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Rebuilding an engine

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Active 50 parts

Whether it is the old trusty beater that has a few too many miles on its rings or the new hot high RPM engine that was tuned too lean for its initial run, you'll need to rebuild an engine in order to get the maximum number of miles out of the same parts. The idea of rebuilding an engine could involve anything from simply replacing the rings to replacing a significant portion of the engine mechanism.


This guide will help you through the process of rebuilding a common two-stroke engine, examining its parts and deciding what does and doesn't need replacing. This guide will provide a high level overview of some steps like disassembling your crank case and removing your flywheel. If you are not prepared to perform these steps, you may want to have a specialist rebuild your engine for you.


Required PartsEdit

During the rebuilding process, you may need several parts and you may actually need none. Everything depends on what you find once you open your engine. One thing you want to be sure you have is a full set of gaskets. If during the rebuilding process you end up tearing a gasket, you will need to replace it. Even reusing gaskets that look like new is risky business given the low price of gaskets and the high price of seizures caused by air leaks.

You will need:

  • Full gasket set

You may need:

  • Piston rings
  • Case or needle bearings
  • Case seals
  • A new cylinder
  • A new piston

Dismantling the engineEdit

The first step to rebuilding an engine is taking it apart to examine its working pieces. In most cases, the only disassembly that will be necessary will the removal of the engine cover and cylinder. Some problems like worn crank case bearings can require that the entire engine be dismantled.

Note: Any gaskets that are stuck can be removed with a razor blade. It is important that all old gasket material is removed. Not doing so can result in the new gasket not sitting right and an air leak being created.

The pistonEdit

Once we have the engine cover removed and the cylinder pulled, we can begin to examine the working order of the piston and rings. In the case that we are rebuilding simply because of the old age of the engine, this will be our only area of interest.


Remove the rings.

Active 50 piston with rings

The ring in the ring groove

Active 50 piston ring pin

The pin in the ring groove

The first thing we need to do is remove the piston's rings. These rings are able to flex like spring steel, but are much more fragile. When removing them, you want to flex them as little as possible in order to pull them over the top of the piston.

It is not necessary to replace the rings every time that you rebuild an engine. Replacing the rings should only be necessary if you notice excessive wear.[1] A sign that your rings are to blame for compression loss is burning or carbon on the sides of the piston beneath the ring grooves. A new set of rings can sometimes restore lost compression which means they can make your engine perform like new again.


Examine the piston.

Active 50 piston carbon

Active 50 piston with excessive carbon buildup

Once the rings are removed, you can examine the piston. The piston should be very smooth around all sides with limited carbon buildup[2] above the rings and no carbon below the rings. The ring grooves should be smooth cuts without any chips or dents. You should also be able to find the two small pins that interrupt the ring groove. They are there to make sure that the rings do not rotate on the piston once they are installed.



Examine the piston pin and needle bearing.

Active 50 piston needle bearing

Active 50 needle bearing

The piston is attached to the crank via a thin, wide bearing with needle shaped rollers. It is common for this bearing to not span the entire side of the piston pin that it rides and it is thus able to slide back and forth a bit on the pin. It should, however fit the pin and crank snugly. A good way to test this connection is to gently try to rock the piston in all four directions. It should rock in two directions along with the rotation of the crank, but it should not rock in the other two perpendicular directions. If it does rock sideways, replacing the piston pin, crank and needle bearing might be necessary.


The cylinderEdit

Active 50 cylinder clean

Active 50 cylinder (like-new)

Examining the cylinder is relatively straightforward. The cylinder wall should be nearly as smooth as glass. Any scratch that is large enough to feel with your finger is large enough to render the cylinder useless. Use a flashlight and your fingers to inspect the whole thing.

It is common for carbon to buildup in your exhaust and combustion chamber as on the top of your piston. A thin layer of carbon will not cause any problems, but as the layer thickens it can decrease performance and lead too much compression and knocking. Carburetor cleaner, a paper towel and a firm rubbing should be able to remove all of the carbon that needs removing. Do not use a wire brush or an SOS pad in the cylinder as it can scratch the cylinder wall. You can use a brush or pad in the exhaust, but you must be very careful to not scratch the cylinder wall and to remove as little material as possible.

If you must, use a plastic brush to get the hard carbon deposits off. The plastic will be softer than the aluminum and steel and shouldn't be able to scratch anything.


The crank caseEdit

The crankEdit

Active 50 crank case

The crank case of an Active 50

The crank assembly is relatively robust and seldom fails or otherwise has problems. The only parts of the crank that could fail are the interface between the crank arm that attaches the piston to the crank shaft and the interface between the bearings and the shaft. We will talk about the bearings next.

There is a needle bearing that connects the crank arm to the crank shaft operates similarly to the needle bearing on the piston pin and you can test its interface in a similar way.


The case bearingsEdit

Checking the bearings without removing the crank. With the cylinder removed, you should be able to rotate the crank by holding the piston carefully over the case so that the piston follows a path similar to that of its path if it were in the cylinder. While rotating the crank, you'll probably feel a specific spot which the crank likes to stay. That is the magnet of the coil interacting with the flywheel and is nothing to worry about. What you should check for is any grinding noises or indication that the crank case bearings have failed. If all is smooth and the crank case is clear of any metal shavings or other foreign elements, you are done.


Dismantling the case.

Active 50 flywheel puller

Simple flywheel puller constructed of 3/8" aluminum

If there is any kind of grinding noise or you just would like to be thorough, it is time to dismantle the case, In all cases, you will need to remove the flywheel and starter pawl in order to dismantle the case. Most flywheels are press fitted with a notch in them set at a specific point to ensure correct timing of the coil. You will need a tool designed to pull the flywheel from the crank shaft in order get it off. Some parts store have them available, but they are quite simple to construct yourself.


Manually checking the bearings. Once the flywheel is removed, dismantling the case should be a simple matter of removing four bolts and maybe a pull start. With the case in pieces, the crank should slide out of the bearings and you can manually check how they roll with your fingers. The bearings should fit the crank shaft snuggly and spin freely. You should see no notable play in the interface been the crank shaft and bearings or within the bearings themselves. If there is any play, the bearings themselves should be easy to replace as they are simply press fitted into the case. If there is any grittiness or catch inside the bearing while you spin it, you must replace it. Bearings are cheap and engines are expensive

The sealsEdit

The seals should not be discolored at any part and should also fit the shaft snuggly with a soft rubbery feel. If they have become hard, cracked or discolored, they will need to be replaced. Most bearing suppliers carry standard sized seals.

ReassemblyEdit

Active 50 piston with oil

Active 50 piston with oil smeared in the ring gooves

When reassembling your engine, you want to make sure that all moving parts get a dab of 2-stroke oil on them to that they don't end up running dry before the fuel-air mixture can properly lubricate them. It is a good idea to smear your cylinder wall, rings needle bearings and case bearings(if you replaced them or took the case apart) with oil.

It is also a good idea to replace all of your gaskets when you rebuild. Gaskets are very inexpensive and new ones could save you the hassle of having to tear the engine down again for a rebuild. Never reuse a torn gasket!



Re-breaking in your engineEdit

If you replaced your piston rings while rebuilding the engine, the new rings will have to break into the cylinder's shape again. It is out of the scope of this guide to tell you how to break your engine in, but these small engines are robust and breaking in the rings will most likely work just fine under normal operation of the engine.

See AlsoEdit

  • [1] Goped Nation (thread about this article][/ref]

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