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I am breaking in an inverter generator with small air-cooled 80cc 4-stroke single cylinder engine. After the break-in, I will change the supplied mineral oil to a high-quality fully synthetic oil.

My plastic oil collecting container withstands temperatures of up to 85 degrees Celsius, which seems a bit low to me.

How many minutes I should wait after the engine has turned off because of fuel starvation, before I change the oil? I understand that the oil should never be changed when cold because it doesn't flow well when cold, but after the engine has been run, it might be very hot and therefore waiting for some amount of time could be beneficial. However, I don't know how long the wait should be, and the manual only says it should be changed "when warm" but doesn't specify what "warm" means (but presumably "warm" is different from "hot").

I understand that oil can easily have temperatures of above 100 degrees Celsius, and even more if the engine has been highly loaded (it my case during break-in I have a varying resistive heater load that is up to 37% of the maximum permissible load -- the manufacturer has specified that 50% load should not be exceeded during break-in). So an immediate oil change after engine has turned off could possibly damage the plastic oil collecting container.

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    With small air cooled motorcycles I wait about 10 minutes after stopping before draining. If you can touch the drain plug momentarily without pain, it should be 60 C or lower. I have to say I usually use a metal container to drain, a metal oil can with one side cut out with a can opener or stout knife.
    – Michael Harvey
    Commented May 17, 2022 at 11:43
  • @MichaelHarvey That sounds like an answer, you should post one! Because of plastic container and because the drain plug is not under the oil but rather over the oil (so the generator needs to be tilted to drain oil) I'm afraid I can't use the drain plug as a "temperature sensor". I think I'll extend the wait time to 15 minutes, because of the plastic container.
    – juhist
    Commented May 17, 2022 at 11:56

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I personally don't think you need to worry about the temperature of the oil too much.

Let it sit for plenty of time to cool to a safe 'warm' temperature. No need to risk a nasty skin burn.

By then, all the oil will be in the sump, so just leave it tilted so that it fully drains.

I also think that you want some viscosity in the oil so that the oil takes as many particles out with it rather than leaving them behind.

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If the engine is too hot to touch then the oil is too hot to safely handle no matter what the container is rated for. Once the engine is cool enough to touch but still warm the oil will still be warm enough to drain. On a hot day you probably don't even need to warm it up for it to drain.

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I changed the oil now. Because there were some contradictory advice (one advice: wait 10min, another advice: wait for long to cool to a safe 'warm' temperature) I decided ultimately on 20 minutes. It seems this 20 minutes was more like waiting to cool to a safe 'warm' temperature.

It seems that after those 20 minutes, when the generator was cooling down in room temperature for that amount of time, the oil wasn't very hot. After pouring all of the oil in the engine (0.5 liter nameplate capacity but it seemed that I only drained about 0.45 liters so some oil probably remained in the engine surfaces because the 0.5 liters of replacement oil I measured seemed to be bit too much), into a 4 liter plastic oil catch container, I measured the temperature of the oil in the container with an infrared thermometer. The reading was 50 degrees Celsius. Presumably when the oil was still in the engine, it was hotter, but after it touched the room temperature oil catch container, it cooled down to an equilibrium.

Next time I reduce the wait time to 15 minutes, and then perhaps to 10 minutes if the oil temperature isn't very hot at 15 minute wait time. I suspect with hotter oil I might get more than 0.45 liters of it out, so the change would be more "complete".

After doing the oil change, I can say that this design of generator has absolutely no danger of burning your hands with hot oil. The oil drain plug and fill hole cover is the same part, and you remove it and tilt the generator to drain oil. The generator in my case was on the first step of a staircase and the oil catch container below it on the floor (below the oil catch container I placed an insulating piece of wood and a newspaper to catch any spilled oil). Because you can remove the plug without any oil dripping out, and oil starts dripping only after tilting the generator, this eliminates any danger of burning your hands with hot oil even if the oil is burning hot.

