How to Adapt a Small Gas Engine to a Bicycle

Adapting a gas engine to a bicycle is the type of project that requires some ingenuity and a fair degree of mechanical aptitude. Even small gas engines generate heat, so you will need to select a location for the engine that's going to eliminate the possibility of burning yourself. With regard to engine types, a 25cc garden trimmer engine is compact and has an integral gas tank, while RC engines are available up to 50cc and require an accessory gas tank. Once you have installed the engine in the bike frame, the rest of the project will come together in a relatively short time.

Things You'll Need

  • Miter saw with metal cutting blade
  • 1/8-inch plate steel
  • Electric drill
  • Assorted drill bits
  • 2 1/2-inch u-bolts with lock nuts
  • Socket and ratchet
  • 2-inch by 2-inch angle brackets
  • 3/16-inch machine bolts with lock nuts
  • Self-tapping screws
  • 3-inch centrifugal clutch
  • 6-inch belt pulley
  • Fan belt
  • Allen wrench
  • Hand throttle lever
  • Throttle cable
Show More

Instructions

  1. Engine

    • 1

      Select a location for an engine platform as close to the rear of the main frame as possible. One possibility is on the front side of the seat tube, about halfway between the bottom bracket and the upper tube of the frame. Another possibility is a welded piece of tubing that extends horizontally from the seat tube over the rear wheel sprocket. The platform would attach to the top edge of the tubing.

    • 2

      Set up a miter saw with a steel cutting blade. Always wear safety glasses and gloves when cutting any type of metal. Measure and cut a piece of 1/8-inch steel plate as an engine platform. Size the platform to the smallest dimensions possible, and still ample to support the engine.

    • 3

      Mount the platform to the bike frame by welding or devising attachments for 2 1/2-inch u-bolts that fit around the frame tubes. Drill the platform for u-bolts using an electric drill and the appropriate drill bit. Slide the u-bolts around the frame tubes and attach the platform using a socket and ratchet to tighten lock nuts on the u-bolts.

    • 4

      Position the engine on the platform with the drive shaft on the opposite side of the rear wheel sprocket. Make sure the drive shaft is perpendicular to the bike frame, and not at an angle that will cause drive-belt wear and reduce engine performance. Mark the location of the engine on the platform and set the engine aside.

    • 5

      Mark locations for 2-inch by 2-inch angle brackets that secure the engine to the platform. Plan to install two brackets on each side of the engine, and one at the front of the engine. Position the angle brackets on the platform and mark the locations of the pre-drilled holes at the base of each bracket. Drill 3/16-inch holes at each mark and attach the brackets using 3/16-inch machine bolts with lock nuts.

    • 6

      Position the engine on the platform against the angle brackets. Secure the engine in place using machine screws or self-tapping screws through the bracket holes and into the engine case.

    Drive Train and Throttle

    • 7

      Slide a 3-inch diameter centrifugal clutch pulley onto the engine shaft. Align the flat spot on the engine shaft with the Allen bolt on the pulley. Secure the pulley to the shaft using an Allen wrench.

    • 8

      Loosen the axle nuts on each side of the rear wheel. Lift the chain off the rear wheel sprocket. Remove the rear wheel from the bicycle and place it on a worktable with the sprocket side down. Remove the hexagon axle nut that secures the bearing cone to the wheel hub using a socket and ratchet.

    • 9

      Put a 6-inch diameter belt pulley on the rear wheel axle and against the wheel hub. Drill four 3/16-inch holes, evenly spaced, in the face of the pulley for machine bolts that attach the pulley to the rear wheel hub. Attach the pulley to the hub using 3/16-inch machine bolts with lock nuts. Tighten the nuts securely with a socket and ratchet.

    • 10

      Replace the rear wheel into the frame arms on the bicycle. Install a 3/16-inch wide fan belt on the rear wheel pulley and the centrifugal pulley. Loop the bicycle chain over the rear wheel sprocket. Pull the rear wheel back in the frame arms until the fan belt has 1/4-inch deflection up and down. Tighten the axle nuts on each side of the rear wheel.

    • 11

      Attach a throttle control to the handlebars using an Allen wrench. Connect a throttle cable between the throttle control and the throttle linkage on the engine. Tighten the cable stops at each end of the pulley to complete the installation.