The Cessna 120-140 Association Forum

Hey Guys,

When is it necessary to preheat engine. How do you do it? I leave a 100 watt light bulb in mine on a drop lamp or hit with hair dryer for 15 minutes if it gets below about 40. Usually let it idle about 10 minutes before takeoff. I have c-85 on a 1946 140. Any thoughts?

My knowledge is all based off the googling research and reading points and counterpoints on forums, not actual experience. Its what im basing my starting knowledge of the topic on, so my understanding may evolve, just as a disclaimer...

Preheating is far more complex and in depth topic than i undedstood it to be! Its not really about having goey oil.. its bearing clearance in the bottom and keeping pistons from scoring up top and condensation issues...

From what i can gather under 40 its time, but wouldn't hurt at any temp though in actuality, and its a mortal sin in the 20s and under to not preheat...

So heating bottom end is vital as cold oil isnt as much an issue w multi viscocity aviation oil as the fact that the aluminum block and steel crank dont proportiontly shrink with the temp, so the bearing clearances are squeezed below their factory designed minimums with the aluminum shrinking more than the steel to the point its so tight that their is no room for oil and metal is kissing metal... so gotta make sure bottom is warm.

Now up top our piston walls are not parrallel with each other at cold temps, instead there is a slight conical shape to our bore with the top being narrower. This is built in by design because air cooled cylders dont hold the same temp at bottom as they do the top under normal operting condition. So our cylinder walls lose their cone shape as they reach operating temp and their tops are hotter than their bottoms... so started cold, that piston is pushing up into a cylder too small at top for low wear operation. One engine "expert" i read said a single cold start can put as much wear on cylder walls as 500 hours of cruise flight...

Now condensation... the ford vs chevy argument in preheat is: on all the time vs on just as long before you need it as neccesary..the issue is an engine that's toasty down bottom but freezing up top is somewhere in that engine is an area at dew point now... engines are humid inside, a byproduct of combustion and some blow by and such... so theres risk with a continually heated engine if theres a significant temp differential within it of creating condensation to sit on engine parts-not good. Thus some preheat just long enough to get to temp, length dependent on environment and equipment capabilities, minimizing the time that condesation is in there on parts that are daily loosing thier oil coating....

I think the truth is in the middle, if you can heat your engine to an even amount and out of dewpoint danger, preheat around the clock. If you can't, preheat when necessary just before flight. And if one does the just long enough method and cancels flight, better to try to get even temp on engine and let preheat stay on till next flight however long that may be, as cooling it again brings it through dew point after being unused so not getting coated in oil again, so best to leave it on till next flight.

I going to go for "on all the time" and try for even temp, mine has a sump heater, and im going to sew her a cowl cover custom to wrap cowl including the front, and make propeller covers as well, as the prop and spinner act as a huge aluminum heat sink for the engine. A heat sink is the metal cooling device used on electronics to passivly cool them by radiating off heat, prop does same thing to our engine in preaheat... im then going to monitor temp somehow to ensure im keeping above dew point and that with cowl wrapped and prop covered the sump heater is warming whole area sufficiently. If i cant get an even enough temp then i'll go to the "just enough time before" method. To make sure i dont rush the preheat im going to install a cell phone activated switch so i can turn it on hours and hours before i go...

So long and short of it: its never really too warm to preheat, when in doubt do it. A preheat thats truly effective will get top and bottom warm whatever method one chooses. Be conscous of condensation and find a way to best mitigate its danger. There's also a thread on engine dehydrators on here, thats not an entirely unreleated topic...

Again this is off google research (though a lot of it) and triangulating forum arguments on method not first hand knowledge yet. Take it for what its worth...

That's a pretty good explanation.

That said, how do you determine the dew point? It's not the one you see on the ATIS. It depends on the relative humidity inside the engine.

