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Classic K, Coolant Change...
Coolant types, including water-less. Heat
insulation under the tank. Heat on your legs.
Changes when installing the 700 watt alternator.
© Copyright 2023, R. Fleischer
K bike section, #K13
https://bmwmotorcycletech.info/k-coolant.htm
BMW began manufacture of the 1993 K bikes in early 1992, which was unusual for BMW, as normally BMW begins the next calendar year production right after the annual holiday of August. Through Summer, the 1992 production of 1993 model year K bikes had a Bosch 400 watt alternator. Late in 1992, BMW began shipping the K motorcycles with 700 watt Bosch alternators. There were a number of other changes to the motorcycles from near the end of 1992.
If you are planning on installing a 700 watt alternator, which became a stock item from approximately the end of 1992, a good time to do that is at coolant change as you may want to install the later design overflow tank and new longer hose to it. The later overflow tank has the hose nipple to the rear compared to the original ...which gives more room for your overflow hose & wiring that passes by a close frame area. There was a factory service bulletin (SI) on some of this, including the longer hose, 1 meter of which is 17-11-1-152-866. If you have not already closed off the system that vents fuel tank fumes to the crankcase; & installed the overflow cup for the tank at the frame backbone, you should consider doing those modifications at the same time.
The earliest coolant radiators were not as efficient as the later ones. BMW increased the number of cooling fins on its K bike radiators, but this did not happen until the K1100 was in production for quite awhile ...I think the change was in late 1992 ...to 80 fins.
I suggest you consider adding some reflective insulation to reduce engine & radiator heat getting to the fuel, & also reducing the amount of heat thrown back at you, the rider. This means improving the insulating/sealing areas done by BMW under the fuel tank (the K1100 was better than earlier models, but certainly can also do with improvements). Information on doing insulation work, & alternator conversions, etc., is located at various places on the Internet, including the www.IBMWR.org website.
I suggest you install, if missing or badly damaged, reflective aluminized insulation material on the underside of the fuel tank. This is NOT the big thick super-soft fiberglass insulation that is used in home walls & attics. BMW sells an insulation kit for the underside of the fuel tank. You do not have to use BMW's insulating item. You can, at such places as Lowe's, Home Depot, etc., get a roll of Duct Insulation; which is much less expensive than the BMW stuff. Get the type that has a self-adhesive foil & foam. This product is used on heating & air conditioning ducts, & the type I purchased is 12" wide, 1/8" thick. It has a reflective layer. If you can find this stuff thicker than 1/8", DO use it, as it will work even better. The roll I purchased was 15 feet long ...plenty for lots of K bike jobs, & other things around your vehicles & home. There is another product you might be interested in, for areas that need heat reflective tape, but not an insulating tape. This is HVAC industrial aluminum tape, a foil tape. Whatever product(s) you get, be sure the glue/adhesive backing is strong. Otherwise, get a strong adhesive to use, such as SUPERweatherstrip Adhesive, made by 3M and probably Permatex now, this is the yellow stuff mechanics often call Gorilla Snot, but Gorilla Snot is NOT a name on the tube. It is also available in black.
I suggest you insulate the fuel lines too.
It is beyond the purpose of this article for an in-depth discussion of various insulating schemes, installation of the larger alternator, overflow cup, etc.
For excessive heat on your legs, refer to articles on https://ibmwr.org/, and other places, regarding closing off areas and using deflectors, etc. I previously had a long section on the subject here, and decided to remove it.
The subject of which coolant to use has become much more complicated ...and often quite confusing ...due to later model cars using a variety of coolants, & coolant colors; and most owners seem to, very wrongly, just purchase coolant from autoparts stores ...without knowing enough about the coolants and what their vehicle requires. Some coolants (Like GM specified EX-Cool in the USA) are not compatible with each other, & mixing them (by mistake, or, perhaps by not thoroughly cleaning out the old stuff) can result in a gooey & clogging gel in the system. The wrong coolants can damage your water pump. This happens to BMW motorcycle cooling systems. You may have heard of the costly damage to Honda GoldWing bikes due to use of wrong coolant. The BMW Classic K bikes do not have a large reserve for cooling the engine, this is particularly noticeable in hot weather. As I have mentioned earlier in this article, BMW installed a more efficient radiator in late 1992 on the K bikes, but using a wrong coolant can still 'plate' your cooling system, badly too, and result in easy over-heating in hot weather. I will get into coolant selection, and the WHY information, just below.
