PRD Fireball Shop Manual/Bottom End


The bottom end of the engine requires periodic inspection and replacement when worn beyond allowable tolerances. A bottom end job is the most involved regular maintenance procedure required by the engine, and may not be a task suitable for the "home mechanic" as many specialized tools are required. The following parts of the motor are qualified as "bottom-end" parts:

  1. Crankshaft.
  2. Crankshaft pin.
  3. Crankshaft bearings.
  4. Bottom-end/Big-end bearing.
  5. Connecting rod.

When replacing or addressing maintenance to these parts, this procedure is especially relevant.

What You'll NeedEdit


  1. Metric Allen head wrenches
  2. Metric hex head wrenches (sockets or other)
  3. Torque wrench (0-200 in.lbs.)
  4. Non-marring hammer (rubber or nylon)
  5. Copper (or brass) hammer
  6. Clutch removal tool (specialized tool)
  7. Ignition rotor removal tool (specialized tool)
  8. Crankshaft pressing fixture (specialized tool)
  9. Fixture to align crankshaft
    i.e. Dead-centers in a lathe.
  10. Shop press (5 tons)

Replacement PartsEdit

  1. O-rings for head - Optional but highly recommended.
  2. Jug base gasket - Optional but highly recommended.
  3. Reed cage gaskets - Optional but highly recommended.
  4. Bottom end kit
  5. Crank pin - Optional - Dependent on hours of use/condition.

Consumable MaterialsEdit

  1. Anaerobic flange sealant
  2. Thread locker (blue)
  3. Multi-purpose oil or 2-stroke oil
  4. Gasoline
  5. Paint-thinner


The following items are recommended to reduce risks generally associated with the activities described in this manual.

  1. Eye protection - splashes and flying parts.
  2. Nitrile gloves - hazardous chemicals.
  3. Tactile work gloves - damage to hands and fingers from heavy equipment or tools.
  4. VOC filtering mask - inhalation of hazardous chemicals.


Remove HeadEdit

  1. Remove the spark plug.
  2. Rotate the piston to Top Dead Center.
  3. Drain the engine coolant.
  4. Remove the hose clamp from the head outlet.
  5. Remove the hose from the head outlet.
  6. Remove the nuts from the head - try to progressively unload the nuts instead of loosening each nut completely.
  7. Remove the washers from the studs.
  8. Pull the head upward and off - it shouldn't give any resistance to removal.


Remove Exhaust (optional)Edit

  1. Unhook the springs that wrap around the exhaust pipe.
  2. Remove the nuts from the head - try to progressively unload the nuts instead of loosening each nut completely.
  3. Pull the exhaust outward from the studs.

Remove JugEdit

  1. Pull upward on the jug - you will notice the piston sliding downward - have your hand underneath to catch the piston and rod from banging into the crankcase.


Remove ClutchEdit

  1. Remove the clutch guard.
  2. Install a clutch removal tool or other specialized tool to enable locking of the crankshaft.
  3. Remove the crankshaft end nut.
  4. Gently slide the clutch drum and needle bearing off of the crankshaft - the needle rollers are captured in the cage and shouldn't spill out.
  5. Install your clutch removal tool - it is advisable to add a dab of grease or oil to the end of the crankshaft, and the tip of the pin inside the clutch puller tool.
  6. Remove the clutch - it may take quite a considerable amount of torque on the tool.

Remove Ignition RotorEdit

Note: The ignition rotor does not require the ignition coil/ignition sensor to be removed.

  1. Install a clutch removal tool or other specialized tool to enable locking of the crankshaft.
  2. Install your ignition rotor removal tool.
  3. Remove the ignition rotor - be advised that there is a Woodruff key captured between the shaft and the ignition rotor.


Remove Reed CageEdit

  1. Remove the four 10mm nuts surrounding the flange that the carburetor is mounted to.
  2. Extract the flange and carburetor.
  3. Extract the reed cage.
  4. Remove any gaskets still attached to the crankcase.


Split Crankcase HalvesEdit

If you have not already removed the engine from the engine mount, you must do before proceeding.

  1. Remove the eight button head allen bolts from the left half of the crankcase.
    At this point the crankcase isn't fixed together by bolts, but will require some coercion to split apart; there are studs locating each half, and sealant between the gap.
  2. Split the crankcase
    • Use a non-marring hammer at first to break the sealant: hammer on the crankcase joint until a visible gap emerges.
    • Pull as evenly as possible on opposing sides of the crankcase, to prevent binding.


Press Crankshaft ApartEdit

The following procedure requires a specialized tool; the procedure to arrive at the expected result should be dictated by that tool's instructions.

  1. Utilize a crankshaft pressing fixture to press one half of the crankshaft apart.


Remove Bottom EndEdit

  1. Remove bottom end components:
    1. Washers.
    2. Connecting rod.
    3. Needle roller bearing - the rollers are not captured by the bearing cage and will fall out when the bearing is removed from pin.
  2. Inspect the crankshaft pin with a micrometer. If the crankpin is worn beyond allowable tolerances, follow the next section, otherwise proceed to Replace Bottom End.

Replace Crank Pin (optional)Edit

The following procedure is necessary when the crankshaft pin has worn beyond allowable tolerances.

  1. Utilize a crankshaft pressing fixture to remove the crankshaft pin.
  2. Press a new crankshaft pin into the crankshaft.