I used gloves, but I can't imagine ever accidentally touching hot oil in this design of generator, so next time I might do the oil change without gloves. With work gloves, there's always the danger that the gloves are dirty and could theoretically introduce dirt to the engine.

Since the plastic oil drain plug is above the oil, and doesn't really touch oil in use (except the integrated thin dipstick is under oil surface), feeling its temperature can't be used as a reliable indicator of oil temperature.

Edit: a second data point.

I changed my lawnmower oil (a different engine than the inverter generator). This time I waited for 10 minutes. After the oil change which required few minutes of waiting for the hot oil to be poured, my inframeter thermometer measured 60 degrees Celsius for the temperature of the oil. Obviously during those few minutes, the oil cooled, so when it was pouring out from the engine, it was hotter than 60 degrees Celsius. But by how much, I don't know.

About the possibility of burning my fingers, this seems a bit remote. Tipping the equipment isn't hard (but on a Briggs&Stratton it would be hard since Briggs&Stratton oil can be changed only from below or from above, not from the side as on these Chinese engines), and I obviously wear oven mittens when tipping the equipment and holding the oil collecting container.

At this 10 minute waiting temperature (probably somewhat above 60 degrees Celsius but by how much, I don't know, as the oil had cooled for few minutes), I managed to collect 381 grams of oil. I measured that 0.5 liters of new oil is 417 g, so this 381 grams is around 0.46 liters. The specified oil capacity is 0.5 liters, so I managed to collect about 92% of it.

So, I think 10 minutes is probably an optimal waiting time for small engines. The temperature is sufficiently far away from the 85 degrees Celsius at which the oil collecting container can be damaged, and yet still warm that it flows easily. Also, less time wasted for waiting for the oil change. Most of the 10 minutes can be spent by collecting the equipment and materials needed for the oil change.

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    Don't overlook accidents. There is no guarantee that something won't go wrong, especially with a makeshift workspace on the stairs. So wait until the oil has cooled enough not burn you if there is a mistake.
    – Weather Vane
    Commented May 17, 2022 at 18:08
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    This is a QA site, not a typical forum. Please don't post updates as answer. Edit the question instead.
    – Jeffrey
    Commented May 18, 2022 at 3:19
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    Updates should not be included in the question. This isn't a helpdesk either. Answering the question with a process and information that does respond to the requirements and queries raised, is absolutely appropriate.
    – Nij
    Commented May 18, 2022 at 8:00
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    In this case, I considered it better to answer the question than to edit the question, since the update is in fact an answer.
    – juhist
    Commented May 18, 2022 at 8:46
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    @Jeffrey - And changing the question after the fact to suit some new criteria usually F's up all of the answers which have been posted. If those who have answered have answered the question in good faith, the OP should create a new question to cover the new subject and not change the gist of the question to suit their needs. If the question is unanswered, then changing the question is still fair game (in my approximation).
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented May 18, 2022 at 12:19
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Waiting before the oil change of any machine is a compromise between:

  • draining too hot oil (unsafe)
  • failing to drain the wear products with the oil (this is what the break-in is all about, but when left for a while, the particles precipitate on the bottom of the sump) and leaving more residual oil over the machine parts (hotter oil flows better).

The "unsafe" part can be mitigated by setting up your workplace in a reasonable way.

I am yet to see a plastic container that cannot withstand a momentarily heating up to 100-120C, even if the nameplate says 85C (or whatever). There are simply no plastics that much sensitive in production these days.

Most engines (even air cooled) are not designed to reach an oil temperature much higher, because one will need a higher grade (expensive) oil to run the engine. High-performance motorcycles are different, but we talk about a generator here and they are similar to either lower-class motorcycle engines or to tractor engines.

This is why one can safely assume 100C as an upper limit of the oil temperature, reachable only under heavy load and high ambient temperatures. Still unsafe for your skin, but manageable and absolutely safe for whatever container you happen to use.

Use gloves. Even if the hot oil reaches your hands, the heat exchange will be somewhat limited and you could quickly remove the glove.

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