Normal operating temperature for the engine oil is about 180-200°. Normal operating temperature for the head and piston is dependent on how much debris is packed into the cooling fins under the tins covering the head and cylinder, how many fins are broken off the fan portion of the flywheel that supplies the cooling air, the fuel/air mix ratio, and the load on the engine at a specific point in time. Also, the temperature is not consistent across the cylinder head as the cool air/fuel mix enters at the intake valve, and the mix is still burning as it exits the exhaust valve right beside the intake valve.

I'm sure that someone has the info that you want, but the temperature depends on the above conditions and can vary by several degrees above and below any specific "normal" temperature. An engine working hard is not going to achieve the same operating temperature at below zero winter temperatures that an engine doing moderate work at summertime ambient temps of 85°+ will achieve.

The best move is to ensure the cooling fins are all present and clear of debris, the carb is supplying the correct fuel/air mix, and the timing is correct. I think that mouse nests in the cooling fins cause more overheated engines than anything else, and a combination of lean fuel/air mix and early ignition timing cause most burned pistons.

If the maintenance is done properly, small engines rarely overheat.

 

The one thing unmentioned(I think) is to keep the rpms up. The cooling air flow caused by the flywheel blower diminishes a lot with lower rpms. The flow can be reduced too much, causing slow airflow, and allowing the cylinder and head to overheat. IOW, don't run for any significant length of time at an idle. That is a more likely condition than overheat at normal rpms.
tom

 

I don't think so. Unless it's pulling a load at idle, it's just sipping gas, and the combustion of the gas is where most of the heat comes from.

As an example, I ran in the cam on a rebuilt 327 SBC at 2500 rpm sitting on the garage floor. Fuel consumption was 1.5 gph. A week later, the engine was in my Chevy Van pulling an 18' travel trailer across the prairies against a head wind. Engine speed was about 1500 rpm and fuel consumption was over 9 gph. Should have downshifted to second gear to put more horses to work! (Youth and inexperience!)

Considering that about 1/3 of the heat from combustion goes out the exhaust, about 1/3 gets rejected by the cooling system, and only about 1/3 actually does the work intended, I wouldn't worry about a small engine that is only sipping fuel at idle overheating when almost 1/3 of maximum cooling air flow is available.

 

You're both right... although an engine isn't producing a lot of heat at idle, oil distribution throughout suffers somewhat, especially on a splash-lubricated engine. Extended idle times lead to uneven heat and therefore localized variations in oil and cylinder temps, and thus can cause issues.

Saying it badly... imagine an air-cooled splash-lubed engine at extended low idle. Splash isn't really reaching the whole cylinder wall, whether horizontal or vertical, so the un-lubricated portion tends to warm up excessively, which can lead to the oil gooping up (technical term) around the rings. Which will eventually cause them to stick/carbon up in the ring grooves, resulting in excessive cylinder wear or damage. Pressure lubed engines also, but less so as oil is flung 360* from the crank and con rod bearings. Valve stems and guides may also suffer.

Lack of air flow is definitely more of an issue under load, yes. But the role of oil in cooling and heat distribution is vital, in addition to lubrication. If the engine is idling excessively, you're risking hurting it two ways.

Ballz to the wall for duty keeps temperatures and lubrication even throughout the engine when it's working, limited warm-up and cool down idle times ftw.

 

Good points! The tendency is to not remember the important part that oil plays in keeping the engine innards cooled.

 

Just generally ,I'm not that familiar with your Kohler engine, the hottest part of an air-cooled engine is the area of the exhaust valve. The intake area is kept a bit cooler by the gas mixture each stroke I started using the "infra-red" temp hand tool when they came out, I was working on aircraft at the time ,and a missing cylinder could be identified by cooler running temp. It's important to test the same area each time. On the old "flathead" engines the head close to the exhaust ,but not the exhaust port or the thin exhaust pipe. on the OHV engines the area near the spark would work. The "air/fuel mixture" ( leaner hotter rich cooler) seems to have a more and faster effect ,on air cooled vs liquid cooled, with the head temp leading oil temp.

 

There's quite a big difference between water cooled Auto engines with circulating coolant even at low RPMs vs air-cooled small engines with a flywheel and a shroud system that actually blows air to cool it. Look at all the motorcycles that were air-cooled but have extensive fins on each cylinder to dissipate the heat because they do not have the cooling fins from the flywheel and the shroud system. Of course many of those are getting airflow when they're moving and the temperature will increase when they're sitting in idle without airflow from moving down the road. All these little engines have for the most part is the movement of the flywheel so it's much more critical that the RPMs be kept up to keep the airflow strong.
It's also important said the transmission cooling fan can stay at a speed to move as much air as it can to cool the transmission also. And what some people forget is when they turn off the blades and slow down there engine when they're done cutting the grass to drive it back three or four hundred feet to their barn or garage Etc is that they still have a pretty good load on the machine. It's still moving about 600 pounds of weight but now the engine speed is at a third of what it was.
I don't lower the engine speed on my equipment until 10 to 15 seconds before I turn off the key.

