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Thermo-Bob Coolant Thermostat

  • Dennis Urban
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2 years 11 months ago #9502 by Dennis Urban
Dennis Urban created the topic: Thermo-Bob Coolant Thermostat
To all the Thermo-Bob users: How is your system working? Have you had any problems?

This is how Thermo-Bob works:

[url=http://]http://www.watt-man.com/uploads/TB_FAQ.pdf[/url]

This is the Thermo-Bob that fits a Rotax 912:

[url=http://]http://shop.watt-man.com/Thermo-Bob-1-Universal-Kit-with-One-Inch-Fittings-TB1.htm[/url]

Thanks,
Dennis

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2 years 11 months ago #9503 by Kevin Machniak
Kevin Machniak replied the topic: Thermo-Bob Coolant Thermostat
Hi,
I have been using the thermobob for two years now . It has worked great. 912 uls in a kitfox5.

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2 years 11 months ago #9507 by Mike Goodrich
Mike Goodrich replied the topic: Thermo-Bob Coolant Thermostat
I haven't used thermo bob but made my own system up using a bypass thermostat from a Land Rover (lightweight ABS plastic housing, 25$ on ebay). It ran very well and I coupled it up to a Mocal oil/water heat exchanger so that when cold the water heated the oil and when hot it cooled the oil. On a 0°C (32°F) day I could get the oil temperature up to 50°C (122°F) in 3 to 4 minutes and in flight the water temperature was a constant 90°C (194°F) and the oil temperature was a constant 100°C (212°F) for all flight conditions.

I have since sold that machine (autogyro) and was in the process of making up the same system for my new autogyro when it occurred to me that my design had a serious flaw that is probably also in the thermo bob design.

If/when the thermostat fails it fails in the cold position which shuts off the radiator and leads to a very rapid overheating of the engine. This isn't a big problem for a motor bike (for which thermo bob was designed) you simply stop by the side of the road but could be a very big issue for an aircraft.

Do you really want the security of your flight to hang on a 12$ (thermo bob price) thermostat????

Mike G

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  • Dennis Urban
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2 years 11 months ago #9515 by Dennis Urban
Dennis Urban replied the topic: Thermo-Bob Coolant Thermostat
Mike, I like your oil to water heat exchanger idea as well as the full bypass thermostat. I have a couple questions.

Did you find a Land Rover full bypass thermostat that fits our 1” hoses (if so, do you recall the part number), or did you make adapters? Do you recall which Mocal you used, and how much it weighed? I’ve looked at them now that you told me they exist, and I think the smallest would work for us, but could not find how much it weighs. Where in the system did you mount the Mocal, was it in series with an air-based oil cooler? Or was the Mocal the only oil cooler you used? Any info you can provide will be appreciated.

You are right that if any thermostat fails in the closed position, it will rapidly overheat the engine. On the other hand, how likely is this, and does this overweigh the consequences of running an engine too cold over and over again. I don’t hear of many automobiles failing this way. Perhaps the answer might be to go back to taping over the radiator or putting in adjustable cowl flaps to control air over the radiator - if this concern overweighs the anticipated benefits.

I think, for me, I will use some type of bypass thermostat (and take my chances). Right now I use Thermo-Bob but am worried about hot summer days and the permanent 3/8” bypass line. If I have problems, I will consider switching to a full bypass like the Land Rover or BMW (but I have not found any for 1” hoses and I don’t like lots of adapters due to lower reliability).

I do like your idea of how to heat the oil more quickly and want to pursue this further.

