How To Build A Stock Moto Engine
Part VI: Finishing Touches

This final section of the Stock Moto guide focuses on assembly of the top end of the engine. The rules in Stock Moto are pretty strict, but there are some little things that can be done that will go a long ways in terms of reliability and power.

This is not a mandatory modification, but it is certainly recommended. The stock Honda piston does not come with any holes drilled in it to lubricate the exhaust rib. The exhaust rib is the hottest part of the cylinder. Keepign the rib lubed will help minimize the damage done if the engine seizes due to being run too lean. I installed the cylinder almost all the way down to find the center of the piston.
Using a Wiseco piston as a template, I drilled a pair of 1/16" holes that are roughly 1/2" and 1" from the top of the piston. After drilling, be absolutely certain that there are no raised edges on the piston and that there are no metal shavings trapped anywhere.

Honda CR125 Top End "kit"
Sources: Fastech Racing, Honda of Troy
Cost: $82.95

Being installed are the new piston, new wristpin, new wristpin bearing, and new clips (all included in the piston kit). A standard Honda .020" thick base gasket is used. This will generally make about 95% of CR125 engines legal in terms of combustion chamber volume. We'll check later.
Before the cylinder can be installed, the power valve plugs need to go in. Here's another pic of the SwedeTech plugs that I'm using. The o-ring design makes sealing with the messy orange stuff a thing of the past.

SwedeTech O-ringed Power Valve Plugs
Sources: Fastech Racing, SwedeTech Racing Engines
Cost: $65

Some people just toss the power valve plugs in and then leave them alone. Big mistake! As you can see, about .040" of the roof of the port is obstructed. This will have very negative consequences on the engine power above 11,000 RPM...the range that you most need it! The edges of the plug need to be "knife edged" so that they do not obstruct the exhaust flow.
Here's a close-up of a plug that has been grinded a bit. Notice the sharp edge vs the unmodified one on the right. You cannot grind the plugs while they are in the cylinder. In addition to being very tricky, you would likely inadvertently grind the cylinder itself. That would instantly make it illegal. The plugs need to be installed without the o-rings (to make installation/removal easier) and then marked with a Sharpie. Pull them back out and then grind the areas that are marked by the Sharpie. A variable speed dremel tool, sanding drum, and some patience will do the trick.
Here's the extremely nice o-ring'd exhaust manifold from Fastech Racing. This setup is far superior to the stock manifold with the crush gasket. The manifold can be ported if it obstructs exhaust flow, but as you can see, this manifold does not obstruct at all.

CR125 O-ringed Exhaust Manifold
Sources: Fastech Racing
Cost: $31.95!

Here's the cylinder installed along with a new headgasket.
You can run any reeds you want. This engine got a set of CarbonTech low tension reeds. They are awesome reeds that will last a long time.

When installing the the reed cage, rock it up and down to see if there is any slop. If there is, remove the reed stoppers and bend them outward a bit (do this in a vice). Repeat until the reed cage fits very snugly into the cylinder. It should not be loose, but you should not have to force it in. This mod will allow the reeds to open to their fullest and will help give you maximum airflow through the engine.

CarbonTech Reeds
Sources: Fastech Racing, eBay
Cost: $48.95

In stock form the CR125 uses a pair of small radiators. There are two 1/2" outlets on the head. Some people prefer to use a "Y" shaped hose, but plugging one hole (the inner one) and running a larger fitting will give the driver more clearance, require less plumbing, and will allow you to use the larger hose that is more commonly used with today's karting radiators.

This is a simple fixture I made using a 1"x4" piece of wood. The head is bolted to the piece of wood and then squared up in the vice. Using the chuck of the drill press, I use a 3/8" NPT tap to recut each hole. The inner hole is plugged with a 3/8" NPT brass plug. The outer hole now has a hose fitting that is 3/8" NPT on the threaded side with a 3/4" flared fitting on the outlet side. This fitting can be found at Napa Auto Parts. Ask for part number 660-1523. Thank you to JR Clasen of Power Source Racing for helping with this information.

Next we'll check the combustion chamber volume to ensure that we are legal. In 95% of the cases, this step is not necessary, but it's always good to check. Pictured below is a 15ml buret and some Marvel Mystery Oil.
With the piston just below TDC, dispense exactly 11cc of Marvel Mystery Oil into the engine. Bring the piston up to TDC. As long as the oil does not spill out, the engine is legal. As you can see, the oil only went about 3/4 of the way up the spark plug threads. This engine is probably around 11.4cc's which is plenty legal.
With the clutch cable, J-arm, engine sprocket, and ignition cover installed, this engine is almost ready to rock and roll!

Clutch cable, J-arm, engine sprocket
Source: Fastech-Racing
Prices: Clutch Cable: $20, J-arm: $35, Sprockets: $20

A popular place to mount the coil is directly to the head. An 8mm bolt passes through the head to hold the engine into the bike. This bolt hole can be utilized, but your coil will generally not last if you don't use rubber washers to help isolate the vibration. The vibration usually takes its toll on the wire connections, eventually causing them to break. This is a very common cause of a DNF, so spend the time to mount your coil correctly. Make sure there is no tension on the wires and connectors.

The CDI can be mounted to seat. Use of the factory rubber CDI holder will make this job a little easier.

We're nearing the end! RLV's R2 pipe is a legal Stock Moto pipe in all parts of the country. Some other parts of the country allow the RCE T3. The R2 does not have the hard hitting powerband of the RCE T3, but its wide and forgiving powerband make it a good choice for beginning and intermediate drivers. You can save a lot of money by buying a used pipe, but they don't come up for sale very often.

RLV R2 Pipe Kit
Sources: Fastech Racing
Price: $349.95

The 2 legal Stock Moto carbs are the Keihin PWK and the Keihin PWM. The PWM features a shorter inlet tract that gives it a slight edge in top end performance.

The PWK (pictured here) is generally considered to be a bit easier to tune. It's also the factory carb for the late 90's Honda CR250. The PWK pictured has had the "pumparound" mod performed on it. This makes the carb far easier to work with. You can either buy a new pre-modded carb or send an existing one in to be modified. The pumparound mod isn't super cheap, but it pays for itself quickly since fuel delivery problems are a no longer an issue.

Keihin PWK/PWM with Pumparound Mod
Source: Fastech Racing
Cost: $349

The final step in our build is to install the airbox. RLV's 4-hole airbox is a great choice and with the RLV angled filter, mounting it is piece of cake.

RLV Shifter Airbox
Sources: Fastech Racing
Cost: $42.95

That's it! If you took the time to do things right, you will now have a Stock Moto engine that will be legal, fast, and reliable! You also saved enough money to buy a club membership and pay for entry fee for a whole season at most clubs!

Please direct questions/comments/feeback to me using this link. You can also contact me if you need help locating an engine, engine parts, or a fully assembled engine.

Introduction/Search Criteria Rebuild Overview Teardown Gearbox Conversion Assembly Finishing Touches