicable) should be drained. If at all possible, the fluids should be left overnight to drain to ensure as much as possible has been removed from the engine, etc. (Also, it is good practice to soak it with WD40, or its equivalent, the header and muffler pipe bolts/nuts overnight as they are often seized). However, you must observe workshop safety when leaving a machine to drain in this manner such as no open flame heaters and adequate capacity in the catch container.​

Note: The individual fluids should be kept separate for environmental reasons (dealers are liable to substantial fines for not correctly handling waste fluids).

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Disconnect the Battery

For safety reasons, it is best to disconnect the battery. It is very important to disconnect the ground lead first when removing or disabling a battery and, conversely, it is equally important to connect the hot lead first when refitting a battery.

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Remove the Fuel Tank

To gain access to many engines it is best to remove the fuel tank. If the bike is likely to be off the road for some time (a winter rebuild, for example), a fuel stabilizer should be added to the fuel.

On motorcycles with evaporative control systems, the vent lines should be clearly labeled. If the mechanic is not sure what every line does he must, as a minimum, mark each line and its relative location, ‘A’ to ‘A’ for example.

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Remove the Muffler and Header Pipe(s)

The hardware (nuts, bolts, clamps, springs, etc.) associated with mufflers and header pipes should be loosened evenly so as not to put excessive pressure on adjacent parts. For example, all of the header pipe bolts screwed into the cylinder head should be backed off slightly rather than anyone bolt removed before moving onto the next.

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Remove the Air Box and Carburetors

Before removing carbs, it is good practice to drain the float chambers. Ideally, this will have been done during the fluid draining process.

If the carbs will not be refitted for some time (again during a winter rebuild, for example), they should be thoroughly cleaned and WD40 should be sprayed into the float chambers. They should then be placed inside a sealable plastic bag.

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Removing the Final Drive

On chain driven motorcycles, the chain must be removed to allow the engine to be removed. However, it is sometimes possible (even desirable) to keep the chain assembled (hard link type) and remove the gearbox output sprocket. Note: It may be necessary to back off the chain adjustment to give sufficient clearance at the sprocket.

Shaft drive systems differ in their attachment to the gearbox on most motorcycles. However the typical system for driveshaft removal is to disconnect the rubber gaiter at the front section to gain access to the shaft, then unbolt, at the universal joint, the shaft.

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Remove Cases

Eliminating the cases at this point will help the mechanic to disassemble the engine later, as it is much easier to loosen the bolts when the engine is in the frame. On motorcycles with multiple retaining screws on cases (most Japanese machines), it is important to loosen the screws a small amount before removal so as not to warp the cases.

Note: It may be helpful to remove the oil filter canister on some engines at this point.

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Remove the Clutch, Alternator and Drive Gear

The clutch plates must be removed first to access the clutch’s retaining nut. However, it is very important to use a special clutch cage holding tool when backing off the nut. 

Due to the vulnerability of oil lines and their fittings, it is good practice to remove them (where fitted) before attempting to remove the engine. Note: The lines will often have a small amount of oil in them.

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Disconnect All Electrical Plugs

The vast majority of motorcycle electrical systems have color-coded wires that ensure the correct wires will be reattached upon assembly. However, if there is any doubt, the mechanic should label the wires as required. Multi-pin plugs typically have a locating groove which only allows the plug to be reattached to its appropriate opposite receptacle (male to female).

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Loosen All Engine Mounting Bolts

To remove the engine, it is necessary to loosen then remove the engine mounting bolts and associated plates. However, the mechanic must exercise caution during this process as the engine will at some point drop under its own weight.

Before the final bolts are removed, prepare a suitable space on a nearby bench. In addition, the mechanic should enlist the help of another person at this point for safety reasons. For most engine removal operations, a mechanic will straddle the bike and lift the engine to one side first (have the assistant balance the engine at this point) before coming to the side where the engine will be removed from.

Before continuing any work on the engine, the mechanic should inspect the frame and engine mounting plates at this point as parts may need to be ordered to complete the reassembly.