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'92 K75S radiator leak

robsryder

Motorcycleton
Yesterday while riding my 92 K75S I could smell the anti-freeze from my radiator. Small droplets would hit the exhaust and create a noticeable smell. This morning I could see a few drops of coolant on the floor beneath the bike. I removed the fuel tank and many of the body parts. I had hoped to be able to spot a location from where the leak originated, e.g. cracked hose or clamp that needs tightening. Alas, the leak is coming from the center of the radiator.

I last repaired a leaking radiator core in 1974 by using a torch and solder on a removed radiator from a car (64 Triumph TR4) that I was working on. I wonder if a similar method would work on the K75S radiator. I have done some looking on the 'net and found some other threads with various repair options. Some have indicated success with a "stop leak" additive to the coolant. I would be a bit concerned about such a product negatively affecting the water pump.

One respondent indicated making a repair by using an externally applied putty-like material. Several folks indicated a lack of success in getting automotive radiator repair places to attempt a repair to the K75S radiator.

Following are links to the past threads that I've found related to this topic -
https://forums.bmwmoa.org/showthread.php?19850-1987-k75s-leaking-coolant
https://forums.bmwmoa.org/showthread.php?73177-87-K75s-Coolant-Dripping
https://forums.bmwmoa.org/showthread.php?52201-Coolant-Antifreeze-for-1994-K75
https://forums.bmwmoa.org/showthread.php?6657-Radiator-Leak-amp-Part-s
https://www.motobrick.com/index.php?topic=10735.0
https://www.k-bikes.com/forums/56-other-k-bikes-discussion/18987-how-much-coolant-leak-normal.html
http://www.k100-forum.com/t4231-we-ve-got-a-coolant-leak
 
Rob,

You might just consider replacing it with a good used one. It must be out of a K75 C, S, or standard, not an RT. K100s and K1100 will not work.


:dance:dance:dance
 
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I had a client who had a situation similar to yours. He could smell it but not find the leak. I finally determined that the cooling fan had walked its way down its shaft and would come to rest against the back side of the radiator. The fan's spinning eventually wore a pinhole in the radiator leading to the leak.
The radiator repair shops in my area had all evaporated as modern radiators don't lend themselves to repair, so I ended up having to find him a used one online. If there had been local repair shops available, I'd have first tried that route.
 
I found a used K75S radiator for sale on ebay, said to be "good". It is on its way here.

Just to be sure of the leak location I obtained the radiator pressure test kit from Harbor Freight. Previously I had seen several good reviews about this kit.

https://www.harborfreight.com/radiator-pressure-tester-kit-63862.html

63862_I.jpg

The yellow cap in the kit fit quite well on the K75S inlet. Pinched closed the overflow tube and pressurized the coolant system. At 15 psi I could hear a slight leak and could see coolant coming from the front center of the radiator. See photo below -

k75s-rad-leak.jpg
 
If you have a local radiator shop, they can pinch seal the leaking tube at both ends.

However, if you have a replacement on the way, replace it and then play around with solder/brazing the leak.
 
FWIW
Which is not much granted.
About 10 years ago I developed a radiator leak in my 1978 BMW 633 CSi car similar to the picture shown in this thread.
I was/ am lazy and poured a tube of that silver granular STOPLEAK in there and drove it. It stopped leaking and hasn't leaked since. No change in temp gauge.
So now I carry one of those tubes of STOPLEAK in my 1990 K75RT tailcone, along with a fuel filter and one of those Z hoses. And a cellphone and credit card. And a Garmin InReach satellite tracker.
With that Garmin you can communicate anywhere with a text. And the wife and friends can see where I'm at.
I ride alone a lot and If I blew off the road and ended up hurt in a ditch they could find me.
Sorry to hijack this thread.
Ride Safe in 2020!
 
I found a used K75S radiator for sale on ebay, said to be "good". It is on its way here.