IMPORTANT! ....>>>using the proper coolant:
For the BMW Classic K bikes, in my opinion, there are several things that are quite important, and using the wrong coolant will have long-term lasting deleterious effects! It really is important to use non-mineral-containing water. You motorcycle will be much better off by you going to the SuperMarket or WalMart & buying some DISTILLED water (I suggest you avoid Purified Water). The type of coolant is even more important.
For the COOLANT, which is often just called Antifreeze, use only one whose formula DOES NOT CONTAIN nitrites, silicates, phosphates, and borates. The proper coolant will not tend to plate-out, nor otherwise damage your cooling system. Plating-out is BAD, and it means that the above named No-No additives have deposited and formed a very thin insulating chemical blanket or plating on the metal parts of the system. You want maximum heat transfer, NOT an internal insulating blanket. Another problem with plating-out, is that those additives are no longer properly doing their original purpose ...which includes anti-foaming, anti-corrosion, etc. When water with a lot of mineral content is used with phosphate additives, the additive reacts with those minerals, which are primarily calcium and magnesium, and make nasty deposits, typically looking and being what is called 'scale'. This is BAD, hard to remove, and is the reason you SHOULD use only distilled water. If not available, use purified water. My strong advice: Use ONLY DISTILLED water! DO NOT USE TAP WATER, particularly not from where tap water is what is called 'hard water'....which means it has lots of minerals.
BMW says ...in some publications, ...and in K owner's manuals, ...that you must not use coolants containing certain ingredients. BMW has not used the same coolants in its cars over the years, as in the motorcycles ...so I suggest you NOT use BMW car coolants.
BMW sells a coolant for its MOTORCYCLES. If you are worried about what coolant to use, you can certainly purchase BMW's MOTORCYCLE coolant, BUT YOU DO NOT HAVE TO. More information later in this article.
Understanding all the various available coolants is not easy. I used to have a much longer treatise on it here. Then I found an older article that was quite good. https://www.machinerylubrication.com/Read/841/coolant-fundamentals. With a quite modest amount of reading and comprehension, you will have the needed facts. Below, I will leave some of my original comments on coolants:
If you have previously mixed coolants of various types for your motorcycle, or used the wrong coolants and have ...or have not ... found gelling, or other problems, my suggestion is to do a very thorough CHEMICAL treatment & flushing of the cooling system, BEFORE installing the proper new coolant mixture. A chemical treatment and flushing will REQUIRE you to run the engine enough to have the thermostat open and then run the engine somewhat longer. Because of that, I suggest, depending on outside temperature, conditions, etc., that you may want to cover the radiator, and go for a ride (a lengthy garage warm-up CAN overheat the exhaust system, etc.). The ride will ensure that the water gets hot enough to open the thermostat more than briefly. Let the chemicals and water heat up to the point the fan comes on ...then continue on a bit. If any overheating, simply uncover the radiator. This is easier to accomplish if your bike has the factory temperature gauge, but it is not a must. I recommend this same FULL process for each of at least TWO plain water flushings after the chemical treatment. Use de-mineralized or distilled water, even for the flushings, since you will be heating the system between treatment and flushings. Yes, this process takes time and effort such as lifting the fuel tank upwards for access, and draining the system, each time. Do not forget to clean the overflow tank chemically, and flush it too. If you do this chemical treatment and flushing correctly, and use the correct coolant AND use distilled water to create the proper mixture, you will likely never have to do this full procedure again, and your cooling system should work almost like brand-new, assuming you use the proper coolant mixture from then on. Coolant mixture, here, means the proper concentrated coolant diluted to the proper percentage with distilled water. It is OK to purchase the proper coolant PRE-diluted, as the commercial proper coolants do use distilled or purified water.
Failure to do it fully & properly, on a bike that has not been previously serviced correctly, will result in less cooling ability, more wear on the fan motor, etc. Whether ...or not ...your bike has had cooling problems ...I advise you to use my full procedure, and thereby get your cooling system well cleaned-out.
There are reasons to NOT to want silicates. Silicates (and/or borates) ...while they have worthwhile anti-corrosion benefits ...are abrasive when deposited or plated-out. The primary abrasiveness is not from them IN SOLUTION, but when they plate-out onto sealing surfaces. This was/is particularly important for such as Honda's Gold Wing, with ALUMINUM sealing parts in the water pump ...the silicates plated-out, the plating IS an abrasive coating, and the sealing was injured, eventually badly. Silicates tend to gel and settle in the coolest parts of the cooling system. This can cause radiator plugging and overheating. This is above and beyond the plating and abrasive effects. One of the other problems with silicates (their primary purpose is anti-corrosion) is that they are not stabilized very well, so coolant life is rather limited ....one of the reasons that long life coolants have no silicates.
