This is rather simple query for a rather convoluted issue. I'll try and stick to the simple and add the background after. The back rack found on K-75's and some K-100's is made of what material? I do not believe it is steel or other ferrous metal as it is not magnetic. Can the piece be brazed or otherwise mended? And not by epoxy, glue or such material.

Background, I previously posted a long narrative of trying to mount a top box on a K75 (https://www.k100-forum.com/t17928-back-racks-and-further-adventures). MartinW posted a really nice description with photos of how he had done just that and I tried to copy his example. Obviously I did it wrong or badly as after a rather short period the OEM back rack broke. I will attempt to post a picture of the broken piece with the fissure revealed by wire brushing at the break. So now I'm trying to repair the broken part. This is not Martin's problem, I likely did something wrong, and in addition I have another unbroken piece. But for several reasons I'm trying to restore the broken part so I can use it in another project.  Hence the question of repair. Here is a photo of the broken part:

It's die cast in some aluminum/zinc alloy and powder coated.  It might be weldable, but I wouldn't bet on it.

Point-Seven-five wrote:It's die cast in some aluminum/zinc alloy and powder coated.  It might be weldable, but I wouldn't bet on it.

How about brazing?

I can't see how you could have done anything that caused it to break, especially with where it broke. Possibly a casting fault, was it mounted with big flat washers in the tail unit?
Regards Martin.

I would take it to a good welding shop and see what they say. 

It might be cheaper to get a replacement at eBay.  After the repair it will need to be repainted and that could be a problem with the powder coating that I think is on it.

I would not braze it as I doubt it would hold any strength.  

From what I can see and there appear to be two breaks, and I would make a simple bracket around both breaks.

If it was my situation I would use an aluminium or even a thin stainless steels sheet fitted to both sides of the break and drill two holes, one either side of the break and find a suitable pan/mushroom head screw/bolt and drill a hole through to the other side.  At the same time use an alloy adhesive on both surfaces of the break.  Only a small amount would be used, assuming you can jimmy both surfaces apart.  

An alternative to this would be to use the same aluminium/stainless material bent into a U shape with the above repair.  

The important thing here is to ensure the sides of the aluminium are flush with the upper surface, so as not to rub against the lower surface of the top box which would apply an unnecessary stress on the repair.   

Another option is to make the same repair but to use rivets instead of a bolt.  You could use the usual soft aluminium pop rivets but I would prefer to use aircraft structural rivets, simply because they are so much stronger.  However to use those you need an air rivet gun because you will break a hand held riveter using one.  

If you are using pop rivets you need to be accurate in drilling the holes for the bracket and sheet, to make it all hold together.  I would manage it in a way that ensures it pulls both pieces together as if it is loose then the repair will eventually work loose.  A bit hard to explain here.  

Make everything, then assemble, then glue and rivet/bolt straight afterwards.  And make sure you deburr the holes!!!  

Again, this is how I would repair it but I have access to aeroplane repair stuff.  And the aircraft rivets are a lot more expensive that pop rivets.

MartinW wrote:I can't see how you could have done anything that caused it to break, especially with where it broke. Possibly a casting fault, was it mounted with big flat washers in the tail unit?
Regards Martin.

There was a rubber washer under the mount and a washer inside the tail unit (as for each of the four mounts). As I said it is not particularly an issue as I had another one which was damaged but not in the same way that I replaced this broken one with. And to top it, I have an undamaged unit as well. The reason for my inquiry is that I've another project that would make use of this (the one in the photo) damaged one if I can weld/braze or otherwise fix it.

True Confessions: when this unit broke - I decided to make a total replacement of steel 1/2" square tubes. Well, I measured, measured, measured, then cut and welded. And the standoffs/mounts are just a bit off.   I could go on but it would undoubtedly bore everyone. But it's been an excuse to break out my flux welder, ox-acetylene rig, my powder coating station and lots of stuff that's been taking up space in my garage for the past several years.  And saving me from completing the K100 restoration project. 

The only thing about it breaking is that it was never loaded with any particular weight. I used it to hold the helmet when I was parked and occasionally to carry pastries home. My buddy Greg commented that it must have been vibration that caused the break but I've not identified any obvious cause. But it did make an odd noise that took forever to track down due to the location and black color of the rack. So a casting fault with vibration is probably as good a guess as any as to why it broke.

Mine has large round metal washers over rubber washers.
Regards Martin.

Here is a photo of the top box mount I made up. Now don't hurt yourself guffawing at the welding. It was an excuse to try welding with flux cored wire welding machine that I've had. The welds are reasonable penetration for this thickness of steel but the splatter was unexpected. Think I'm better with a simple ox-acetylene rig. Also the instant on welding helmet sucked big time and required development of some skills in welding without being able to see. But all the holes are threaded for 6mm bolts and there are 6 mm nuts welded into the bottom of the stand offs.