Just to be sure of the leak location I obtained the radiator pressure test kit from Harbor Freight. Previously I had seen several good reviews about this kit.

https://www.harborfreight.com/radiator-pressure-tester-kit-63862.html

View attachment 76853

The yellow cap in the kit fit quite well on the K75S inlet. Pinched closed the overflow tube and pressurized the coolant system. At 15 psi I could hear a slight leak and could see coolant coming from the front center of the radiator. See photo below -

View attachment 76854

A valuable tool :thumb
You will be able to test the replacement you have coming as well.
OM
 
I removed the radiator from my 92 K75S in anticipation of installing the (hopefully) "good" used radiator recently purchased on ebay. When I removed the fan from the back of the radiator, the cause of the leak became apparent. The problem was similar to that mentioned in a previous post; the plastic fan blade had "walked up" the shaft and started rubbing against the back side of the radiator. Eventually a small leak occurred. See photo below for the damage location. I think that this leak will be pretty easy to repair (if one believes the various YouTube videos :) ).

I pushed the plastic fan back onto the shaft sufficiently to expose part of the shaft. It is just smooth steel. There is nothing other than friction to keep the fan from again "walking up" the shaft and causing further damage. I am considering modifying the shaft to preclude further damage by the fan. Options that I am considering include drilling a small hole near the edge of the shaft and placing a small washer and cotter pin in place; another option is to cut a small groove in the shaft and fitting a washer and circlip in place. For me, the easier option may be drilling the hole as I don't have a lathe handy. If this is successful I will plan on doing something similar to my other K75S (an 88 with over 100k miles).

Photos below show a close up of the damage on the backside of the radiator and the end of the shaft upon which the fan rests.

back of k75s rad.jpg
back of k75s rad - damage.jpg
rad fan shaft.jpg
 
Robsryder
Thanks for that heads up on the fan blade moving on the shaft issue.
I'll have to keep an eye on that
Best Regards
Nick
 
I think the most straightforward fix for the fan blade moving forward would be to drill a small hole in the shaft and installing a cotter pin in the hole.
 
Drilling the shaft will certainly work, but it's a small rod and even center-punching it, I think the small bit will tend to walk off it. You could file a flat on the shaft first to alleviate that. Another option might be to find a piece of metal (brass) tubing that is close to the shaft diameter, then epoxy the short tube section onto the shaft. Someone with a lathe could make a long cap with a flange that would ride against the fan, but that doesn't sound like an option for you. A collar with a set screw and a flat on the shaft would probably also work, again saving the drilling.
 
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I was able to drill a 1/16-inch diameter hole near the end of the shaft (see photo). The key to doing this is to use a drill press with the work piece held in a small vise. I used a small center drill bit, then the 1/16 inch drill bit. As the shaft is carbon steel, I applied a small dab of paint in the hole to slow oxidation. I will use a small brass cotter pin to keep the plastic fan from again walking down the shaft.
https://www.amazon.com/Anytime-Tool..._4?keywords=center+bits&qid=1579643359&sr=8-4

center drill bit.jpg

k75s rad fan shaft with hole in end-1.jpg
 
As I mentioned I've already ordered a used radiator from ebay and will in all likelihood install that in a day or so. But I wanted to try and repair the small hole in the back of the radiator. There are various options for repair of alumnum radiator cores.

Several years ago at the AMA Vintage Days a vendor was demonstrating an aluminum "welding" (really soldering or brazing, not welding) method. He was selling the metal rods used to accomplish this repair / joining of aluminum. The vendor was demonstrating the method on broken pieces of motorcycle engines and aluminum cans. He made this look very easy. I have come to learn that it is not as easy as it appears. It is very important to remove the aluminum oxide layer that forms on the outer surface of aluminum. Real aluminum welding uses inert gas (e.g. argon) to allow the aluminum metals to join in a strong manner. The soldering or brazing method is claimed to be very strong as well. My first attempt to use this stuff was to repair a broken fin on an airhead cylinder. I was not successful.