Dennis

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2 years 11 months ago #9529 by Mike Goodrich
Mike Goodrich replied the topic: Thermo-Bob Coolant Thermostat
Denis
The land rover thermostat I used was like this one,
www.ebay.com.au/itm/NEW-LAND-ROVER-DISCO...100990-/320863542343
it might be the same but I can't remember the number. Mine was an 85°C thermostat and I found it too cold so still had to blank off the rad a bit. I might try a 92°C thermostat next time. It had 1.25" (32mm) connections unfortunately and I used silicon hoses from a race car supplier, they include things like reducers but I've just realised that if you slip a 11/4" silicon hose over a 1" hose over a 1" piece of aluminium tube a single hose clip would seal the three pieces together. It would be shorter in length (if you have a space problem) and save having to buy reducers (if, like me, you're cheap). I'm going to try it on my next installation.
I used a Laminova heat exchanger, they're made in Norway and I think Mocal use the Laminova cooler matrix in the heat exchanger they supply. I used a Laminova C43 182 (182 is the length in mm) that I bought direct from them. I am in discussions with Laminova at the moment for my new installation and they recommend a C45 182 which is a lightweight version for the 912/914 Rotax.
You can get these with 1"or 11/4" water connections, my first installation used 11/4" because I was concerned about the extra pressure drop I was adding to the water cooling circuit but Laminova tell me that most 912/914 installations are using 1" connections so I'll go that way for my new installation.
You must specify that you want a "plug" (part number TB00176) in the heat exchanger.
You can order with just about any oil connection imaginable and it weighs 722 grams.
You don't need an air based oil cooler the heat exchanger does the job but if your water radiator is at its limit this might be a problem because the water rad now has to dissipate the heat from the oil as well as the cylinder heads.
I mounted the "Mocal" in the suction line to the water pump.

Here is an alternative made by a French company that includes the heat exchanger, thermostat and 1" connections.
loravia.com/shop/fr/-nouveautes-/echange...e-thermostat-3-voies

It weighs 1.1 kg, it's 576 euros which isn't bad but I prefer to engineer my own solutions (and I'm cheap).

I agree that thermostats don't fail very often but then neither do ignition systems and we want a double ignition system in our aero engines. It's a personal decision, each owner has to decide for himself.

I'm looking into Motorad "fail safe" thermostats, they might be a good solution but it means I have to make a by pass thermostat housing.

Good luck

Mike G

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2 years 11 months ago #9531 by Rob Seaton
Rob Seaton replied the topic: Thermo-Bob Coolant Thermostat
Mike,
I just have to speak up before others get the wrong idea idea.
you are using a coolant thermostat so have increased your failure points, then you are using a heat exchanger that cools the oil via the coolant.
- so now if you have a coolant system leak your oil will have no cooling.
The Rotax was certified with fins on the cylinders as it is a "get-home" redundant feature; if the coolant system fails you can operate at a lower power setting and the engine will run for hours. If you have a coolant failure with your system you will very quickly lose the engine due to oil over-temp.
I do not recommend installing a coolant thermostat. It is not necessary.
For sure do not use the Laminova heat exchanger.
Rob

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2 years 11 months ago #9537 by Mike Goodrich
Mike Goodrich replied the topic: Thermo-Bob Coolant Thermostat
Rob
As I said every owner has to make his own decision based on his operating practice.
I fly an autogyro, I'm never higher than 500' and can land within a few yards so any problems I have with the cooling system can be resolved by landing straight away. Also I cruise at low speeds with low power settings so getting the engine up to temperature is my biggest problem especially during winter.

That being said I agree with you, a thermostat is a another potential failure point and (as I said earlier) they don't fail safe, making their use even more debatable. I probably would not install one in a aeroplane that didn't have the stol possibility of my gyro or if I was flying over water. I'd install a thermostatically controlled flap on the water and oil coolers with a manual overide to force the flap open if necessary. I'd do it on my gyro but it wouldn't solve the problem of oil warm-up time in winter.

You raise an interesting point about the use of the Laminova/Mocal heat exchanger, how long can a 912/914 run without water?

