How to Build a Triumph T120 Race Engine

Updated February 21, 2017

The Triumph T120 comes with a stock 650cc Bonneville engine that you build out to 675cc for flat track racing or road racing. The engine build requires mechanical your expertise along with the services of a racing shop to modify the cylinder bore and upgrade the valves. You disassemble the stock engine on your workbench and upgrade the clutch, ignition timing and carburettors as part of the racing engine assembly. Once the engine build is complete, you can set up the exhaust, sprockets, brakes and tires for the type of racing you plan to do.

Drain the engine oil. Follow the procedures in section B-1 of your workshop manual and remove the engine from the motorcycle. Place the engine on the workbench and remove both carburettors. Remove and discard both carburettors and the intake gaskets. Remove and discard both spark plugs.

Remove the right and left rocker boxes with the valves intact. Remove the twin cylinder head and discard the head gasket. Lift the push rods out of the twin cylinder block.

Pry the twin cylinder block off the crankcase and discard the crankcase gasket. Remove the wrist pin clips from both pistons. Tap the wrist pins out and take the pistons off each rod. Discard the pistons, wrist pins and clips.

Loosen and remove the clutch cover bolts at the right primary case and remove the cover. Take the stock pressure plate, springs, clutch discs and clutch basket out. Save the clutch cover and bolts. Discard the stock clutch components.

Turn the engine around. Remove and save the timing cover and the cover bolts. Detach and discard the stator, points and timing plate assembly.

Take the twin cylinder block, twin cylinder head and both rocker boxes to a racing shop. Have the cylinders bored to the maximum 2.84 inches each. Have the head serviced and polished. Have the shop service the rocker boxes and install racing valves and valve springs. Ask the shop to provide the oversized pistons, rings, wrist pins and wrist pin clips.

Attach the new pistons to the piston rods using the new wrist pins and clips. Put the new compression rings and oil rings on each piston. Place the new cylinder block gasket from the racing kit atop the crankcase.

Install the pistons into the cylinder block sleeves using a ring compressor tool. Torque the cylinder block bolts to the specified foot-lbs. given in your workshop manual.

Place the head gasket from the racing kit atop the twin cylinder block. Install the newly serviced twin cylinder head and torque the head bolts in a crisscross pattern. Install the new rocker box gaskets atop the twin head. Attach the right and left rocker boxes and valve assemblies to the head and torque the bolts in a crisscross pattern.

Turn to section B-28 in the workshop manual and follow the procedures to install the racing ignition timing components. Follow the instructions in the racing kit and install the slipper clutch components. Reattach the saved timing cover and clutch cover using the saved bolts.

Position the new carburettor intake gaskets at the right and left intake ports on the cylinder head. Attach the twin Keihin racing carburettors to the head and torque the bolts to the specified torque. Install new spark plugs. Follow the procedures in section B-1 of your workshop manual and install the racing engine in the motorcycle. Fill the crankcase with racing oil.


Check the technical regulations of the particular race class to ensure conformity before building the race engine. Save all bolts in a glass jar for easier reference during the assembly. Clean your tools before assembling the racing engine. Include a high-volume fuel pump and high-output ignition coil to complement the racing engine.

Things You'll Need

  • Triumph T120 workshop manual
  • Standard gauge engine tools
  • Triumph Daytona 675 race kit
  • Ring compressor tool
  • Torque wrench
  • Keihin racing carburettor kit
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About the Author

William Machin began work in construction at the age of 15, while still in high school. In 35 years, he's gained expertise in all phases of residential construction, retrofit and remodeling. His hobbies include horses, motorcycles, road racing and sport fishing. He studied architecture at Taft Junior College.