BMW K bikes (Bricks)

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jjefferies

jjefferies
Gold member
Gold member
The K75 Radiator Cowling (also known as a Shroud or in the parts lists as Radiator Trim Panel p/n  46 63 0 026 168) is found on naked K75, K75C, K75T and possibly other variations. It is not the same part as found on the K75S, K75RT or K75RS which fall more in the genre of the K100RS/RT family. The reason I'm writing is I've had a few frustrations with this part and thought to pass along the information I've acquired. A common failure point I've encountered is the mounting tab on the right side. The cowling itself can be broken in a crash or the occasional tip over. But I presently have 4 sets (the cowling is made of two distinct halves joined by two bolts) which have the same failure  and have just acquired a fifth one with the same issue. The cause from what I've learned is the rider/repair person forgetting to to remove the cowling before removing the gas tank. And of course removing the gas tank is necessary to get to any electrics under it. So the tank gets popped up and for as yet unidentified reason the right side, not the left, tab on the unmounted cowling gets stressed/bent/broken/torn off the primary plastic cowling part. The cowling itself does come up for sale regularly on ebay. I just bought a whole one (both halves) for USD $47 which with shipping and tax was USD $63. Unfortunately it came with the right mounting tab damaged. There is also, at this time, a right half piece for sale on ebay for $53.  And I understand there are still 100 new of the right side parts available in the world from BMW in Germany priced USD $85.


But for the home mechanic determined to repair his/her own machine, the following information and suggestions may be of some use. I took a broken cowling half to TAP plastic for identification of the constituent plastic. For folks not familiar with TAP, they sell many different types of plastic, solvents, glues etc for plastic etc. When you can find an knowledgeable individual they can be very helpful. Looking at the broken piece and testing it with a solvent the fellow tentatively identified the plastic the cowling is made of as Polyethylene. Here is a list of similar or very common plastics along with a flame test to help identify them:


One of the simplest ways to carry out a flame test is by cutting a sample from the plastic and igniting it in a fume cupboard. The color of flame, scent and characteristics of burning can give an indication of the type of plastic:

    Polyethylene (PE) - Drips, smells like candlewax
    Polypropylene (PP) - Drips, smells mostly of dirty engine oil and undertones of candlewax
    Polymethylmethacrylate (PMMA, "Perspex") - Bubbles, crackles, sweet aromatic smell
    Polyvinylchloride (PVC-U, Unplasticised) - Self extinguishing flame
    Polyvinylchloride (PVC, Plasticised) - Green tinge to flame and white acrid fume
    Polyamide or "Nylon" (PA) - Sooty flame, smells of marigolds
    Acrylonitrilebutadienestyrene (ABS) - Not transparent, sooty flame, smells of marigolds
    Polycarbonate (PC) - No drips, phenolic smell
    Polyurethane foam (PU) - Yellow flame, acrid smell, plastic crumbles
    Polyethylene foam (PE) - Drips, smells of candlewax

You can also conduct a burn test, checking for signs such as a yellow flame with blue edges or a sharp smell to indicate that it's ABS plastic.



Even after the examination at TAP plastic I am not absolutely sure that the cowling is Polyethelene. The biggest issue is getting anything to adhere to the plastic. As the chap at TAP plastic commented the fact that paint (primer and color coat as well as body putty) were adhering was suggestive that it might be something else. But once home I decided to experiment with plastic welding assuming it was Polyethylene and that I could salvage the broken piece. I tried using two different heat sources with a feed stock of tie wraps. I had read that our ordinary tie wraps used to hold cables and wires together on our bikes works as a source of material, i.e. feed stock, for plastic welding. The first heat source was a small butane torch. This failed for two reasons, 1.) it didn't put out enough heat without directing the flame at the target area 2.) it would cause the tie wraps, i.e. feed stock, to catch fire. Once the material started to burn it created a crusty material which had no strength and would peel away from the target area. The second heat source tried was soldering irons. I quickly learned that a 140 watt iron didn't provide enough heat to melt the feed stock and underlying cowling plastic. Moving up to a 250 watt iron with a flat tip would melt the tie wrap and partially melt the cowling plastic. This worked well enough that I was able to cover a 1 inch diameter hole with melted plastic. Testing the covered hole by twisting and bending it the cowling - this is pretty much what happens when the cowling is being placed on the bike - caused the repaired piece to crack. Investigation of the cracked pieces showed that the plastic welding had inclusions, small space holes, in the material which may be the reason it was brittle. While generally a failure if one were trying to weld back the mounting tabs, the welding process does show some promise if you were trying to fill a gap. I doubt it would be of interest in repairing a broken mounting tab.


Regarding repair to or replacing the mounting tabs I found 1 repair method and 2 replacement methods. Which to choose should be determined by how damaged the tab is.

1.) Repair: If the mounting tab is not completely broken and the surrounding cowling plastic is intact and fairly sturdy, filling the box like structure under the mounting tab flange (the part that like a spear fits into the rubber grommet on the gas tank mount point) with epoxy is possible. I also put additional epoxy outside of the box to give as broad a base of support as possible. I suggest the areas be cleaned with alcohol and the use of J-B Weld original. I understand that J-B Weld makes epoxy specifically for plastic but several experimenters on youtube have reported the original works better even with plastic.
2.) If the mounting tab has been completely broken off but the cowling plastic itself is intact then the use of industrial velcro with adhesive backing is an alternative. I'd suggest the hook (harder side of the velcro) be mounted on the tank with use of a fastener like a bolt through the mounting hole. The softer fluff side of the velcro being mounted on the cowling with the adhesive.