I thought that I would clean the area about the leak on my radiator. I used a Dremel and a small brass, then stainless brush. This cleaned area is shown in the photo below -

k75s rad with clean holes in back-1.jpg

But recalling my prior lack of success, I though that I would perhaps practice on something else first. I chose an empty aluminum can. I punched a small hole in the bottom of the can, similar in size to that in my radiator. I also cleaned the area using the same brushes with the Dremel.

al beer can with cleaned hole-1.jpg

I used a MAAP gas torch to heat the area about the hole in the can. I melted the aluminum can. This is really more difficult than the vendor at Vintage Days made it look -

al beer can with melted hole-1.jpg

I kept trying to heat the can and apply the low-temp "welding" material. Finally I got some to melt and stick to the can. This stuff seemed to adher quite well to the can and couldn't be pulled off with some pliers. Alas, the can with my attempted patch still leaked. So more practice or a different method is needed.

al beer can with lo temp solder patch-1.jpg
 
Great pictures :thumb
I know what you mean regarding “wonder rods”. You have it nice an clean, bet someone with a tig machine could do it.
OM
 
I pressed the plastic fan back onto the motor shaft, but stopped when the shaft was flush with the edge of the plastic fan. The bottom edge of the plastic fan body is quite close to the clips holding the fan motor to the plastic mount. If I continue pressing the plastic fan onto the shaft to expose the hole for insertion of the cotter pin, the fan might hit the clips. I may have to go back and cut some slots in the outer edge of the fan "nipple" to allow the cotter pin to be inserted. Since I painted the area about the hole drilled for the cotter pin, the plastic fan is very tight on the shaft. Perhaps this is sufficiently tight so that no further "walking" of the fan outward on the shaft will occur.

plastic rad fan on shaft-1.jpg

plastic rad fan to motor clip right-1.jpg
 
As an alternate to "welding" the aluminum radiator with metal rods to make the repair I am considering using an epoxy material. Stik-Tite is a low melting temperature epoxy intended for the repair of aluminum radiators. I obtained some of this stuff and did another experiment on an aluminum can. This time I used a small butane pencil torch. I created holes smaller and larger than in my K75S radiator. The area about the holes was cleaned using the Dremel and stainless brush. The area was heated with the butane torch and I let the Stic-Tite melt around the holes. This was very easy. Afterwards the aluminum can held water (not at pressure).

https://www.northernfactory.com/Product/Z12156
https://www.amazon.com/Northern-Rad...PPMG24STCQR&psc=1&refRID=H5RM261TNPPMG24STCQR

stik-tite on small hole-1.jpg

stik-tite on larger hole-1.jpg
 
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Since the root cause of the leaking radiator on my K75S was the plastic fan walking forward on the motor shaft I decided to fix the issue so that it wouldn't happen again. I used a 3/32-inch drill bit to make a "wallered out" hole ithrough the nipple of the plastic fan. This allowed a stainless steel cotter pin to be placed through the hole in the plastic fan and motor shaft. The small red dots on the top of the shaft were to help me with alignment.

k75s rad fan secured to motor shaft-1.jpg
 
Since the root cause of the leaking radiator on my K75S was the plastic fan walking forward on the motor shaft I decided to fix the issue so that it wouldn't happen again. I used a 3/32-inch drill bit to make a "wallered out" hole ithrough the nipple of the plastic fan. This allowed a stainless steel cotter pin to be placed through the hole in the plastic fan and motor shaft. The small red dots on the top of the shaft were to help me with alignment.

View attachment 76928

That'll fix it for sure. Excellent!
 
I was able to successfully repair the radiator on my 92 K75S. I will describe the things that I did (some worked, some did not). I did not start this repair until I received the used radiator from ebay; the used radiator was to be my back up in the event of catastrophic failure.

