Stock Moto is one of the fastest growing karting classes in the US! Stock
Moto offers impressive speed, close racing, outstanding reliability, and best
of all, requires a budget that will not break the bank since parts last a long time. A brand new Stock Moto engine package will run you close to
$4000 after you purchase the engine, pipe, carb, and all supporting mods.
However, this guide will show you how to build a totally legal and fast Stock
Moto engine for about half that price. In addition to saving a lot of
money, you will learn just about everything about the inner workings of
Sourcing a good used engine is probably the trickiest part of this
project. In general, the CR125 is a very stout engine and will last
a very long time if properly maintained. In many cases, you'll be buying
an engine over the Internet. This is often an easy way to find a great
deal, but there are a couple of things that you need to watch out for.
Sourcing a good used engine is probably the trickiest part of this project. In general, the CR125 is a very stout engine and will last a very long time if properly maintained. In many cases, you'll be buying an engine over the Internet. This is often an easy way to find a great deal, but there are a couple of things that you need to watch out for.
What year is it?|
The engine we want is the 1999 CR125. A rebuild, gearbox swap, and some accessories will make this engine legal in Stock Moto. To find the serial number, look directly above the gearshift spindle. 1999 CR125s serial numbers start with JEO1E-6000001. 1998 CR125s started with JE01E-5900001.
Notes about other CR125s:
|Ignition - Another easy way to identify the 1999 CR125 is by the ignition. It is the only CR125 with large dual molex plugs that will fit this CDI.|
|Cylinder Head - Ask your seller to remove the cylinder head and take a picture of it. Here is what you want to see. Notice that the head is not machined in any way and that there is no detonation damage.|
|Here is a cylinder head showing signs of detonation damage. The damage to this head is pretty minor, but since Stock Moto does not allow machining of the head, it would be tricky to fix this without making the head look modified. A CR125 head is relatively inexpensive, so if everything else checks out, this engine would be a good candidate.|
|Cylinder - The condition of the cylinder is very important since a replacement cylinder is expensive and due to the national backorder, they are extremely hard to get! Here is a good cylinder. Note that the Nikasil coating is not worn off anywhere. There are few "scuff" marks that are normal, but no grooves or scratches.|
|Here is a cylinder that is in trouble. Note that the Nikasil is worn off around the exhaust port. Since the Nikasil is worn off there the aluminum will get very hot. The resulting expansion can seize the engine. In other words, this cylinder would make this engine a very unlikely candidate. Keep in mind that if there is obvious damage such as this, a seller will often make you a fantastic deal on the engine. If this '99 CR125 could be purchased for $250, it would still be a great buy.|
Other Questions To Ask
Do the cases have any cracks?
Case cracks are bad news and are generally a deal breaker since case halves are expensive.
When was the engine last run?
If an engine has sat for many years, generally corrosion will develop in the water jackets and water pump assembly. It is desirable to find an engine that has been regularly (and recently) run.
Is the engine modified in any way?
Internal engine modifications are far less common in the MX scene than in the karting scene. However, modified MX engines do exist and a modified one will not be suitable as a base for a Stock Moto powerplant.
With the search criteria out of the way, we're ready to start building!
|Introduction/Search Criteria||Rebuild Overview||Teardown||Gearbox Conversion||Assembly||Finishing Touches|