Force 12 Engine Modifications

By Eric Perez

  1. Intro
  2. Engine Mod intro
  3. Engine Mod Tools
  4. Preparing the Engine
  5. Porting The Sleeve
  6. Working the Connecting Rod
  7. Other Engine Mods
  8. Putting it all Together
  9. Engine Performance
  10. Engine Tuning


That's a picture of my brand new Force 12 Engine. Its been in storage for a year, waiting for its older brother to die! Let us refer to the old brother as the Red Energizer Bunny.

Article Intro:

This engine is know as the Ofna Force 12, the Force 12 "Red Head" and the Colt 12. The original engine had a blue heat sink head and a bad carb. They fixed the carb and changed the head color to red. This engine is considered one of the lowest cost "sport" engines available. Street value for these engines is less than $80. One less know fact is the that the Duratrax Velocity 15 is also the same engine as the Ofna Force 15. The only difference between the 12 and 15 is the sleeve, piston and head-button; all other parts are identical. For a relative inexpensive engine, this thing packs a punch! ABC sleeve/piston construction, rubber sealed front engine bearing, 100% machined transfer ports on the crank case, O-ring sealed crank-shaft, Double O-ring sealed carb with pinch bolt. The feature list goes on and on... One of the most unique aspects of this engine is a unique pull starter system. The starter-shaft actually disengages while the engine is running! This means, no more wore out or slipping one-way bearing or leaking starter-shafts. With a little TLC, this engine can be made to run better than engines costing a lot more. If I had to use a pull start engine this would be it! Note: The engine is also available as a non-pullstart.


The Red Energizer Bunny Engine. Still racing after more than 4 gallons of fuel.






Engine modification Intro:

Most people start by modifying an older engine to get a feel for how to do the mods, later they move on to newer engines that can benefit more from the modifications. This is not rocket science, engines use fuel and air to make power. The available power is related to how much mixture the engine can suck in, this available power is reduced by the work needed to draw the mixture in through the carb and out the exhaust. If you are an "expert" engine tuner you can increase power by as much as 15%. Even if this is your fist engine mod -you'll see the difference at the track. Once you modify the engine you can't go back. You'll need a new piston/sleeve.

These engine modifications are very easy to make. The porting work is done with a rotary tool (Dremel), equipped with a metal cutting bit. After the porting job, everything was polished to a mirror finish with a polishing bit. Before we start the hacking and grinding lets give you the legal stuff first:

Let me start by saying this. If you can't afford to mess your engine up don't try this. You must know how to operate a Dremel and understand how to set the speed for the bit that you using based on the material that your cutting. Always wear eye protection when using the dremel. Follow all safety procedures for operating the items/tools needed to modify your engine. once you port an engine you can't undo it. Follow these instructions at your own risk.


Engine Modification Tools:

To modify your engine, you will need a multi-speed Dremel tool. Depending on the bit that you are using you will need to either increase or decease the RPM. When using the cutting tools on aluminum you need to increase the speed until the bit does not jump or chatter. If the RPM is to high the tool will be hard to control and it will want to wander inside the engine -not a good thing. The polishing/grinding tools can be used at lower RPM, this will vary depending on the force being applied to the tool.

Dremel Metal Cutting Bits

1. Round cutting
2. Small round engraver
3. Cylindrical cutter
4. Small cylindrical cutter

For more information on Dremel cutting bits go to:

You will also need your regular RC car maintenance tools. This includes regular tools like hex-wrenches, and some engine specifics like piston locking tools and flywheel wrenches.

Preparing the Engine:

Before doing any work on the engine, make sure the engine is as clean as possible. Any dirt on the outside could easily get inside the engine -dirt and internal engine parts don't mix very well. I like to clean the engine with Simple Green and a tooth brush. Make sure you cover the carb inlet and the fuel inlet, before you spray anything on the engine. After the engine is clean, take it apart and remove the piston and sleeve from the engine. If you are not sure how to take the engine apart -Check the instructions that came with your engine. If you are having problems taking the engine apart get some help from a more experienced racer. Some Hobby Shops offer engine porting services. If you decide to go that route; print the article out and get a quote.

This is how the engine should look like at this stage.






Porting the Engine Sleeve:

Once you get the engine sleeve out, clean it and inspect it for any scratches or other wear marks that might be precursors of a total engine failure. If the piston will travel to the top of the sleeve easily, then the engine is worn out and needs to be rebuilt. You can always try out the mods on wore out engine parts to practice Dremel skills -but you'll have to repeat the process again on good engine parts.

This will be the legend for the sleeve that we will use from now on. I don't have any before and after pictures of the sleeve, it will be very obvious to you how the stock sleeve looked like. The sleeve was polished for "show" it's not necessary, but it gives it a more professional look.

Note: The sleeve is in great shape, no scratches or flaking of the chrome. This is what 4 gallons of good quality fuel w/ 18% lubricants will do for you!

Exhaust Port 0 deg Intake port 90 deg Boost Port 180 deg Intake Port 270 deg

We are not modifying the exhaust port, for those engine gurus out there -Notice how the exhaust port is angled toward the top. This increases the duration of the exhaust in degrees and raises the exhaust timing. This is one of the reasons this engine can trade punches with engines costing a lot more. A lot of work went into the port design of this engine and it shows on, and off the track. We will only work on the three intake ports of the sleeve.