Thanks for your input, I always appreciate your comments on the forum
Mike G

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2 years 11 months ago #9538 by Rob Seaton
Rob Seaton replied the topic: Thermo-Bob Coolant Thermostat
sounds like you have it under control Mike, certainly a different circumstance with the gyro.
The engine should run for several hours with no coolant, at low power settings. I have heard of one aircraft that ran for 4+hrs as he had to make it over inhospitable terrain before landing at the nearest airport. I will ask the story-teller for more details...
In some aircraft such as the Diamond DA20-A1 the engine is running so hot it would not last long without coolant.
The trick is to not go to full power, like a take off, as this is when the combustion gasses will blow out between the cylinder and now-soft/warped cylinder head; the result being a loss of power and possibly blow-torching hoses. The most common area for this blow-torch effect is in the pushrod tube area.

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  • Dennis Urban
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2 years 11 months ago #9546 by Dennis Urban
Dennis Urban replied the topic: Thermo-Bob Coolant Thermostat
A lot to think about! I appreciate the good advice.

There is a problem I am trying to resolve, or maybe I am making too much of the engine temperatures? With no thermostats, when I start the engine in autumn, not even winter:

1) I can idle for 10 minutes and the oil temps won’t even move up to 120 while the oil pressure remains around 75 psi. Coolant does begin to come up to 120 during that warmup time. Then, with these readings, I take off. Is that ok?

2) During flight, the oil and coolant temps might reach 170. During the dead of winter, I expect they are likely to be lower. Is this ok?

My grass strip does not have electric power. And I don’t really want to go through an elaborate pre-flight heating of the engine with external heaters every time I fly. Am I wrong to not do this?

My questions are obvious.

1) Aren’t these low temps both during takeoff and during flight going to have an accumulative negative effect? Or should I just change the oil more often?

2) For that matter, during extreme cold, does even starting without heating the engine damage it?

3) Will I be able to tape over the cooler and radiator enough during winter to keep temperatures high enough during flight, and not overheat during normal air temperature fluctuations? Based on the reliability discussion, I am leaning this way.

Thinking about what you say, I might permanently remove all the thermostats after winter, and leave them off. You’ve all made a very good point about this being an airplane, and what if either thermostat (oil or coolant) fails in a way that bypasses the cooler/radiatior and heat gets out of control, especially with no airport nearby. But I still like the idea of an oil to water heat exchanger in addition to, not in replacement of, the oil cooler, strictly to heat the oil more quickly during warmup, which does not occur during idle even with a thermostat.

Having already said I lean toward removal of both thermostats, I still want to discuss them. With the Thermo-Bob, coolant temperatures come up nicely to 180 in a 10 minute warmup. It would be really nice to bring the oil temps up as well. The PermaCool does raise inflight temperatures, and seems to do a great job with the new Stant 195 degree in lieu of the factory 170 waxstat, but does not seem to raise oil temps during warmup (too much oil mass, not enough heat, at idle).

And, oh by the way, I contacted PermaCool to find out which manufacturer waxstat they are using, but they refused to tell me. So I replaced the 170 with a 195 (a Stant I mentioned earlier – but I plan on replacing that Stant with what follows). I looked at all the engineering drawings I could find online and found it is most likely a MotoRad. The 195 degree MotoRad P7200-195 has identical dimensions right down to the casting marks on the bottom of the waxstat. And, in support of the loss of reliability argument, I put the 170 (just removed) and MotoRad 195 in boiling water. The 170 failed! It would not expand at all. But the new 195 worked fine. But this makes me wonder what would happen in flight if the Permacool were letting this much oil bypass the cooler all the time. When did the 170 fail and why did I not notice that inflight. Temps inflight with the 170 had been very low, as expected.

I still am thinking about reliability overall, while keeping operating temps in mind.

