Mechanical raison or not ?

A good question.  I Googled it and found this:  https://www.rotax-owner.com/en/912-914-technical-questions/5518-why-is-912-called-912

Google is a good place to start if you are curious about anything.;-)

Mirek, your response is a true copy of a discussion on the Vans aircraft forum from March 30, 2021.

https://vansairforce.net/threads/rotax-912-history.193838/

I'm rather waiting for a less "google" answer.
but it's just curiosity, my engine runs well, even without knowing the origin of the name "912"

A philosopher might say.... 😊

The digit nine is the highest single-digit number in numerology, representing spiritual enlightenment and universal love, and is associated with rebirth and spiritual awakening in Tarot cards.  The digit 1 symbolized the beginning of everything. The meaning of the number 2 is about duality, cooperation, adaptability, and sensitivity in relationships with others. It’s about finding equilibrium and peace, both within oneself and in one’s interactions with others. Think of it as the number of balance, bringing a sense of peace and stability. 

(sorry, this was also from google searches on the meaning of digits)

OK, so this is fun.  In the beginning of using Rotax (2 stokes) they used derivatives of the same type with a change to the last number of the type to reflect aircraft version.  The type 277 single was  both an aircraft version (the serial had the letters UL on the plate) for certified gliders it had modifications and became a type 275.  The type 503 had at least 3 variants, they typical 503 with UL on the tag was for experimental and the versions with type 501 and 505 were special for glider versions.  There was also a type 532 version, letters UL for aircraft and a version for gliders called a 535.  Obviously there were others, the main ID for aircraft was the addition of letters UL to the data plate to indicate unlisted to a certified TC (yes in the beginning it was for Ultralight) The UL stuck on the line as a moniker for aircraft experimental versions.  Any done to a TC were given a slightly different type number also. 

4 stroke engines for aircraft were completely different develop lines.  The first aircraft version was in released as a prototype in 1985 as a type 906.  It had opposed 2 cylinder, dry sump with gearbox and electronic ignition.  (yes looked like a ½ 912.) it however only had 36 HP and was a flop right out of the gate with high torque but low HP it was not what anyone wanted.  That never made it into serial production.  The next and more successful was the type 508.  This upright twin had overhead cam and looked promising in that it made some 43 HP.  It was developed from designs being worked on for motorcycle engines at the time. The 508 was a bit heavy but had great fuel consumption numbers but never was that popular in the USA.  It found some market in the UK and a few other european countries. 

In late 1988 Rotax did a pre-production run of the all new 912, the 12 did indeed stand for the 1211 cc displacement (906 was as you guessed ½ the displacement of the 912) Full serial production started in 1989 and at that time the 912 sold retail for 6000 USD and there were almost no sales.  In the USA, perhaps the world, the first OEM to switch from the Rotax 2 strokes to 4 stroke in volume was Kitfox aircraft.  They were the first to sell over 100 aircraft with the new engines and other OEM quickly made the switch for many light experimental kit aircraft. 

Fun to think back to the early days as we could never have imagined where we would go at that time. 

Cheers

Thank you very much for this response and this very informative history.

"Wizard" , I really appreciate your answers on the different subjects

Jacques