The repair needed to have two main attributes. One seal the leak and two be strong enough to last as a permanent fix. I thought that the aluminum brazing method might work best, but as described above, I initially melted the aluminum can and made a leaky patch. More practice was needed.

I went back to the aluminum can as my test platform. I punched a small hole in the can approximately the size of the hole in the radiator. Then, using a dremel with wire brush, cleaned the area about the hole.

can-test1.jpg

Rather than use the hotter MAAP gas torch, I decided to use a propane torch with a less hot flame. Again, I melted the aluminum can -

can-test2.jpg

I continued practicing on the aluminum can and tried to be more careful in heating the can. Eventually I was able to glop some of the lo-melt-temp alloy onto the can -

can-test3.jpg

More practice and I was able to get a better appearing patch over a hole without melting the can. These latest patches were securely attached to the can, I couldn't pry them off. -

can-test4.jpg

These patches were tested by filling the aluminum can with water and checking for leaks. No leaks with my better looking patches.

So I cleaned the area on the backside of the radiator using the dremel with wire brush and used the aluminum brazing rods with the propane torch. The patch still looked a bit "gooped" onto the holes in the radiator. And when cool, the aluminum patch could easily be flicked off with a screwdriver. I expect that I didn't heat the aluminum radiator sufficiently with the torch. I was too concerned about melting the aluminum radiator.

rad-fix1.jpg

I went back to the Stik-Tite epoxy. I called the manufacturer and asked about success in repairing aluminum radiators. The customer rep. indicated that Stik-Tite was widely used by radiator repair shops and was used to make quick repairs to race car radiators. I used a butane pencil torch to heat the area around the radiator holes and applied some Stik-Tite. This stuff is very easy to work with. -

rad-fix2.jpg

I believed that I created a patch that would seal the leak, but I was concerned about the long-term strength of the patch. I decided to make a hybrid patch.

I took a hot air gun and heated the Stik-Tite until it started to melt. I used a bit of wire to push the Stik-Tite material into the holes in the radiator. Some needle nose pliers were used to remove some of the fins from the core about the holes. I wanted to create a larger surface area for the patch material. The dremel and wire brush was used to clean the exterior area around the holes. I mixed up some JB Weld and dabbed some about the holes. A small strip of aluminum was cut from the can and formed into a "V-shape" and placed (like a tent) over the holes into the soft JB Weld material. I then placed more JB Weld over the aluminum tent and about the cleaned area.

In making repairs on other bikes I have found the JB Weld epoxy and aluminum "sandwich" to make a pretty strong patch.

rad-fix3.jpg

The patch was allowed to cure for 24 hours. I inverted the patched radiator and used the hot air gun to blow hot air upwards into the patch. The idea is to re-melt the Stik-Tite in the hole to more fully seal the hole. When cool the radiator and fan were installed on the bike and all hoses re-connected. Distilled water was placed in the radiator and the system was pressure tested. At 20 psi there were no leaks and the system held the pressure for my hour long test. The water was drained and replaced with a 50/50 mixture of anti-freeze and distilled water. I used the same silicate-free anti-freeze as before.

The bike was started and allowed to run for a while. I let the bike run until the radiator fan cycled on and off a few times. I took a little ride and observed no leaks. Thus, I am declaring the leak to be fixed.

I was able to re-use all of the radiator hoses except for the over-flow hose. Fortunately, a replacement was available for about $3 at the local auto parts place -

rad-ovfl-hose.jpg

I will make a further suggestion to anyone removing their radiator, either for repair or to make a prophylatic fix to the fan blade (see above).

Take some time to examine the many wire connections that live in the area below the fuel tank behind the radiator. I pulled each connection apart and used some WD-40 to clean the connectors and then placed di-electric grease about the pins before securely re-connecting things.
 
Thanks for the great documentation on your repair project.

Your brazing jobs mimic most of my soldering attempts.

I too have had luck using J B Weld.
 
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