Rectifier Temperature

That schematic is a shunt regulator.  Battery voltage is sensed at the "C" terminal.  If it exceeds the set point, the Darlington transistor pair triggers the MCR16N SCRs, which shunt the generator's stator's output ("G" terminals) directly to ground.  Output is on the "+B" and "R" terminals.

The rectifier (shown as four discrete diodes inside the dotted rectangle) contains no active switching components.  Datasheet: https://www.comchiptech.com/admin/files/product/20181106093830.PDF

Shunt regulators have the odd behavior of running hotter as less demand is made on the system.  If the battery is fully charged and the system has low demand, the regulator has to shunt maximum current from the generator stator to ground in order to hold battery voltage at the set point.  This generates maximum heat in the SCRs.  If the battery is low or there is high demand, more current flows from the regulator's output and there is less shunted through the SCRs, generating less heat.

The opposite behavior (more current demand = more heat) is a feature of linear, and, to a somewhat lesser degree due to their efficiency, switching regulators.

Is that an official Rotax schematic, or something that was drawn from a reverse-engineered regulator?  It has a couple of curious errors:

- The battery symbol (just above the "+B" terminal) is backwards, suggesting a positive-ground system.

- The symbol for the 22nF 1kV capacitor between the "G" terminals shows a polarized capacitor.  In that position it would be non-polarized, probably ceramic or film.

[Edited to change "generator" to "stator."]

Yes, that diagram is a shunt regulator with the shunt on the DC side, but it is single phase.  The 912iS uses a three phase alternator and regulator/rectifier and I suspect may shunt the voltage on the AC side, similar to the diagram below.  I don’t know this for sure, and I would like to see a schematic of the actual regulator/rectifier used in the 912iS.  

The shunt is on the AC side in that diagram.  Notice that the SCRs are connected directly to the "G" terminals, which are fed from the stator (the squiggle-in-a-circle at the top of the schematic is the symbol for an AC voltage source).  DC begins at the node in the middle of the rectifier, between the diodes, feeding the "+B" and "R" terminals.

Back to Sean's original question.
I find nothing that Directly addresses the Temperature question in the ULS documents.

My apologies to Sean,  You did not specify what engine /Regulator you were referring to, and my mind defaults to my 912is.
I see you have a ULS so, you can ignore all the Shunting arguments.

The 912ULS manual specifies a 90°C Ambient Max for the External Alternator.
The 912is manual specifies 80°C Max on the body surface of the Regulators.
And 80-90°C  would be reasonable working values.  But, indeed, too Hot to touch for more than an instant.

The spec sheets of the SCRs in the Mystery Schematic list 150°C for their Max values.  Way beyond Touchy Feely HOT!

Solid-state components can tolerate way more heat than the average person would consider reasonable.
In my production days, a rule-of-thumb (Actually Finger) was that if you wet your finger and quickly touched a part and it did not instantly boil (Pssss!), it wasn't too hot. 
If it took more than a half-second for the moisture to evaporate, it wasn't even worth worrying about.
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Now for that Mystery Schematic.

Of most interest is that there is no name of the creator in the sheet. 
No one is taking credit for it. ???
I think the 22nF cap just has the wrong symbol. No harm, No foul!
22nF Caps are not available in 1kV polarized versions.
I don't think they are available in ANY polarized versions. Why would you want one?
22nF is mighty small.
Battery Symbol?  Another Typo. or would that be a Drafto?

I do NOT see a shunt regulator.
I see what are essentially two, half-wave, duty-cycle controlled rectifier circuits.

I see the Bridge rectifier supplying the V+ to the Battery, but the V- side does not go to the ground, it is used to bias the Fault Lamp circuit OFF.
There will be a minuscule current (<10uA) on the V- side.
So, where is the Generator's Ground path?
It is through the two SCRs that are controlled by the regulator circuit originating from the "C" lead.
This is a classic, duty-cycle-controlled, lamp dimmer circuit.
The duty-cycle controlled ground path is supplied whenever a current is needed to raise the voltage to the desired level.
The output is extremely spikey and noisy in the sonic range. 
This is why the regulator MUST be connected to a battery, or at least a Large (22,000uF) Capacitor to filter the output.

Mike Miller had it right!  ?
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And I too would love to see the 912is three-phase shunt regulator's schematic.

And, Happy Holidays to you too, Roger.