How To Rebuild the TRX 2.5, 2.5R and 3.3 Engine

Article #34788

TRX Racing Engine
Rebuild Instructions

 


The instructions below are for the rebuilding of the Traxxas TRX 2.5, 2.5R, and 3.3 Racing Engine once the engine and EZ-Start® system have been removed from the chassis. Before you remove your engine for a rebuild, consider trading it in for a new engine using the Power-Up Program. Rebuilding engines from parts can be costly and time consuming, and can be outside the mechanical skills of many enthusiasts. With the Power-Up Program, a worn out or damaged engine can be traded in for a brand-new, factory-fresh TRX® Racing Engine right away by simply exchanging your worn engine at your local hobby store. The cost is 1/2 of the retail price of the new replacement engine. It’s less expensive than rebuilding the engine yourself and you get the peace of mind knowing your engine is brand new (not refurbished), and ready to deliver the power and reliability you expect. In many cases, installing the new engine only takes minutes and you're back in action.


1. Start by removing exhaust pipe and header from the engine by removing the two 3x15CS screws.

Remove the three 3x12BCS screws that secure the EZ-Start® assembly to the engine and separate the gear housing from the engine.

2. Remove the 5.0 E-clip from the crankshaft and slide the 5x8x0.5TW with the clutch bell off the crankshaft. Caution: it's important to use eye protection when removing the E-clip from the shaft to prevent the clip from causing an injury. Pull the clutch shoes off the flywheel pins.

Remove the flywheel nut from the crankshaft using an 8mm socket and disconnect the flywheel from the crankshaft by lightly tapping the back of the flywheel with a rubber or plastic mallet. Place a small flat blade screwdriver into the slot of the split cone washer and twist slightly to loosen the split cone from the crankshaft. Slide the split cone washer off of the crankshaft.

The engine is now ready to be rebuilt.

3. Remove the glow plug from the cooling head and discard the plug. The glow plug should be replaced with a new one any time the engine is rebuilt. Remove the four 3x6CS screws from the back plate of the engine and pull the back plate from the crankcase.

Remove the cooling head from the engine case by removing the five 3x12CS head bolts. Be careful not to lose or damage the copper head gasket. If the head gasket is damaged, replace with a new one.

4. Locate the bottom of the sleeve through the rear opening of the crankcase. Press the bottom of the sleeve upward with a plastic or wooden stick.

This is to prevent internal damage to the crankcase. The sleeve should slide up and out of the crankcase. Note: If the sleeve does not slide out of the crankcase with reasonable effort, slightly warm the crankcase with a hair dryer to about 130 F.  Do Not use an open flame to warm the engine!

The sleeve should slide easily at this temperature.

Use gloves to protect against burns. Rotate the crank to where the rod and piston is located at TDC (top dead center). Gently pull on the bottom of the connecting rod with needle nose pliers, sliding the rod off of the crank pin. Be careful not to damage the connecting rod if the rod is being reused.

The rod and piston assembly will exit the top of the crankcase.

5. Push the end of the pilot shaft through the crankcase.

The crankshaft should exit the rear of the crankcase. Loosen the 3.0NL just below the base of the carburetor. Slide the carburetor up and out of the crankcase. Now the crankcase is ready to be flushed clean and inspected.

6. Flush the crankcase and bearings out with denatured alcohol or electric motor spray. Dry the crankcase out with compressed air. Caution: Always use eye protection when using compressed air. Inspect the crankcase and bearings for damage or missing parts.

Make sure that the bearing cages are not broken and check for corrosion inside of the crankcase. The bearings should feel smooth and have little play. Inspect the exhaust O-ring gasket and the back plate O-ring gasket for tears. Replace these O-rings if needed. Note: Never use engine sealant or RTV on a TRX engine. They are precisely engineered to use special O'rings for a proper seal.

7. Apply a few drops of 'Mobil 1' synthetic motor oil to the bearings. Inspect the crankshaft for corrosion and also check for scratches or score marks around the intake port of the crankshaft. Check the crank pin for wear. The connecting rod should fit and rotate smoothly on the crank with no play between the rod and the crank pin. If there is play between a new connecting rod and the crank pin the crank pin is worn and the crankshaft will need to be replaced.

IMPORTANT: Do not reuse a damaged or worn crankshaft. This will only cause more problems inside the engine. If there are damaged or worn parts beyond the piston, sleeve and connecting rod it may be best to take advantage of the ERP program.

8. Slide the crankshaft back through the crank bearings until it stops. Inspect the carburetor O-rings for damage and the rubber throttle arm boot for tears. Note that on carburetors that have a lower O-ring, it is necessary to replace it when the carb is removed.

Unscrew the high-speed needle all the way out of the needle housing and flush out the high-speed needle housing and the carburetor body with denatured alcohol or electric motor spray.

