"Quote" My gut feeling is although Tohatsu make engines for other more expensive manufacturers their own may be pared down a little somewhere so that they sell cheaper . ie although for instance the merc 3.3 is a tohatsu 3.5 is it assembled and finished by merc to a higher standard.
At present time this is not so, because Tohatsu makes not assembles engines for Mercury, Mariner & Nissan with same components and parts. It's known that many manufacturers have technical collaborations in between brands, that is, some Mercury/Mariner models are Tohatsu and Yamaha made, Suzuki/Johnson, etc, will depend on engine manufactured year.
Before representing Tohatsu, used to work with Mariner/Mercury, have personally seen shipping boxes with stickers saying "engine assembled in Japan" , that is with Japanese parts and components.
If compared Tohatsu is cheaper because it's the manufacturer, remember all engines are exported to USA and then reexported to other countries and the supposed engine manufacturer has a wider price range because of larger import costs, handling, brand name, etc.
Here is a letter from the manufacturer, does not specify from what year or how long have been working together.
Dear Luis Montero :
Smaller engines are manufactured by TMC, Tohatsu Marine Corporation which has been invested by Brunswick Corporation (Mercury Marine Corp) Tohatsu Corporation is actually producing core parts for bigger engines than 70 HP.
Tohatsu Corporation presently manufactures engines for Mercury//Mariner/in the range of: 2.5 to 50 HP in 2 strokes and from 2.5 to 30 HP in 4 strokes engines with their own colors and logos in our new plant at Komagane Factory in Japan.
Hope this information answers your questions and doubts.
Sales Department Oversea Section