Some coolants contain borates ...these can cause pitting corrosion in hot areas like some parts of the cylinder head. BMW does NOT want you to have silicates and borates in your cooling system.
The wrong coolant PLATES the inside of the cooling system; and, in effect, puts a very thin blanket on the internal metal, making the cooling system less efficient. For your Classic K bike, the plating makes the fan motor come on more often, can even let hot spots develop in the valves area (and maybe around the upper areas of the cylinders ....). If bad enough, the entire system, including the radiator, does not cool nearly as well as it did originally. You really need all the cooling you can get for many riding situations. BMW had cooling problems with the early K bikes, and now you know why; and, the increased radiator surface area (more fins) was only 'some' help.
There are certain ingredients that are needed in a coolant, and the most important ones are anti-corrosives or corrosion suppressors, and anti-foaming/aeration types. Corrosion does more than just eat away at metals ...those particles have to go someplace ...so corrosion, while liberating particles, lets such become abrasive everyplace in the system. There are three types of corrosion:
(1) cavitation caused by tiny air bubbles (which have MONSTROUS forces when they implode);
(2) pitting caused by aeration;
(3) galvanic/electrolysis from dissimilar metals.
I suggest you drain and flush the old system rather well. It is a bit of a bother to drain and flush correctly ...particularly since the thermostat is closed unless the coolant is hot. Use a commercial (autoparts stores) system cleaner if you think there is any chance of plating or other deposits or filth. Clean the system really REALLY well after any commercial chemical cleaner is used. Use only distilled water (de-mineralized water if you absolutely have to) when mixing the coolant mixture from concentrate, if you intend to purchase a concentrate. It is often CHEAPER, and BETTER to purchase a concentrated coolant).
Be sure the cap and its rubber gaskets are OK. Any leaking of the cap gaskets WILL cause problems. Hoses usually last a very long time, as does the thermostat, but DO A GOOD INSPECTION. You really do NOT want to have hose problems while riding.
The equivalent or better coolant than BMW's, at a much better price (especially when on sale!), is specified in this article, as Prestone LL. Once you clean and flush your system with commercial chemical treatments, NEVER EVER use anything afterwards but the same brand and version of a quality coolant, and only use distilled water. The coolant should be the long life type with NO nitrates nor silicates, as noted above. Prestone LL coolant is good for this. Use 40% coolant, balance is distilled water.
DO NOT USE TAP WATER, even if you have a 'water softener' at your house. I suggest you NOT use "purified" water. DO NOT USE any other coolant but the Prestone LL; or, BMW's. I have not tested but half a dozen coolants, and those two are the only ones I approve of. The Prestone LL coolant MAY now be available in pre-mix, you want 40%. I mix the concentrated coolant and distilled water myself.***BMW has published Service Information bulletins on problems with using the wrong antifreeze. This is not just Snowbum's ideas. One very pertinent bulletin about the gelling and heat blanket coating is here: SI 00 055 88 (2333).
Cleaning/flushing the cooling system. Coolant change:
I suggest you have on hand 1 gallon of Prestone LL antifreeze-coolant, in the concentrated version; and, at least a few gallons of DISTILLED water, some for flushing, some for mixing the dilution of the coolant. The Radiator cap has a small gasket I have had to sometimes replace, it is 17-11-1-460-759. The larger radiator cap gasket was usually OK, but if you need one, it is 17-11-1-460-316. It is a good idea to have these both on hand before you start work on the cooling system. You want to be sure the rubber gaskets are OK ...not deformed or broken, so that those cap valves work properly...and, that the cap does not leak. The radiator cap has both a pressure valve and a vacuum valve built into it. Cap releases at 1. BAR at 120°C; and the vacuum releases at -0.1BAR. I don't keep the hoses in stock, but you could consider replacing your hoses if they are quite old and noticeably poor (squeeze test and look-see).There are minor variations between models, most of what follows is for K1100LT. An example of one of these minor variations is that the K1100RS is drained at a screw plug on the water pump, while the K1100LT is drained via the water temperature sensor removal. There are more differences with earlier models. When I did the coolant RR job to a K1100 I kept detailed notes on what & where parts were used. The RS & LT were different, as the LT has its water temperature sender up front below the oil sender as noted above. The oil sender has a 15/16" hex, the water sender is a 14 mm hex. Don't mix them up. The RS has a PLUG in place of the water sender. Don't unfasten the oil sender by mistake. No harm will be done, but you won't see water draining, because it won't! I think BMW made size changes to sensor & plugs in late 1993 or into 1994 ...so, CHECK YOURS.
