How to Convert a Harley-Davidson 883 Sportster to 1200

Updated April 07, 2017

The 883 Sportster is a low, nimble Harley, but it still weighs 261 Kilogram; with something less than 45 horsepower, most people consider it dramatically underpowered. More than 100 horsepower can be coaxed out of these engines. Two common improvements will reliably boost the 883's horsepower to around 62. The first is to pay the so-called Harley Tax and upgrade the bike's intake and exhaust systems. The second upgrade is to bore the bike out to 1200 cubic centimetres. Most owners buy a conversion kit for around £390 and convert the bike themselves.

With your shop manual close at hand, begin disassembly. Close your petcock. Put the bike in fifth gear and depress the clutch. Start your motorcycle and run the engine until the carburettor is out of gas. Remove your key. Remove your air cleaner including the backing plate and your complete exhaust system. Remove the lower gas line where it joins your carburettor. Drain your gas tank into a gas can; it helps to use rubber tubing. Disconnect the choke from the left side of the bike. Disconnect and mark the throttle cables. Remove the carburettor from its rubber boot. Disconnect the spark plug wires from the coil and the plugs.

Continue disassembly by raising the motorcycle on a motorcycle jack. Ensure the bike is stable. Disconnect your coil and horn from the wiring harness. Remove and mark the location of each tank mount bolt one by one. Remove in order the gas tank, coil, seat, battery, ignition switch, Vacuum Operated Electrical Switch (V.O.E.S.), rear motor mount, intake manifold, spark plugs and front motor mount. Tie off all loose wires with cable ties.

Work on one head at a time and begin with the front head. Loosen the rocker box screws with an Allen wrench. Remove the rocker box covers. Working on the right side of the engine first, loosen the rocker arm nuts with a box wrench. Loosen the Allen screws on the third rocker box. Loosen the inner rocker cover and remove the lower rocker box and rocker arms. Unless otherwise specified, assume all fasteners are hex socket nuts. Loosen them with open-ended box wrenches, closed box wrenches and socket wrenches.

Mark the location of each pushrod, then remove the intake and exhaust pushrods. Consult your shop manual and remove the head bolts exactly how and in the exact order specified in the manual. Remove the head and set it aside on a bed of clean rags. Mark and remove the pushrod tubes. Remove the head gasket and O-rings.

Turn the rear wheel until the front piston is at bottom dead centre. Slowly lift the cylinder straight up. Turn the rear wheel again until the piston moves up. Use a small screwdriver to remove the wrist pin retaining wire. Tap the wrist pin out of the piston with a rubber mallet and a wood dowel.

Repeat the last three steps on the rear cylinder.

Reassemble the engine top end using the replacement cylinders and pistons in your conversion kit. Reuse your old heads. Be sure to tighten the head bolts in the exact sequence and to the exact torque specified in your shop manual. Change your oil and filter. Refuel the motorcycle.

Things You'll Need

  • Mechanics tool set
  • Motorcycle jack
  • 883 to 1200cc conversion piston, cylinder and gasket kit
  • Gas can
  • Rubber tubing
  • Cable ties
  • Clean shop rags
  • Gasket scraper (optional)
  • Piston ring compressor (optional, depending on your replacement parts)
  • Rubber mallet
  • Wood dowel
  • Torque wrench
  • Four quarts of motor oil
  • Replacement oil filter
  • Shop manual for your motorcycle
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About the Author

Don Davis has been a professional writer since 1977. He has had numerous writing jobs, including writing news and features for the "Metrowest Daily News" and "Los Angeles Herald-Examiner." Davis has a Bachelor of Arts in English and history from Indiana State University.