As a result of work spanning several years, a new set of engines was introduced. These were the Unit Construction Engines (UCE). The 350 cc UCE found use in the domestic model Thunderbird TwinSpark in a configuration with two sparkplugs per cylinder, with integrated 5-speed left-shift gearbox. It has not been featured on any of the Bullet models, domestic or otherwise. By 2011, the old cast-iron engine had been completely phased out, including in the Standard 350 model. Now all Royal Enfield bikes are only available with the all-aluminium UCE engine.
The 500 cc UCE features fuel injection, and has greater power than any Royal Enfield 500 cc motor. The 500 cc UCE, with an integrated five-speed gearbox, powers the current Bullet Classic model. Starting in 2009, this engine was available only in the EU to satisfy emissions regulations,[1] but as of 2010 it is available in the United States under two frame models, the Bullet Classic C5,[8] or the Bullet G5, which looks similar and shares paneling with the earlier AVL Electra models. In 2011 a third export model, the B5, was introduced combining the newer 500 cc UCE engine with the traditional Indian domestic tank and frame.