You will need a new water temperature sensor gasket (crush ring washer) if you have the model that uses its removal, such as the K1100LT. On a K1100LT I did recently, the sender was the small one, it had a COPPER washer #07-11-9-996-072. There is an aluminum washer -073 used on the Airheads. I suggest you use the proper -072 copper version. When re-installing the water temperature sender, tighten to about 9 Nm.
The below procedure can be modified, and the engine run with the fuel tank slightly elevated, especially on the right side. If you have a 2-valve K100 or a K75, modify appropriately.
Drain your K bike's coolant system when it is still warm from the engine having been run. Unfasten the front of engine located water temperature sensor and drain from there for the LT; for RS it is the PLUG. Your bike may be different. Remove the radiator cap to relieve any developing vacuum while draining. Inspect the rubber gaskets at the cap carefully ...and if at all doubtful, replace them. The earliest caps were not as good, and the rubber gasket didn't stay in position very well, could be squeezed out. These gaskets MUST SEAL or the system will give you problems!
Flush the now mostly empty system at the radiator cap neck, with a considerable amount of quite hot water. I use a teakettle and near boiling water for this. If your system is very badly gunked-up, obtain a commercial cleaner used for that purpose, and follow the instructions. Be SURE to flush VERY well after using such a cleaner (that includes the engine being HOT!). If you use such a cleaner, take your time to use the product properly. The more 'plated' hard insulating 'stuff' you can remove from the system, the better your system will cool ...and the less your fan motor will come on, and the longer the fan motor will last!
When flushing, you will have to put the fluid in QUITE SLOWLY. It is also important to install the fresh antifreeze fluid in VERY SLOWLY.
Remove the contents of the overflow tank using something like a turkey baster, but not the one in the kitchen! I also like to clean and flush that tank if it is grungy. You can do it all with the baster, tank remaining in place. Purchase a baster just for this job, and keep it in the garage ...not the kitchen! Flush the baster when you are ready to store it. If the baster barrel tip is too large, add a piece of small hose to the tip.
Inspect both ends and middle of the large hose from the radiator; & check the small hose to the overflow tank.
If you have a grungy system with built-up deposits, you will have to use a commercial cleaner product in order to obtain the best cooling. You need only do this once, as you WILL be using a proper long-life coolant as recommended; ...correct? Flush the system truly thoroughly after using such a product! Flush also AT LEAST once WITH the engine quite HOT! ...so the thermostat is positively OPEN!
After draining, and a thorough flushing with very hot water, using a cleaning solution if required, then repeatedly flushing with hot water, ...then, replacing the sender (or plug) with a new crush gasket, torque properly ...& you are ready to refill the system.
Use a proper antifreeze mixture, NEVER just plain water, even if you live all the time in hot weather! Use 40% or 50% concentration of coolant, make the mix with DISTILLED water. The % of coolant in the mixture you make up depends on conditions you store/ride in. I recommend the 40% coolant mixture that BMW recommends, which will be OK to -28°C. (-18.4°F). Increasing the concentration to 50% could be done if you store your motorcycle where it can experience extremely cold temperatures. For storage in arctic conditions, below those temperatures, consult the coolant container.
Do NOT use a stronger % of coolant if the motorcycle is not exposed to temperatures nearly at or below those temperatures .....more coolant does not mean better cooling. Coolants RAISE the boiling point, not just LOWER the freezing point. You need BOTH these effects, and your K bike system is PRESSURIZED by the radiator cap sealing and pressure valve (and vacuum valve to keep the system filled from the overflow tank) to enable an even higher operating temperature, when needed. The vacuum valve must be operable. The overflow container needs some fluid to enable suck-back as the engine cools down, to keep the system full, without air pockets. This means that use of an 'antifreeze' mixture is helpful for both hot weather riding as well as cold weather riding/storage. You should NEVER run plain (or even just purified or just distilled) water in your K-bike, NO MATTER THE WEATHER CONDITIONS. COOLANTS of the PROPER type also lubricate and protect parts in the cooling system.