Thanks,
Dennis

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  • Roger Lee
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2 years 11 months ago #9547 by Roger Lee
Roger Lee replied the topic: Thermo-Bob Coolant Thermostat
I had a pilot with an Aztec with over heating problems. He had the heat exchange. I had him put everything back to a more stock configuration. Over heating disappeared. It's the only heat exchanger I have seen on a Rotax. I would think using only a heat exchanger one temp is dependent on the other and maybe not the best option for separate temp help in case you loose a coolant hose.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell

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2 years 11 months ago #9551 by Mike Goodrich
Mike Goodrich replied the topic: Thermo-Bob Coolant Thermostat
I've just found out that the thermostat/heat exchanger offered by the French company that I mentioned in an earlier post comes from a German company Silent Hectik ( www.silent-hektik.com/UL_912_Thermo.htm ) that also offer other mods for Rotax engines.

As I've already said and Roger and Rob have warned "Buyer beware".

Mike G

PS Roger, an Aztec with a Rotax??? We can't be thinking about the same Aztec

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2 years 11 months ago #9556 by James Ott
James Ott replied the topic: Thermo-Bob Coolant Thermostat
FWIW, I have a 912s in a Kitfox 7. The oil cooler system has a thermostat, the coolant system does not. The oil cooler is mounted right in front of the coolant radiator. For winter flying I just put a 2" strip of aluminum tape clear across the oil cooler, which also blocks air flow to the radiator. I just did this two weeks ago and got a temperature comparison: BEFORE-oil temp in 5000 rpm cruise was 175 F. AFTER-oil temp was 225 F. In both cases the cyl head/coolant temp was about 5 degrees below the oil temp. A pretty simple way to get temps up into a good operating range.

Warm-up time to 120 F after cold start at 40 F is about 5-7 minutes.

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  • Dennis Urban
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2 years 11 months ago #9557 by Dennis Urban
Dennis Urban replied the topic: Thermo-Bob Coolant Thermostat
Roger, Rob, Mike and all,
You have convinced me. After thinking about what all of you are saying, I do believe it is an unacceptable critical reduction in reliability to Rotax 912 aircraft engines to add (“currently available”) thermostats to either oil or coolant systems. The problem, as you put it, is that if the thermostats fail to send adequate hot liquid to a cooler, destructive high temperatures will occur.

I also want to thank everyone who is running thermostats for your responses as well. Very helpful.

Just for discussion:

Perhaps on the coolant side this might not do damage if quickly recognized and countered by reduction to very low power until an airport can be reached. I say this but I don’t know if this is the case, if that is what would happen, e.g. if the ThermoBob were to only route all coolant via the 3/8” bypass while blocking off the cooler. The resultant hot coolant might raise pressure enough to blow by the pressure cap even at low power settings until it becomes a case of no coolant, which Rotax seems able to handle even if it is not something we want to have happen.

But on the oil side, both Thermostasis and PermaCool continue to send some oil through the cooler in all configurations and temperatures (and failure modes). The questions here might be: is this enough cooling to proceed on reduced power (as with the loss of coolant) to an airport, and how quickly and how high would oil temperatures rise with 90% reduction of oil going through the heat exchanger?

Wouldn’t our engines benefit from truly reliable (or fail-safe – which needs to be defined based on our Rotax engine needs) thermostats. I do not know what is inside the Thermostasis, but I do know an automotive waxstat is inside the PermaCool as well as the Thermo-Bob. I suspect all coolant thermostats have a waxstats. I have not seen very many automotive thermostats fail, but my stock Permacool waxstat failed. Did I make this happen by placing the room temperature 170 degree waxstat directly into boiling water? My oil in summer will get hotter than 212, but it would do so more gradually than when I tossed that room temperature waxstat into the boiling water.

In absence of knowing if any ‘truly reliable (or fail-safe)’ thermostats exist, I think it is better to tape over heat exchangers. What I would like to know is what is happening inside the engine when running at too-cold take-off power or during sustained too-cold cruise – and this goes for both coolant and oil. I would also like to know how many people are running thermostats and what have been their experiences; how many have had failures and what happened.

In summary, after all the time I spent researching thermostats, and the money spent on parts and miscellaneous fittings and new hoses, as well as effort to cut into lines and neatly configure and locate automatic temperature controls on my Rotax (and being proud of the results), I am now concluding that I need to tear all that crap off and go to taping over heat exchangers. Oh well. What is safest = what is best.

