Suggested coolant for '86 K75C

What suggested coolant is preferable for the subject bike?

For my water-cooled motorcycles, including K75S bikes, I use a silicate-free coolant. I mix the coolant with distilled water in a 50/50 mixture. Usually when replacing the coolant I will drain the old coolant; then fill the radiator and engine with pure distilled water. I run the bike for a short while to circulate the water with the old coolant. The water is drained. I might do this a couple of times. Then the new 50/50 coolant mix is added to the bike. After a brief running of the engine (before the bike gets hot, just a few seconds of running) I remove the radiator fill cap and check if the coolant needs to be topped up. If the bike is allowed to warm up WAIT BEFORE OPENING the rediator cap!!!!

I have in the past used the Honda coolant because the place where I purchased many motorcycles was also a Honda dealer and they carried this stuff. This stuff is green and should not be mixed with water (it is already diluted with water). The picture below is of a newer version of what I used. Also below is an image of a silicate-free version that is readily available at most nearby FLAPS (Friendly Local Auto Parts Stores). A link is given for a pdf file that shows many different types of coolants and the various constituents.

https://images.oreillyauto.com/uploads/pdf/ad/SD_672 AFZ Comparison Chart-1.pdf

any name brand coolant. nothing special required. I use prestone (yellow or blue, doesn't matter which) in all my bikes and cars.

When I rebuilt my water pump on my K75 a couple years ago, I went to a BMW auto dealership and got a gallon of BMW antifreeze from their parts department.

The figure below is taken from the BMW Service Manual for K-75 and K-100 models. I am unfamiliar with any of the approved coolants -

That is a list of coolant brands available in Germany in the early 1980s. It isn't very useful in the US in 2021.

I did some research into K75 running temps several years ago. This all started because I had installed temperature gauges and could now see that on a hot day on the Interstate the indicated temperature was approaching the level where the fan comes on in city traffic. Thus, the dangers of too much information.

After a lot of experimentations with and without the gaiters, and about a half-dozen different coolant ratio mixture changes, the short store is that I found the fork gaiters I had retrofitted on both bikes were disrupting the air flow at high speeds and causing the bikes to run hotter by about two needle's width worth. Enough to keep the fan off at highway speeds some days. (FWI the "S" belly pan didn't seem to make a measurable difference.) Secondly, I selected a ratio of 65% water to 35% coolant. Water is a better coolant than "anti-freeze", so that mixture gives a higher Specific Heat Capacity than 50/50 or the original BMW recommendation of 60/40. It also provides freeze protection down to about -4F degrees. Since I don't ride in those conditions and my bikes are kept in a garage where it never gets below freezing, I when for the better cooling in the summer.

Finally, although I think almost all modern coolants are silicate-free, do make sure what you've purchased specifically says so on the bottle. I went with a long life coolant and check the chemistry with test strips periodically.

BASF is the world's largest chemical company and their antifreeze brand is Glysantin.

Their current recommendation is G48.

https://www.glysantin.de/en/product-finder?tags=9-103-362-363

https://www.valvoline.com/our-products/heavy-duty/zerex-g-48

Good to know the difference between "suggested" and "specifies." BMW specifies, which means use what they list.

My 2 cents...

If you want superior cooling, and you don't subject your bike to freezing temperatures, try Red line Water Wetter with straight distilled water. The difference in running temperatures is noticeable.

BC1100S said:

My 2 cents...

If you want superior cooling, and you don't subject your bike to freezing temperatures, try Red line Water Wetter with straight distilled water. The difference in running temperatures is noticeable.

Click to expand...

I know many people swear by Water Wetter as an additive, but I'm not aware that it has the anticorrosion and water pump lubrication additives necessary to keep the inner cooling system healthy and happy, especially since Red Line recommends replacing it every 15,000 miles. I looked, and did not see any such recommended application on the Red Line web site. https://www.redlineoil.com/waterwetter. Go down that page to the "White Paper" link to find a very interesting document about WW and how cooling systems work. The paragraph below is from that White Paper. Personally, in street-use only I've found that stock cooling systems in proper condition work fine, at least under normal conditions. However, given the extreme heat we are seeing so far this year in major portions of the US, perhaps some "helper" additive might well be worth it.


USE DIRECTIONS
One 12 ounce bottle treats 12-16 quarts of water or a 50% ethylene or propylene glycol solution. In smaller
cooling systems, use 4-5 caps per quart. Add directly through the cooling system fill cap into the radiator or
into the overflow tank. Do not open a cooling system while hot. For best protection for aluminum, replenish
or replace every 15,000 miles. The anti-scaling ingredients in Red Line WaterWetter® allow its use with
ordinary tap water. However, using with distilled or deionized water will accomplish some scale removal
in the cylinder head area. Plain water with or without WaterWetter® should not be used in cooling systems
containing magnesium - antifreeze should be used - with WaterWetter®. For maximum temperature
reductions use the most water and the least antifreeze possible to prevent freezing in your climate.
Even in summertime the use of air-conditioning can blow freezing air through the heater and cause
freezing of the heater core unless approximately 20% antifreeze is used. Red Line WaterWetter® is available
in 12 ounce containers.