Use compressed air to dry the components. Reinstall the high-speed needle into the needle housing and return the mixture setting back to the factory recommended break-in setting (four turns out from fully closed). Carefully insert the carburetor into the crankcase until fully seated. While pressing the carburetor firmly into the crankcase, tighten the pinch bolt. This process ensures that the upper O-ring forms a good seal between the carburetor and the crankcase.

9. Apply a drop of oil in the piston's wrist pin bore before installing the wrist pin. Locate the oiler hole in the connecting rod and the skirt relief side of the new piston. Insert the connecting rod into the piston with the piston skirt relief and the oiler hole on the same side as shown. Line up the wrist pin bore in the rod with the wrist pin bore in the piston and slide the wrist pin through the larger opening of the piston. Note that the closed end of the wrist pin should be inserted first. The orientation of the wrist pin is to help keep wrist pin cooler.

Install the wrist pin clip (G clip) into the groove of the wrist pin bore of the piston. This will retain the wrist pin inside of the piston. Be sure to fully seat the G clip in the groove in the piston!

Lower the piston and connecting rod assembly into the top of the engine case with the oiler hole of the connecting rod facing toward the crankshaft. Apply a drop of oil onto the crank pin before installing the connecting rod onto the crank pin. Slide the connecting rod over the crank pin and rotate the crank pin to the bottom center of the crankcase.

10. Lube the outside of the piston and slide the new sleeve into the engine case. Make sure the piston fits into the sleeve without binding. Press the sleeve all the way down into the engine case. Important: The slot (in the lip) at the back of the sleeve must key into the pin at the back of the engine case opening. Do not install the cooling head until these two items are keyed into each other.

Place the copper head gasket onto the sleeve and set the cooling head into the sleeve. Note that there are two slots notched through the cooling fins on each side of the cylinder head. These slots are for access to the engine mounting screws.

The five head bolts should line up with the five holes on top of the engine case and the access slots on the cooling head should line up with the slots manufactured into the engine case.

Be careful not to damage the copper head gasket (see picture for orientation). While pressing the head squarely onto the sleeve, thread the head bolts into the engine case just until they stop. Do not tighten down these bolts individually. It is very important to attain even pressure around the cooling head. Slowly tighten each screw just a little at a time in a crossing pattern, (see picture) until each bolt becomes snug. Finish each bolt in the same pattern with 9.7in/lbs of torque.

11. Connect the back plate with the starter shaft to the engine case. Important: The reliefs that are manufactured into the back plate must face up to match the ports inside of the engine case (see picture for orientation).

Key the starter shaft to the crank pin by slowly rotating the starter shaft as pressure is applied towards the crankshaft.

Once the starter shaft is keyed into the crank pin, join the back plate to the engine case and secure the assembly with the four 3x6CS screws. Install a new glow plug (#3232 is recommended for the TRX 2.5 Racing Engine, and 3232X for the 3.3) into the cooling head. The engine is now rebuilt and ready for the installation of the clutch assembly and the EZ-Start® system.

12. Slide the split cone washer over the pilot shaft with the larger opening toward the engine case.

Slide the flywheel over the pilot shaft and join it up with the split cone washer. The split cone washer should fit inside of the tapered hole in the flywheel. Thread the flywheel nut all the way up to the flywheel and tighten the flywheel with 62.0 in-lbs. of torque against the flywheel.

13. Slide the clutch shoes onto the flywheel. Key the flywheel pins into the holes of the clutch shoes (See picture for orientation of the clutch shoes).

Racer Tip: By reversing the direction of the clutch shoes, the clutch shoes will provide a little firmer bite against the interior wall of the clutch bell. This is useful on higher bite surfaces that offer a higher level of traction.

14. Clean and lube both clutch bell bearings and install a ball bearing into each side the clutch bell and slide the clutch bell over the pilot shaft followed by the 5x8x0.5TW. Fasten the 5.0 E-clip (wearing safety glasses) around the pilot shaft to retain the clutch assembly.

15. Clean the roller clutch with denatured alcohol and lube the roller clutch with Mobil 1 synthetic motor oil. Slide the EZ-Start® system (with the roller clutch) onto the starter shaft that exits the back plate of the engine. Note that the roller clutch is to be installed onto the starter shaft so that the clutch engages the starter shaft clockwise and rotates freely counter-clockwise.

Note that the engraved text on roller clutch faces the engine backplate for correct rotation. Fasten the EZ-Start® gear case to the back plate with the three 3x12BCS screws.

16. Mount the exhaust header and pipe assembly back onto the rear of the engine case. With the two 3x15CS screws.

The engine, clutch, exhaust system and EZ-Start® assembly is now ready for installation into the chassis.

NOTE:  Newly rebuilt engines require break-in.
Break-in instructions are located in the owners manuals (TRX 2.5/R or TRX 3.3).


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