 

I, too, am very interested in the engine’s operating temperature. The range that I am most familiar with is 180F – 210F. I previously owned a Porsche 911SC which was air/oil cooled. The oil system held 13 quarts! The engine-driven cooling fan provided massive amounts of air to cool the cylinder heads as well as the engine-mounted oil cooler. Still, in our hot Alabama summers, the oil temperatures rose to 210 F is traffic but quickly settled to a more comfortable 180F on the road.

After purchasing an Ariens Zoom 42 zero turn mower, I ordered and installed an oil temperature gauge. This machine, at least in the hot summer time, almost always maintained 180F. Of course, the ambient temperature had a significant impact of the realized oil temperature – still, in 90F+ weather, it held to 180F. In opposition to what the operator’s manual stated, I never ran the unit in the full throttle position (3,600 rpm); instead, I operated it at 2,400 rpm +/- 100. It always did a great job of cutting and our yard looked great.

... And what some people forget is when they turn off the blades and slow down there engine when they're done cutting the grass to drive it back three or four hundred feet to their barn or garage Etc is that they still have a pretty good load on the machine. It's still moving about 600 pounds of weight but now the engine speed is at a third of what it was. I don't lower the engine speed on my equipment until 10 to 15 seconds before I turn off the key.

I am in TOTAL agreement with TobyU here! One of the things that surprised me the most was NOT reducing the speed of the engine after stopping the mower deck. My previous practice (for the first five years of ownership) was immediately take the engine's speed down to 1,800 rpm for my drive to the shed and for a two minute cooldown after completing my days’ mowing. The oil temperature may have dropped just enough to be noticed on the gauge. One afternoon in the middle of mowing, my wife flagged me down to tell me something. I disengaged the mower blades but did not touch the engine’s throttle. When I came back a few minutes later, the oil temperature had dropped 15F. After that experience, I never touched the throttle again after disengaging the mower blades – this was also true for the engine’s new and improved cooldown procedure. I concluded that reducing the load alone had minimal effect on the realized oil temperatures – to maximize the cooldown, the load removal must be in conjunction with higher air cooling flow rates (which, of course, are provided by higher engine speeds). My new methodology still maintained the cooldown rpms well below 3000 and lowered the engine’s overall temperature. Just like TobyU again, I reduce the rpms only just before shutting the mower's engine down for the day.

Now, to my final (on going) experiment. Last year I picked up two old Gravely Two Wheel Tractors (a 1957 and 1974 model). After getting these units running, I was really intrigued by how cool these engines ran. The old Gravelys are air cooled by a fan driven engine’s crankshaft via a belt. Three air shrouds direct the cooling air around the cylinder and over the cylinder head. These machines are extreme workhorses and EVEN operating these machines in excess of 90F outside temperature, they never got hot. I can easy rest my hand on the oil filter and it is quite comfortable – warm, but by no means hot. I have pulled out my infrared temperature device and shot a laser at many spots on the machine. Although there is a plethora of different temperature ranges on the engine depending upon where you are measuring, the cylinder’s temperature was at 200F (the cylinder head presumably was hotter). The engine case, just like the oil filter, was warm but not hot… and this is after two hours of running these machines under load. If I remember correctly, the engine is designed to operate at 2,600 – 2,800 rpm… which is significantly lower than today’s mower’s engines.

With all this stated, there is one more vital piece of information that I wish to share. I ALWAYS use synthetic oils in these air-cooled engines regardless of whether it is a Porsche 3.0 liter engine, a 16.5 HP Briggs & Stratton mower engine, or a 5.5 and 7.6 Gravely engine. The synthetics are noted for their ability to deal with the higher engine operating temperatures that are experienced by air cooled engines. There has been some research indicating that engines run cooler with synthetic oils.

In the end, I do not have a definitive answer for your original question. What I can say is that you will see many different temperature readings from different parts of the engine – I have also pondered, at length, at where (and how) the best place to record engine temperatures. To this, I still have no answer… However, I would recommend researching the usage of synthetics is your engine BUT be forewarned there is much debate and many arguments both for and against this practice! There is also much debate regarding the proper viscosity to use - I 'mostly' adhere to the viscosity ratings from the manufacturer but I seek out the synthetic version of that weight.