I do NOT purchase 'pre-mixed' coolants, they often cost much more, and you cannot 100% depend on the total formula with SOME brands ....but, Prestone is supposed to eventually, maybe now, sell the LL coolant, properly mixed, if you prefer premix. Check the percentage listed on the container.
I use Prestone Extended Life; also called Long Life or Prestone LL, because it is a trustworthy brand. I know what the ingredients are; and have had new and used coolant TESTED for the various additives, AND aluminum from the engine, ETC. I change the coolant every 3 to 7 years on my own K1100LT, depending on my usage. That coolant does NOT contain silicates NOR nitrates NOR borates. YOUR change interval should depend on how many short trips, how much total mileage, & time since last change. I drive my K sidecar rig in the Winter, & the 40% has always worked fine for ME, since my garage is lightly heated, & I do not ride below +20°F. I use the BMW recommended 40% concentration; that is, it is 60% DISTILLED water. I do not use common tap water, I believe it simply ages the coolant faster & increases carbonate deposits, which also act like an insulating blanket.
Because I thoroughly cleaned the cooling system after I purchased the motorcycle, I will likely never have to re-clean it, just change the coolant on a schedule.The trick to refilling & proper operation is to be as sure as you can that the system is refilled as much as possible without air bubbles. This means dribbling the coolant mixture into the neck VERY slowly; 'burping' the system a number of times by squeezing the hose as you very slowly refill. I have found that it helps for the final burping's if the neck is filled to the ledge & the cap FULLY tightened, before the squeezing. I then remove the cap & continue refilling until I can't do more filling without the coolant mixture going into the overflow hose.
DOING IT .....what follows after the burping & filling:
Fill the cleaned overflow tank to not quite half-full with coolant mixture. Do not fill it too high. The overflow tank level will reduce some after the engine is run and cooled. In case you did manage to fully fill the engine cooling system, some overflow may come when the cap pressure releases, so you don't want hot coolant to overfill and overflow the overflow tank. Just how much the overflow tank level will typically go down, after a full heating-cooling cycling of the engine, depends on how well you did your job of filling the engine, and condition of the cap rubbers, etc. Even if you were careful, there may be air bubbles someplace in the engine. Install the radiator cap, inspecting its two rubber seals carefully ...first.
Using a dowel, or other long tool, carefully rotate the cooling fan blades a turn or two, to be sure the fan motor is free to rotate. K bike fan motors are notorious for freezing up and then burning up the connections area. Some find a way to put a drop or two of lubricating oil into the fan motor forward bearing. Rotating the fan should especially be done after any storage period. I typically block most of the front of the radiator using a piece of cardboard, to be sure the fan will eventually energize. The temperature gauge will be high at the time the fan comes on, because what you are after is to be sure the thermostat opens and therefore the water pump ensures circulation throughout the engine and radiator, etc. If you have a problem with the fan or thermostat or sensor driving the fan circuit, you want to be very careful. After the engine heats enough that the fan comes on, you can remove the cardboard, or partially, to keep excessive overheating down ....and ride home, and park the bike on the center-stand, to cool-off.
I recommend you do not run the engine to make the fan come on in a garage setting unless you have a big squirrel-cage fan, as the exhaust system, including header and exhaust ports can get considerably hotter than I think is OK for them, could crack, etc. If you DO use such a fan, DO put cardboard in front of the radiator to block the air flow; which will reduce the amount of time the engine must run. Try to direct the fan output to the underside of the engine ...& the left side exhaust header area. Squirrel cage fans are very handy ...and often available free from heating and cooling companies, who have old worthless house heaters you can remove a fan unit from. You will want to raise the throttle for maybe 3000 or even 4000 rpm a few times as the engine heats up quite hot, to ensure the water pump does a really good circulation job.
After the bike is completely cooled, which can take a fair number of hours, recheck the overflow tank. It is best that the engine cools completely to whatever the outside air temperature is, this promotes opening the radiator cap such-back function. Usually, due to trapped system air, which now should be eliminated, the coolant tank level will usually have gone down some as the system sucks the fluid back. If it did not suck back anything, remove the radiator cap, & see if the system is full. If not full, you may have a bad cap, or bad cap gasket (there are two gaskets at the cap). This is why I said to check the cap gaskets first. If the overflow tank level is quite low or near empty, then refill again to nearly half-full with coolant mixture and repeat the ride and cooling. If you have filled the system slowly and with care in the first place, a second ride is usually not needed. Recheck the coolant tank after any second ride and full cooling-off. Adjust the coolant level in the overflow tank to a bit above the minimum mark ..but not as low as the minimum mark. THIS ALSO MEANS DO NOT FILL IT TO HALF WAY AND HIGHER! The coolant tank level should be above minimum, and maximum half-way, engine fully cold.
