How To Build A Stock Moto Engine
Part I: Introduction/Search Criteria
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
your engine.
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:
1998 - Suitable for Stock Moto, but requires ignition changeover.
Expect an additional cost of $150-$300.
2000-2002: Same requirements as 1998 CR125, but
also requires a '98/'99 head and cylinder. These newer engines are
harder to find cheap. Since the head, cylinder,
and ignition are newer, they have good resale value on the
used parts market. This can put the overall cost of the conversion
on par with the conversion cost of the 1998 CR125.
1990-1997: Cheap and easy to find. Same requirements as
2000-2002, but no gearbox swap is required. The 1998/1999
cylinder's transfer ports do not well match the 1990-1997 cases and
the cases themselves are technically not legal. Generally not worth
converting unless you can find one real cheap and are not
concerned with the transfer port mismatch.
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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.
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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.
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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.
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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.
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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.
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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!