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Thread: '92 K75S radiator leak

  1. #1

    '92 K75S radiator leak

    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...eaking-coolant
    https://forums.bmwmoa.org/showthread...olant-Dripping
    https://forums.bmwmoa.org/showthread...e-for-1994-K75
    https://forums.bmwmoa.org/showthread...eak-amp-Part-s
    https://www.motobrick.com/index.php?topic=10735.0
    https://www.k-bikes.com/forums/56-ot...ak-normal.html
    http://www.k100-forum.com/t4231-we-v...a-coolant-leak

  2. #2
    3 Red Bricks
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    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.


    Last edited by 98lee; 01-17-2020 at 08:36 PM. Reason: Sp.
    LONG MAY YOUR BRICK FLY!

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  3. #3
    Registered User
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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.

  4. #4
    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/radiat...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

  5. #5
    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.

  6. #6
    Nick Kennedy
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    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!

  7. #7
    Fortis Fortuna Adiuvat Omega Man's Avatar
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    Quote Originally Posted by robsryder View Post
    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/radiat...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
    A valuable tool
    You will be able to test the replacement you have coming as well.
    OM
    "You can do good or you can do well. Sooner or later they make you choose." MI5
    Mod Squad
    2009 F800GS 1994 TW200

  8. #8
    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

  9. #9
    Nick Kennedy
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    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

  10. #10
    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.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://web.bigbend.net/~glaves/

  11. #11
    JohnWC
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    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.
    Last edited by jconway607; 01-21-2020 at 06:12 PM.

  12. #12
    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-Tools...9643359&sr=8-4

    center drill bit.jpg

    k75s rad fan shaft with hole in end-1.jpg

  13. #13
    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

  14. #14
    Fortis Fortuna Adiuvat Omega Man's Avatar
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    Great pictures
    I know what you mean regarding “wonder rods”. You have it nice an clean, bet someone with a tig machine could do it.
    OM
    "You can do good or you can do well. Sooner or later they make you choose." MI5
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  15. #15
    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

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