The R18 improved upon the R11 in a variety of ways but kept the same overall setup and looks largely identical to the untrained eye. The R18 was eventually developed into a separate pump for the Unites States denoted with a "Y" at the end of the model number.

The most obvious updates are a larger window to the plunger area allowing much more access to plungers, reduced overall bulk, and additional cooling ribs. The larger window allows easier adjustments during testing while the cooling ribs help regulate temperature.

The R18 was replaced by the R18Z which has ball pressure valves, no oil check valve and richer adjustment. The R18Y, made for increased emissions regulation in the U.S. market, has a stop magnet, modified three-dimensional cam and a guide tube for the full-load adjustment screw.