Thanks for the help,
Dennis Urban

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2 years 11 months ago #9559 by James Ott
James Ott replied the topic: Thermo-Bob Coolant Thermostat
I am somewhat surprised at everyone's assumption that wax bulb type thermostats fail closed. My experience with cars is that when they fail, the coolant emperature stays low as if there was no thermostat in the system. I guess it depends upon how the thermostic valve is designed. I would venture to say that the valve is designed so that failure routes full coolant flow thru the cooler. It is the only design method that makes any sense to me. Another point; I believe the Kitfox factory installs an oil thermostat in all their factory built SLSA's, but not a water thermostat.

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2 years 11 months ago #9563 by Mike Goodrich
Mike Goodrich replied the topic: Thermo-Bob Coolant Thermostat
James
Motorad (makers of thermostats) seem to think that most thermostats fail in the closed (or by-passed) position.

That being said this is obviously an advert for their system so it may be biased.

Most commercially available thermostats work on the principle that the wax pellet expands with temperature and opens a valve to the radiator or closes a valve to the by-pass (or both) and the failure mode is usually the wax leaking out of its container leaving the spring to push the valve(s) into their cold position which is radiator shut off and by-pass (if there is one ) open.

www.uniquecarsandparts.com.au/images/how.../Water_Cooling_8.jpg

I know of many Rotax installations in Europe with the Permacool oil thermostats installed by the aircraft manufacturer, but that doesn't mean it's a good idea. In my opinion many of them just copy the others without any serious engineering of what they’re doing.
Permacool say
“At temperatures below 180°F the valve is open, with 90% of the oil by-passing the cooler. The remaining 10% of the oil flows through the cooler, “
That means that if the thermostat fails in the by-pass position at least 10% of the flow is still going through the cooler so provided you weren’t pushing the engine that might get you home or to a safe landing spot. That’s probably similar to Rob’s scenario where you lose all the coolant and can keep flying at low power settings. I suppose a lot would depend on how big your oil cooler was and the OAT.

Thermostasis seem to have studied the Rotax installation a bit more than Permacool

thermostasis.com/includes/templates/ther...thermostasis-faq.pdf

There’s also some interesting info here:

thermostasis.com/includes/templates/ther...leshooting-rotax.pdf

and that could be reassuring although I haven’t found any figures about how much oil continues to flow through the cooler when the thermostat fails.
Also they raise an important point that I don’t think we”ve talked about here and that is that the oil temperature we measure on the Rotax is the lowest temperature in the oil circuit, it is the temperature after the cooler and before the oil goes into the engine. The oil reservoir is probably nearly the hottest part of the oil circuit and that is where we want the water in the oil to boil off. So we don’t have to see 100°C (212°F) on the gauge in the cockpit. The problem is nobody has an oil reservoir gauge and I don’t know if anybody has ever measured the oil temperature increase as it flows through the engine.

Blanking off the oil and water coolant radiator is the tried and tested method, but on a very cold day you still have to wait for ages before the engine manages to heat up all the oil and water in both circuits. If you operate from a large airfield and know you will spend 10 minutes or more taxying around and waiting then that’s not a big deal, you’ve got the time, but if (like me) your hanger is 100 yards from the runway and there’s never anybody else in the circuit it’s a real pain to sit there burning fuel just to heat up the oil, not to mention it’s often bl..dy cold just sitting there in an open cockpit. Also I’ve found that if I’m just doing circuits on a cold day the oil never gets above 70° even with the oil rad blanked off completely.
The down side of blanking the rad off is that on a cold morning doing circuits on my own the oil won’t be hot enough and in the afternoon when the sun gets up and I take a heavy passenger up for a low level flight the temperature gets too high because I’ve forgotten (yes it happens) to take some of the tape off the radiator.