3.) Another alternative replacement is to use an auto bumper fastener (usually listed as Clips Type: Push Type Radiator Yoke, Fender & Bumper Shield Retainer Clips). You can find these at auto supply stores usually in black plastic. They are like a fat plastic nail with feathered flanges on the side. Where the cowling mount is gone but the cowling plastic is intact you drill a hole smaller than the full width of the fastener. Push it through and into the tank mounting hole. I've not yet found one with the feathered flanges as wide as the hole but with the usual rubber grommet in place the mounting hole should be small enough to hold the bumper fastener. The main concerns are that the length is sufficient and the width of the feathered flanges wide enough to engage the rubber grommet.



So that pretty much covers my attempts to fix K75 Radiator Cowlings. Comments, additions and suggestions always appreciated.

    

Point-Seven-five

Point-Seven-five
Life time member
Life time member
I had some experience molding automotive parts back in the 1980's. Most of the body panels and trim were molded with ABS, an alloy of polyurethane and ABS, and an alloy of polycarbonate and ABS with a trade name of Pulse.

Lenses were mostly acrylic or polycarbonate. Interior panels were ABS, polypropylene, or styrene. Underhood was a lot of nylon, thermoplastic phenolics(polyphenoleneoxide, polyphenolenesulfide) or polysulfone. Coolant and washer tanks were polyethylene. Ductwork was often made of polypropylene.

Heavy parts like steering wheels were made of a material called bulk molding compound which was basically a glass filled bondo that was cured under heat and pressure.

ABS and polycarbonate was sometimes filled with glass fiber or talc to increase strength. My guess is that the radiator cowling is most likely the ABS/polycarbonate alloy. or just ABS.

The cable ties you were using to weld are molded from nylon. I have no experience welding plastics beyond closing up cracks.

An epoxy repair should work, I would reinforce the repair with metal or fiberglass. Make sure to scuff the repair area well with 180 grit paper and clean well with alcohol. Melt some steel wire into the part surrounding the repair extending into the epoxy to tie it all together. I have had some success with using small pieces of fiberglass window screen to reinforce repairs. I've had good success with the window screen and ABS plumbing cement repairing that flimsy plastic piece undr the windshield of my K75RT.


__________________________________________________
Present:
1994 K75RT
1991 K100RS
1988 K100RS SE

Past:
1994 BMW K75S
1992 BMW K100RS
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

jjefferies

jjefferies
Gold member
Gold member
@Point-Seven-five wrote:I had some experience molding automotive parts back in the 1980's.  ..
You are definitely more experienced than I. So I was trying to weld polyethylene with nylon. No wonder it didn't work so well. But  one of the things we need in this group is a call out of what plastic parts are made with what plastics. Seem to remember a thread that did that to some degree but I haven't found it recently. And part of that should be which plastics are amenable tor repair and how. Save us all a lot of time.

    

Point-Seven-five

Point-Seven-five
Life time member
Life time member
I doubt the radiator trim is polyethylene. It's virtually impossible to get a good paint finish on polyethylene. I'm guessing it's made of ABS.


__________________________________________________
Present:
1994 K75RT
1991 K100RS
1988 K100RS SE

Past:
1994 BMW K75S
1992 BMW K100RS
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

jjefferies

jjefferies
Gold member
Gold member
@Point-Seven-five wrote:I doubt the radiator trim is polyethylene.  It's virtually impossible to get a good paint finish on polyethylene.  I'm guessing it's made of ABS.
So how does one ID ABS? I've tried the burn test, checking for signs such as a yellow flame with blue edges or a sharp smell to indicate that it's ABS plastic." But nothing definitive. And any suggestions? Can ABS be plastic welded? Are there any particular epoxy's that will adhere to it?

    

Point-Seven-five

Point-Seven-five
Life time member
Life time member
The only way I know to positively identify the resin is to read the name off the box it came in.

My experience says that our bikes are made of the following resis:

Big fairing pieces, tail cowl - bulk molding compound, a mix of talc, wax, glass fiber and polyester heat cured under pressure in the mold. Repair with epoxy resin, fiberglass and body filler.

Front mudguard, battery covers, mirror pods - ABS, an alloy of several plastic resins. Repair by welding or with Superglue, possible to use ABS plumbing cement depending on the damage.

Seat base - Nylon; can be repaired by welding. Adhesives won't reliably bond to it.

Rear mudguard and non-cosmetic black parts - probably polypropylene or possibly polyethylene. Must be welded, adhesives won't stick reliably to them.

Black cosmetic parts - Probably ABS. Superglue, ABS plumbing cement, or welding.

Belly pans - K75S is Bulk molding compound, others seem to be a polyurethane material similar to what is used for bumper skins. Repair the polyurethane by welding. Adhesives may work with cleaning and scuffing with 100 grit paper.

Windshields, turn lenses - acrylic, possibly polycarbonate. Repair with Superglue.


__________________________________________________
Present:
1994 K75RT
1991 K100RS
1988 K100RS SE

Past:
1994 BMW K75S
1992 BMW K100RS
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

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