Step 1. Port work is done with a round coarse cutting bit#1 on the three intake ports. The tear-drop groove will increase the actual port depth to allow more fuel to reach the piston. Don't make the channel too deep or you'll weaken the sleeve. The groove is carefully blended to the stock port with a round de-burring bit#2. This will allow the fuel to make a smooth transition to the port. When working the side intake ports we only touch half of the port. This creates more swirl in the combustion chamber and allows the engine to use the extra fuel instead of flushing it out the tuned pipe.

Step 2. The groove is then polished with Dremel 421 polishing compound and a rubber tear-drop bit.

Step 3. The entire outer sleeve is polished with a buffing wheel (Optional)

when you are happy with the ports, clean the sleeve with denatured alcohol or simple green, and rinse the parts under running water. You must be 100% sure the sleeve is clean and that no metal particles are left from the grinding operation.

Working the Connecting Rod:

This mod is really not a big performance enhancer, but little things do add up to a better performing engine. The objective here, is to round out the edge of the connecting rod, and give it a a radiused edge. The connecting rod must be separated from the piston, before you work on it. The little G-clips are a pain in the butt to work with; I ended up using an X-acto knife and a pair of tweezers to take them out. Note: Pictures are from a brand new Force 12 Engine.

Note how the oil hole has been slightly enlarged to help it retain more oil. I used a 1/16 drill bit to enlarge the opening. Note: Only do the opening of the hole, do not drive the drill-bit through. When you are done, make sure there are no obstructions (metal particles) clogging the hole. When I first took the Red Energizer Bunny motor apart for inspection before it ever ran. I found a piece of metal plugging up the oil hole. Good thing that I took the time to check the motor before I ran it. You should always open new engines up before they are ever used -You never know what you may find. Ever heard the phrase "life is like a broken engine bearing?" Always do a full pre-inspection on all new engines...


Here you can see the profile of the connecting rod. Don't go overboard here and make the rod too skinny. For first time engine modifiers I usually recommend them not to mess with the connecting rod. After you feel more comfortable with the tools of the trade then you can hack it to death. I heard Tower had an engine tuners special combo. It came with a connecting rod and an assortment of Dremel bits... 




Other Engine Modifications:

There are other modifications that I'm still testing out. These involve modifying the crank-shaft. I'll add them to the article if they pass the torture test...Once again I remind my readers that I will never publish anything unless I have verified it's effectiveness. once again, footing the bill for your R&D.

This piston has seen well over a year of racing and more than 4 gallons of Wildcat 15% Premium Extra (Airplane Fuel). The engine still has good compression and connecting rod is still in great shape. This is the difference between 4 gallons of fuel with 18% lubricants and the 6 month wonder engine that last a little over 2 Gallons with race-car branded fuel that typically contains less than 12% lubricants. If you want a long lasting, well performing engine there is no substitute for using quality fuel w/ plenty of oil.


Putting it all together:

At this stage we clean all the parts again. Inspect engine bearings for smoothness, and flush them out with WD40 and denatured alcohol. If the bearings feel rough, then you might as well replace them. You should be able to get them at the local hobby shop for less than $6 a piece. Coat all parts liberally with Marvel Mystery Oil and re-assemble the engine. Torque all screws in a star shaped pattern; re-install the engine in your chassis and your ready to go!

Engine Performance:

These simple modifications will allow the engine to rev out a lot more than the stock engine. Think of the stock ports as a sort of RPM limiter, they allow the engine to operate in a certain RPM range. The faster the engine goes the less time it has to recharge the fuel/air mixture into the combustion chamber. By enlarging the sleeve port we in a sense, help the engine get fuel faster; this increases fuel consumption and engine power.

A Racing Tale: Let me tell you one of the many stories behind the RED Energizer Bunny Engine. It was the first big 2-day race for my OB4. I still was setting my car up, just slapped some foams and things where not going great. I had a very experienced racer that was helping me with the cars setup. After the first day of the race I took the engine apart and ported it that same night. Next morning I got back on the track and got everything setup. My pit partner could not believe that was the same car and engine that he had seen the day before! I actually finished quite well in that race. It took driving skill to get on top, no doubt more engine power on that large track was a factor in my favor. The point I'm trying to make, was that the increase in engine performance was not just in my head; other people had noticed the the same change as well. The car was launching a lot harder in the corners and straight-a-way speed had improved as well.

Engine Tuning:

I really did not notice any difficulties in tuning the modified engine. When you retune the engine, start on the rich-side, and work your way toward a lean setting until the engine gives you maximum performance. Once you are there, richen the needle slightly, temp the head and if it's less than 250F start racing!

Glow Plug: I use McCoy MC59 hot plug with (2) plug washers as suggested in the engine instructions (when using long plug). The engine seems to like the McCoy plug. Plug longevity is long enough to make me forget to check it :)

You will notice that after the modifications the engine will accelerate faster and reach higher top speeds than before. Fuel economy will be reduced somewhat, my engine will run 6-7 minutes on a tank of 15% nitro.

Hope you enjoyed the Force 12 engine mod article!

Whoop some R/C car butt!!!

Eric Perez


If you have any questions or would like to discuss any of the topics presented in this article, feel free to e-mail me.

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This page last modified: 07/26/11