There doesn’t seem to be an ideal solution on the market but overall I’d say the thermostasis seems to be the supplier who has considered the problem the most and I’d say it, or the Permacool, was a pretty low risk installation. Added to that there are a lot of Permacool installations in Europe and even though that doesn’t mean it’s a good idea we are not hearing of many (any??) failures.

I’m still looking into the Silent Hectik by-pass thermostat/heat exchanger solution and I've been trying to get some information from Silent Hectic in Germany and their sales organisation in France, it's like pulling teeth trying to get serious engineering info and all you get is "I assure you it works well, don't worry". When I asked about the failure mode of the thermostat the answer was "we buy very high quality thermostats from Japan and they don't fail". I’ve asked for photos of installations with 912/914s and no reply. When I say that many of these suppliers don’t really know what they’re doing this is a typical example of the sales hype that we have to deal with.
I think their solution has merit for what I want to do (that’s not necessarily a good idea for others to do) but I need to get more info to really engineer the solution.

Mike G
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2 years 11 months ago #9566 by Rob Seaton
Rob Seaton replied the topic: Thermo-Bob Coolant Thermostat
good info Mike!

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  • Dennis Urban
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2 years 11 months ago #9604 by Dennis Urban
Dennis Urban replied the topic: Thermo-Bob Coolant Thermostat
A couple things to keep in mind. With the Permacool, there are two chambers with two holes each, one is marked “cold” and the other “hot”. The waxstat resides inside the “hot” chamber and is normally collapsed, allowing oil to flow from the “hot” side to the “cold” side until it heats up. When the waxstat is heated, it expands to block off the “cold” side (forcing all the oil to go through the cooler). You need to look at an installation diagram and think about this – or just hold a Permacool in your hands and look inside, then toss it into boiling water and see what happens. All their percentages are just estimates and based on the path of least resistance being the more direct path from tank to oil pump (bypassing the cooler). I don’t think the actual percent number means much. But I do hope and think that the idea of limping home if it fails might work.

PermaCool works differently than the automotive video above (thanks Mike). We rely on the expansion of the waxstat, which I do not know if it is fail safe to both force oil through the cooler and to keep it flowing there (purely by action inside the waxstat bulb). If the waxstat fails to expand, it allows maximum flow between “hot” and “cold” chambers (oil bypassing the cooler as resistance allows). When fully hot, it blocks off the “cold” side, whereas in automotive use, it opens to the radiator.

However, in automotive use, the MotoRad is fail safe because of the spring clips on the side of the spring in the housing (see the video). In our PermaCool application, the MotoRad housing, spring, and clips are cut away and not present. Here is a picture (if my attachment makes it) of the stock waxstat removed from my PermaCool (on the right) and the replacement 195 degree MotoRad P7200-195 on the left (cut out of its housing). Note, even though PermaCool says their ‘system’ is 180 degree, to achieve this they use a 170 degree waxstat. So, I am thinking that using a 195 is a 25 degree F difference (from the 180 degree PermaCool ‘system’ to a 205).


And, as I pointed out earlier, when I tossed both of these waxstats into boiling water, only the new one actuated by expanding. The stock one just sat there. I assume it has failed and done so in the closed position, meaning that inside the PermaCool it would always allow oil to pass between the “hot” and “cold” chambers in maximum heat-up-the-oil mode. If this happened inside a car, I don't see how it would make the fail safe clips work, unless it first expanded too much before failing to expand at all. Perhaps it failed after I removed it, as I never saw high enough oil temps despite this. So I assume this is what happened. But I admit to being unsure.


Dennis

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2 years 1 month ago #11545 by Trevor Doig
Trevor Doig replied the topic: Coolant and oil thermostats
After agonising for 5 years over very low oil and cylinder head temperatures, highlighted by a flight through our snow covered New Zealand mountains, I have fitted both oil and engine coolant thermostats to my 80hp Rotax. I used the Thermostasis for the oil and ThermoBob for the coolant.

Both work really well and my engine runs much smoother.
Trevor Doig, New Zealand

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