Been cold, at what point should you not try to run your motors outside?  They sure seem harder to start when the temp is 50 and below.  Also, how about min temp for breaking-in an engine.  No yankee snow jokes please.

freezin' in Texas
Bruce

The coldest day I flew was -39 C     .  This was at the Winnipeg Perfect Circle Flying
Club's Winter Air Races Cup.  I won it one year .  It's more a matter of how much
one wants to run a motor or go flying, than the weather.  Fuel with all synthetic oil
is best in the cold, add an ounce or so of automobile gas to a gallon of fuel.  You can
also wrap insulation around the fins, and cover that with tin foil.  Preheat,
or keep the engine warm before starting ( keep in car, or use a portable
heater).  If you want to break in a motor, you will likely want to use fuel with castor
oil.  This will be OK, IF you keep the fuel from getting cold, and also the motor.
ie: get everything ready inside where it's warm ( car, garage, house, etc. ) then
go outside and immediately start up your engine.  Do a normal break in type of run.
When you need to stop the motor, go back inside with it if it needs to be warmed
up, before restarting.
   If you go outside when it's cold, and the motor and fuel ( and starting batteries too)
get really cold, then you WILL have problems        .
   A minus 39 C day can be enjoyed outside if one is dressed appropriately, and keeps
active.   You can also keep your flight box with fuel, batteries, etc. in a picnic cooler
with heat packs ( it would now be a 'heater ' instead of a 'cooler ' ).
   Try to avoid getting raw fuel on your fingers and hands, you'll learn quick!  Keep a
heat pack handy to warm up your hands too  !

Stan Bidowski 
The Winnipeg Perfect circle Flying Club