 

I would add one thought or two.
The airflow coming off the cylinder head/cylinder should not be too hot for you to keep your hand in the airflow.
The cast iron engines of 40-50 years past seemed to have more mass to absorb heat, and in addition, many had the mass of the transmission/gearbox that they were mounted to as another absorber of conducted heat. I *think* the operated at lower rpms than the vertical mower engines, for example, on a push mower. Flat head engines had the whole block to absorb the heat of the exhaust valve, not just the mass of the cylinder head as more modern OHV engines have, and the newer OHV engines mostly (all?) have lighter aluminum.
As far as operating engines at a lower rpm than indicated by the slot/groove in most controls, I won't argue, as I always thought it 'better' to run at less than the highest setting. I was set straight by a co-worker who stated the engines run better when run at the set rpm. I have not measured temperatures, but have touched the oil filter after mowing for several hours, and it was not too hot to touch, but hotter than comfortable. I will continue to run at the suggested setting as both CV's I have are over 20 now, one old enough to be served at any bar. No new bits, no rings, no cam, no lifters, nada except an occasional oil and filter change, and air filters, spark plugs on an occasional schedule. IOW, run the beans out of it, 3000 rpms or what ever, and only slow down once on the apron in front of the garage.
I do place my hand under the cylinder to feel the cooling air temperature and flow to reassure myself it is not too hot, and flows freely. I do not idle either engine very much, only to pick up fallen limbs, etc.
tom

 

And that's how you get 20 years plus out of them. Just fixed two Briggs intek twins last week 2003 and 2004 model. Both had slipped exhaust valve guide from overheating. One had excessive debris and grime on that head but the other wasn't that bad. Both owners admitted that they often lower the RPMs or don't mow all the way to the full position notch.
I have had several that were running fine until they were shut off or ran out of gas. Then they would not restart because the guide slipped. Worst thing you can do is get one hot and then slow it down or get one hot and let it run out of fuel. It leans out and gets really hot for the last few seconds.

 

A good cooled engine should be able to be touched on the shrouding by hand if u cant touch it chances you got a clog somewheres

 

Or a possible fuel issue

 

No one has mentioned the 'spit test'

You can remove the spark plug and compare to your engine repair book's 'spark plug condition' pictures, that might tell ya something, especially if your truly concerned of severe overheating. Most lawnmowers that are actually overheating, will do so rapidly, from a cold start, it normally takes a little while to get the thing warmed up. If yours is quickly 'smoking in places it shouldn't be smoking', that might indicate severe overheating--but could be 'just oil leakage'. If the muffler outlet looks 'close to white, not dark beige or darker--you might be overheating, if you see any kind of sparks flying out of the muffler, your concerns are probably real. [although a rusted/worn out muffler could give similar symptoms when no real overheating is occurring, if your setting the field your mowing on fire---your probably overheating!] hahahaha

And of course there is the 'low power', hard to start-hot type condition, that might cause you to think the thing is overheating, and it may well be overheating. The valve's 'job performance' contributes a great deal to intense transfer of heat, when worn & leaky the valve's 'job performance' suffers-- all sorts of hot symptoms appear, there again if your seeing sparks flying out the muffler, or a backfire out the muffler when ya turn the thing off, your probably running too hot. [your old engine shouldn't backfire out the EXH, but low grade or excessively high grade fuel when used can cause different symptoms that might indicate overheating.] Piston detonation damage is very serious, & sometimes attributed to overheating. Galling & scuffing of the piston skirt & cylinder is oft times attributed to overheating.

A general tune-up + valve job is usually a good way to avoid serious engine troubles, as it gives the mechanic a good look at the overall condition of the unit.

If all you have learned from the experienced hands that have replied in this thread, remember it was me that mentioned the old 'spit test' first. hahahaha

wwxx

 

Well wwxx ill agree with you there from the hot start turnoff and back fire situation as my current in use engine is doing this due to valve damage luckily the replacement is almost rdy you solved my issue ill have a gander and video for its tear down and everyone can get a feel for gapping L Head style engines and how to check them for new people

 

Surprised how cool they run being air cooled but they can overheat. Running too lean ruins more air cooled engines more than anything Id say, but idk?
One day back yrs ago I was mowing with my old B100 (10hp Kohler K) and it started slowing down considerably? Looked at fuel filter it had fuel flow so kept going, it suddenly lost a lot more power so I stopped. I raised hood and noticed it was very very hot. Pinging hot you know?..lol
Then I saw it!
A big waud of grass sticking out of my breather. I pulled its cover off and it was simply packed all the way around that sucker. Cleaned it good and it ran perfectly fine but I was so worried Id damaged the head or even worse. Telling you that sucker was hot and I was shocked how much grass it had built up inside it, happened so fast? I always keep filters & fins clean but Id mowed a few times that week cause of prior rains made grass really grow like mad so it got away from me. Too busy mowing not maintaining the ol' girl. Its still doing just fine, dont use it much but' being my first horseI kept it.