Chemical reactions (rusting is a chemical reaction) proceed faster under warmer conditions. For that reason. I would not keep the engine warm all the time. Get yourself a good combustion preheater and preheat it for half an hour (more if it's really cold) and use a multi-weight oil in the winter. (My advise, worth at least twice what you're paying for it.)

Of course, a lot depends on where you live. There's a big difference between 30 degree nights and zero degree nights.

Good point on the dewpoint, i hadnt thought that far ahead, ill move more towards the "just when needed" camp based on that point. My company sells a cell powered wifi hot spot with a switch that can be fired on demand via an app, so i could put that in plug my sump heaterin tht and still wrap cowl/prop. That would be nice option having the switch as it has two outlets controlled seperatly, i could turn sump heater on 8 hrs before and hook second to mini ceramic fan heater in cockpit and fire that just maybe 30 mins before arrival... hmmm

And open to tbe idea of the cumbustion too, going to have to consider that

8322 wrote: ↑Wed Nov 14, 2018 10:31 pmmini ceramic fan heater in cockpit and fire that just maybe 30 mins before arrival...

Where do you live? I'd be careful to balance the risk of an unattended fire hazard against the discomfort of a few minutes sitting in a cold cabin. If you really want to preheat the cabin, do that while you perform your preflight inspection.

Thanks for the replies guys! It doesn’t get below 30 much around here so I don’t need a permanent heating solution. What I was really looking for is someone to tell me this is what I do with my 140 and it seems to work.

8337 wrote: ↑Thu Nov 15, 2018 11:57 am Thanks for the replies guys! It doesn’t get below 30 much around here so I don’t need a permanent heating solution. What I was really looking for is someone to tell me this is what I do with my 140 and it seems to work.

Sorry, got a bit off track... I guess my first message was about making sure we preheat effectively... There are many ways to skin a cat, and there are many individual methods to preheat, and do so effectively...There isn't necessarily a right method, but more about making sure the method used accomplishes an effective preheat. My concern with a hairdryer under the cowl if put in up top would be it would get cylinders probably fairly decent or better, but could leave the crank shaft so cold you get the bearing wear... What could you do to direct some heat "down under"???


If I didn't have a sump heater, or in the past on my club plane when away from juice, I've used a small camp stove and metal dryer duct, put an "S curve" in the dryer duct so any drips from the plane had no access to to open flame, and squished the end of the duct in with the nosewheel and covered the cowl with a blanket for a while. Now that I have a deeper understanding of what needs to happen in an effective preheat, I'm not sure that got the top of cylinders very warm, but I don't know... Now we don't have a nose wheel hole but isn't there a cowl vent on the bottom the dryer duct could be squished into? (I meet my 140 in person Monday- so still new to the model specifics-I forget what the underbelly looks like) Could always pipe a hair dryer in that way too with some flexible metallic duct work... So maybe two hair dryers one up top and one down there...

Knowing what I've recently learned, any method I use I will be investigating to ensure the engine feels warm up top and down below to ensure and not assume that its an effective method.

I don't think a hairdryer (or two) would be worth the effort. Not enough BTU's.

There is an opening between the lower edge of the cowl and the bottom of the firewall you can direct the heat from a preheater into.

Crankshaft bearing clearances are great enough I don't think you need to worry about them. What is important, though, is pumpability of the oil. If it's too thick to circulate it really doesn't matter what the bearing clearances are.

In my opinion You can safely start the engine without preheating down to about 20 degrees F as long as you have an oil rated SAE 20W or lower (20W-50, for example). Below 20 degrees the metal needs to be preheated (cylinders, primarily), and if the "W" rating of the oil is higher, it needs to be warmed so it will flow properly. (If you're using W80 as your winter oil, it has a SAE rating of 40W, so preheat (at least the oil) below 40 degrees.)

8322 made some good points in this discussion reflecting points made by the Tanis Heater rep at the Faribault convention in 2016. I don't mean to hijack this thread, but I will say that I do wish I knew who 8322 is and for that matter who everyone else is that posts on this forum as a number but not as a person with a name. Just saying.