Givi make a suitable product.

I have one. The diagram is a bit misleading as part 9 (washer) doesn't exist and part 10 (nut) is welded directly to part 2 (rack). The good thing about the design is that if you put aside part 1 (top mounting plate) then with not much ingenuity it can be adapted to take any non-Givi topcase.

JGT wrote:Givi make a suitable product.

Yes, but what fun is that?

My OZ-based red RS, now in the capable hands of Charlie99, had an ingenious rear rack, which was built by the previous owner a coupla decades ago, that allowed for an F650 series lockable OEM topbox. It was mounted flat - or level - and was fabbed from a slab of thick wood, painted black, carriage bolts & appropriate spacers with steel rods holding it down to the rear cowl. It even had carved cut-outs for the latch mechanism. The bottom of the topbox was modded with a sheet metal tray that functioned effectively when the topbox was slid backwards onto the rack. Two large Philips head screws penetrated through the sides of the rack and the sheet metal for additional security. It took 1-1/2 minutes to remove from the bike. I don't have a really clear picture but you might pick it up on Chaz's "Refurbish Red" thread. I didn't use it much but it carried a helmet and could easily handle 7 or 8 kgs of weight.

I've recently bought a MIG and your project rack has got me thinking (a dangerous move). I used to be an Instrument Fitter which is a glorified precision plumber in stainless steel tube. Over the years I have bent up fairing brackets, crash bars, pannier frames, rear racks, bow rails, boat ladders, number plate brackets with indicator mounts just to mention a few. However when it came to welding them up  I had the companies really excellent welder TIG them up.  I still have all my benders 1/4, 3/8, 1/2, 3/4" and associated tools. I no longer have access to stainless tube at the right price, however the local steel supply should be able to supply seamless mild steel. I don't know whether my welding is up to scratch, but practice may overcome that problem.
Regards Martin.

MartinW wrote:I've recently bought a MIG and your project rack has got me thinking (a dangerous move). I used to be an Instrument Fitter which is a glorified precision plumber in stainless steel tube. Over the years I have bent up fairing brackets, crash bars, pannier frames, rear racks, bow rails, boat ladders, number plate brackets with indicator mounts just to mention a few. However when it came to welding them up  I had the companies really excellent welder TIG them up.  I still have all my benders 1/4, 3/8, 1/2, 3/4" and associated tools. I no longer have access to stainless tube at the right price, however the local steel supply should be able to supply seamless mild steel. I don't know whether my welding is up to scratch, but practice may overcome that problem.
Regards Martin.

My sentiments also. I went and bought one of the cheap Chinese made welders from Harbor Freight (a US importer of Chinese tools). It works but the flux core wire splatters a bit. Turns out the 1/2 inch tubes are reasonably priced and fairly easy to work with. I admit my welding skills aren't up to my last wife's. You should have seen the morning star she made up. But the piece I turned out is pretty sturdy. And it turns out powder coating ain't that hard. Made myself a powder coating station from an abandoned electric oven and a cardboard box. So some projects are just an excuse to try it all out.

I suggest you simply buy a used replacement. Thom at Kpartsholland.com is my go-to source that I've used many times. If you don't see a part listed on his website that does NOT mean he doesn't have it - simply send him a message and odds are he has it at a decent price. I live in the U.S, and shipping has also been reasonable. Go here to send a message: https://kpartsholland.com/pages/contact-us .

jjefferies wrote: Made myself a powder coating station from an abandoned electric oven and a cardboard box.

Powder, heat, electricity and cardboard—a recipe for magic.

Laitch wrote:

jjefferies wrote: Made myself a powder coating station from an abandoned electric oven and a cardboard box.

Powder, heat, electricity and cardboard—a recipe for magic.

Well the cardboard box is used to provide wind shade. But it has exploded upon occasion. But what else would fun be?

Point-Seven-five wrote:It's die cast in some aluminum/zinc alloy and powder coated.  It might be weldable, but I wouldn't bet on it.

Just an update. I've not been a fan of the latest fashions of brazing aluminum with these aluminum brazing rods. But I got some and tried it out. Turned out reasonably strong. I got the rods from Harbor Freight and used a propane benzomatic torch to heat the piece. The rod melted and flowed onto/into the break. Hardest part as usual was the prep work of wire brushing the powder coating off around the affected portion. Now the question is whether to just spray paint the affected portion or do a lot of work to get all the powder coating off and powder coat it properly.

Bottom line is they can be brazed with aluminum brazing rods. Who knew?

J.