Replace Bottom EndEdit

Do not perform cleaning of these components with solvents; use a compressed oil, or oil from a spray bottle to force any debris from the assembly.

  1. Before reassembly, clean the crankshaft and the crankshaft pin with a soft cloth.
  2. Once the crankshaft and pin are clean, ensure the polished surface of the crankshaft pin is lubricated with a 1:1 mixture of 2 stroke oil and gasoline, or other light machine oil or assembly oil.
  3. Place a new washer.
  4. Assemble the new needle roller bearing.
    Tip: Wrap the bearing cage with a piece of masking or Scotch tape. Insert the rollers into the bearing cage and press them into the tape surface.
  5. Place the assembled needle roller bearing onto the lubricated crankshaft pin.
    If utilized, remove the tape holding the pins in place.
  6. Pre-lubricate the bearing surface of the connecting rod bottom end.
  7. Place the new connecting rod end over the needle roller bearing.
  8. Place a new washer.

Press Crankshaft TogetherEdit

The following procedure requires a specialized tool; the procedure to arrive at the expected result should be dictated by that tool's instructions.

  1. Utilize a crankshaft pressing fixture to press the crankshaft back together.

Align CrankshaftEdit

This procedure requires a fixture to capture the assembled crankshaft at both ends. To capture the crankshaft on it's rotational axis, you need "centers" on both ends, which are rigidly fixed. A lathe with dead-center tools installed can suit this purpose very well, but a suitable tool can be crafted at home from off the shelf materials and repurposed tools.

  1. Fit the crankshaft into the fixture, holding the crankshaft at both ends with tapered steel centers.
  2. Using two machinists travel measurement dials, place the end of the tool on the crankshaft bearing surface. See image below showing one dial on the correct measurement surface.
  3. Test the stiffness of the alignment fixture: while placing moderate radial pressure on the crankshaft, the dial should not render a different reading, especially after pressure is released. If the crankshaft is properly (stiffly) fixed, proceed on.
  4. Verify the crankshaft alignment:
    Rotate the crankshaft through a full rotation and observe the dials. The crankshaft is aligned when the following conditions are verified by the dials.
    1. The dials move in unison, which is to say, when either dial achieves it's high-point or low-point, the other dial reaches that same state at nearly the exact same moment.
    2. The dial range of movement does not correspond to more than 0.001".
    If the crankshaft is out of alignment, apply the technique in the next step to correct improper alignment.
  5. Correcting improper alignment:
    1. Determine which half of the crankshaft to hammer - it is arbitrary, but the crankshaft alignment must be adjusted in the correct direction!
    2. Using a copper (ideal) or brass hammer, hammer the crankshaft half as you hold the other half in your hand.
    3. Repeat the previous step to verify the crankshaft alignment.



It is possible for the crankshaft to be bent. If this condition exists, it is not possible to align the crankshaft. Replace the half of the crankshaft that is bent - this is most likely to be the clutch-side of the crankshaft. Below are the likely scenarios for a crankshaft to bend:

  • The engine spins up very high in RPM as the rear end of the kart catches air. When the axle contacts the track surface, an excessive torque is transmitted through the chain and into the crankshaft.
  • Collisions on or off track.
  • Chain breakage.


In general, follow the same procedure to reassemble the motor that was used to disassemble the motor. Below are some addition steps required for proper reassembly.

Seal the CrankcaseEdit

Before sealing the crankcase, don't forget to reinsert the assembled crankshaft!

  1. Clean the crankcase joining surface with a solvent. The surface must be absolutely perfectly clean and free of deposits and debris before proceeding.
  2. Apply an anaerobic gasketing sealant to the joining surfaces, following the directions on the product chosen. Ex: Loctite 574
  3. Insert the eight button head allen bolts and torque to the manufacturers recommendation.
  4. Wipe the excess sealant from the outside of the motor.
  5. Wipe the excess sealant from the inside of the motor:
    1. Fold a small section of paper towel.
    2. Insert the paper towel into the gap available just after the connecting rod bottom ended moves past the opening reed cage opening on it's downward travel.
    3. Rotate the paper towel through the crankcase, picking up the excess sealant before it has dried.
    4. Repeat this procedure until there is very little excess sealant visible.


Use Fresh Gaskets and O-RingsEdit

It is not always required, but typically a good idea to replace gaskets and even o-rings. Omitting this crucial step may be more trouble than it is worth to conserve a few dollars on reuse of these typically one-time-use components:

  • (Jug) base gasket.
  • Exhaust manifold gasket (if removed).
  • Reed cage and carburetor flange gasket.
  • O-rings: stud (4), inner and outer.

Reinserting the PistonEdit

Use care to reinsert the piston into the sleeve:

  1. Apply lubrication to the piston, ring, and cylinder sleeve before reinserting the piston.
  2. Grasp the piston and compress the ring as much as possible with your fingers.
  3. Gently reinsert the piston into the sleeve.

Installing the HeadEdit

Getting a good seal of the head when the engine has seen much use can be a less than trivial task. Below are some tips and tricks to get the correct seal:

  1. Check for flatness of the head (see below).
  2. Cleanliness of the o-ring grooves.
  3. Fresh o-rings.
  4. Balanced torquing of the head nuts - progressively build up torque on the nuts, and tighten the nuts in an X pattern, akin to tightening wheel lug nuts.