The International Cessna^® 170 Association

hilltop170 wrote:.... A 100W drop light will keep an engine warm ....

If anyone still has 100W incandescent lamps. (I squirreled-away a few before they were “outlawed”...but the church talked me out of them to keep a water-line in an equipment-room from freezing.)

The local home improvement box store sold out of incandescent bulbs years ago for $1 per 4 bulb carton. I bought out everything they had.

I did a trial to get some actual pre-heat data using two 100W drop lights (yes George, incandescent lights) under the engine with minimal insulation of one moving blanket draped over the cowl, starting from a not-so-cold, cold-soaked condition of 45°F ambient air temp with measured 55°F cylinder head and oil temps inside an uninsulated closed T-hangar so no wind.

In 12 hours, the oil and CHT temps went from 55°F to 75°F while the ambient air temp fell to 40°F. Once the engine was started, the oil temp initially remained at 75°F indicating the oil and engine internals were heat-soaked at 75°F, a 20°F increase in 12 hours.

One might consider a practice of Turning your FUEL VALVE to OFF if you use heaters beneath or anything using electricity inside your engine compartment.

i'm going to use the milk house heater with dryer ducting
as i prefer anything that is potentially red hot or prone to break
sitting away from my priceless airplane

https://www.youtube.com/watch?v=5b-4yn2qj-I

the bulbs are a cheap simple solution, but c o u l d destroy your airplane and hangar
it seems logical that the above method would less likely to do this if untended for a few hours

dave