1985 K100RS leaky rad on left hand side tank.

The bike is apart due to what is clearly dried coolant on the left hand rear side of the tank right where the crimping is. It starts about half way up. I noticed it earlier this year and rinsed it off to see if it would reappear which it did. The bike intermittently uses coolant (I marked the clear line on the side of the overflow tank to watch the level) and I can smell it when the engine is hot. It doesn't drip anywhere and only has the slightest amount of residue on the top of the engine right under the bottom left side of the rad. The leak obviously isn't bad enough to actually drip but just evaporates with heat. I want to say it has used about 300-400ml in the 3,600 miles of riding this year.

I'm not interested in adding radiator sealant to the coolant or running a bead of silicone in an attempt to address the issue, neither of which would be long term solutions. I have a spare rad in what appears to be beautiful shape with not a ding in it anywhere although it does have some brownish residue inside after I flushed it out. Not sure what that is as it is an aluminum core and plastic tanks. It does have a 6 month warranty and came out of a 1984 K100RS with just over 14,000 miles.

I have spoken to a shop already that a large dealership takes their rads to. They can pressure test and try re-crimping it.
So, how to proceed? I will most certainly have the spare rad tested, but do I bother to waste my time at all with the original rad by letting them see what they can do with it?

Have any of you guys attempted re-crimping or just replaced the rad when the tank started to leak in this area? Is there some sort of seal or gasket between the tanks and core when it's assembled?

Thoughts?

Personally, if I had a "new" rad with a warranty, I'd probably replace the original with the known-good one. Either way you're going to have to get the radiator out and the cooling system drained, so you're not saving any time or money on those portions of the service.

I'd put the one with the slight leak on the shelf (after giving it a good cleaning) and then in 35 years when the known-good rad has trouble, I'd investigate how to fix one or both.

I had a leak in the radiator in my K75S. I was able to make a repair. My process in coming to the repair is documented in the linked thread below...

https://forums.bmwmoa.org/showthread.php?96841-92-K75S-radiator-leak

Just back from rad shop, and as I was anticipating the suspect rad failed pressure test and leaked. Spare rad passed pressure test and is good to go.

My guess on the brown goop in your radiator would be contamination in the water. My water here is high in minerals, specifically iron. Places it dries up from tend to have a brownish rust residue. I recommend refilling with distilled water.

My experience with a leaky radiator didn't go well. I had a local shop "fix" the leak. The plastic tanks were removed and reinstalled on the core. It subsequently failed the pressure test (which cost what seemed like a lot of $$$). You may be interested to know that there are all alu radiators for K-bikes available now on e-bay for a very, very reasonable price. I'm thinking of buying one just to have a spare. I don't have a lot of faith in the radiators with the plastic tanks crimped on the sides.

walt3022 said:

I don't have a lot of faith in the radiators with the plastic tanks crimped on the sides.

Click to expand...

Me either. Not on a BMW motorcycle, but my Ford Exploder left me stranded when the rubber ring sealing the plastic end cap failed and allowed coolant to spew every which way. It is simply a cheap and cheesy way to build a radiator.