HINT: It is a lot easier to add a small amount of coolant to the overflow tank, than to lift the fuel tank, unfasten the fuel lines, and lift the tank even more in order to refill at the radiator cap. If you followed my instructions carefully, you won't have to add very much coolant mixture to the overflow tank after the first ride, and you likely will not need a second ride test.
HINT: Filling the overflow tank too much will have the tank spit fluid overboard on a hot day. Usually half an inch or so above the minimum mark AFTER full engine cooling, is about correct. That level should hardly change between now and your next coolant fluid change.BMW said in its factory repair manual literature for my K1100LT motorcycle to use 3.4 L of 40% coolant. They mean that the total mixed coolant amount is that figure. That is 3.6 U.S. quarts ....about what I found necessary. I made the mixture from 46 ounces of the concentrated Prestone Long Life Coolant, plus 69 ounces of distilled water. I use an old plastic 1 gallon antifreeze jug I had (with a clear side edge area to see liquid level, which had level graduations) when mixing. I very slowly filled the engine through the system cap, until fluid just started to come from the filler neck and flow to the overflow tank, then I installed the cap. I then filled the overflow tank to 1/2 before starting the engine, since I knew from past jobs that this was not too much, yet it was going to reduce its level the next morning down to a bit above the minimum, but not need refilling; or, little.
The Radiator cap has a small gasket I have had to sometimes replace, it is 17-11-1-460-759. The larger radiator cap gasket is usually OK, but if you need one, it is 17-11-1-460-316. It is probably a good idea to have these on hand before you start work on the cooling system. You want to be sure the rubber gaskets are OK ...not deformed or otherwise bad, particularly not cracked, or broken, ....so that the cap valves can work properly and the cap does not leak fluid, NOR suck in air.
The radiator cap has both a pressure valve and a vacuum valve built into it. Cap pressure valve releases at 1. BAR at 120°C; and the vacuum valve releases at -0.1 BAR.
WARNING: Do NOT unfasten the radiator cap, in any amount, if the system is still quite hot, as there may be up to 1 Bar (about 15 psi) in the system, which could injure you, or damage the overflow system if partially unfastened!
What about water-less coolants?
Some folks promote using such as Evans "water-less" coolants. I am not convinced of their worth. I've being curious about the product for a long time, so I delved into it more deeply. The Evans product is a mixture of a high percentage of ethylene glycol and a lower percentage of propylene glycol. It is, OVERALL, a very high percentage mixture of two common long life antifreeze products. My primary concern, since I knew Evans contains a corrosion inhibitor, was the actual cooling capacity change, if any. Those glycols reduce "specific heat capacity" .... which probably means more fan motor use, and the coolant will work best with higher water pump rpm (will have poorer cooling at low rpm) ...in other words, it will require more air cooling to the radiator and increased liquid flow rate in the engine to enable the same cooling as with other standard coolants.I think the Evans coolant, or other 'waterless' types, will trade off too much cooling ability for their longer life (assuming a good anti-corrosion package in the Evans, and I have very limited information on that). I can NOT recommend such coolants. Since I have somewhat limited cooling capacity in my own K1100LT for the hot SouthWest areas, I'm not inclined to reduce the capacity by using such waterless coolants ....meaning the fan would come on more often ....and fan motor wear is a sore point on Classic K bikes.
I think that standard LL coolants, including BMW's own, & the LL Prestone I have recommended, are likely to only slightly reduce over-all cooling capacity (if at all, which is doubtful), and perform, over-all, very well, considering all other factors..... and have the advantage of long life. I don't expect wearing of the pump nor plating out, nor scale formation ...and other problems of the conventional antifreeze coolants (that contain nitrates, nitrites, silicons, borates, etc).
I will continue to use the LL Prestone, which matches BMW's requirements for its own coolant ...and does not have any plating-out problems, abrasiveness, silicates, nitrites, borates, etc. My testing of that product shows excellent performance. I specifically recommend AGAINST any coolant that contains borates, nitrites, silicates, etc. Use of them will probably...IMO.... result in short water pump life and will result in poorer cooling ....and probably shorter fan motor life.
© Copyright 2023, R. Fleischer
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Last check/edit: Thursday, August 17, 2023