K75RT leaky tank fix + generic maintenance

[samih]

So my newly purchased '92 K75RT turned out to have a leaky fuel tank which I discovered when assessing slight bubbling at the bottom of the tank. As promised in the introduction section going to document the fix here.

This how the tank looked like at the start of the repair.

Mostly harmless:



Not really pretty with a bit of previous tampering present:



Yes, the previous tampering was a bit more serious than expected:



What's going on here? There were localized splattering of what looked like polyurethane foam underneath the paint, both here and also in some places underneath the tank. The reaction of PU with the top coat had turned some areas of the tank into a bad imitation of a balloon. The alumin(i)um underneath was perfect with just a bit of red PU foam remaining. The paint coat here was very thick so most likely the tank was respayed at some stage.

There was also an old patch visible with no signs of attempting to paint over it.



Inside the tank looks largely solid with just tiny pinholes in present on couple of places.



These are the two main culprits:



I'm planning to use HTS-2000 to patch the tank with continuous flow of air through the tank to mitigate the chance of a thermal event. Filling it with water is another possiblity although I'm afraid it might mess with the adhesion. I'm currently waiting for these brazing rods to arrive from UK. As there are multiple potential locations for future leaks I'll also seal inside the tank with epoxy.

The final image shows the bottom of the tank after light sanding and debubbling it. I had to peel off the partially fuel saturated insulation as those darn bubbles were present pretty much all throughout the bottom of the tank. For the repair metal to work the surface needs to be totally fresh alu so plenty of work needed here just before the brazing attempt.



But yes, I'm getting ahead of myself here as usual. The first job will be to practice with the HTS-2000 at the bottom of the tank when those rods appear by post. After that it will be plenty of grinding and sanding to get the tank ready for the paint. I'll need to call to the paint shop next week to order a spray of suitable base coat compatible with bare alu tank and also some high build primer. Then in stage n it will be time for the final colour "675" and a matching clear coat. The inside epoxy coating will probably be added right after HTS before the painting process starts.

The tank was a bit faded anyway and has a scratch on the right hand side so might as well just spray it all.

Apart from this tank saga everything else seems pretty good. The front brake light switch was broken and under the seat, ABS is flashing constantly if not for other reason than this missing switch. The driveline splines were in perfect condition and well greased sometime in the past. All three spark plugs were under torqued which I guess is better than over torqued. Brake fluid looked murky and coolant good. I also took out the thermostat which I'll test later on.

[Laitch]

Thanks for the useful photographs. Members here will certainly be interested in the outcome of using HTS-2000. Good luck with this task.

[samih]

Thanks Laitch, will definitely keep this thread up to date. In general I'm super impressed about the quality of everything on this old girl. The only difficult fastener so far has been the speed sensor screw on the final drive and even that came out with plenty of penetrating oil and patience. Looks like my bike was hardly ever used in "winter" which helps a lot.

[samih]

Still no sign of the HTS-2000 delivery but I have been "productive" and have largely reassembled the bike apart from the side fairings and the tank of course. I have been running this contraption for the past week or so to get rid of all of the remaining fuel vapours.



It pushes in the fresh air and exhausts it via the fuel sender aperture where I inserted a length of flexible tubing snaking into towards the front of the fuel tank. This is to ensure that all the volume of the tank is ventilated. The exhausted air doesn't have any smell of fuel now after a week which fills me with confidence that the tank won't explode on me later on.

I also disassembled cleaned and resealed the gear position switch. As reported here the rubber covering of it had started to fail and had lead to water ingress with sign of slight corrosion within. Apart from dropping 2/3 of the pins and springs on a dusty concrete floor and used 45 minutes searching before I found the last spring, I sealed the switch using Loctite anaerobic instant gasket gel (which is really meant for metal surfaces but that's all I had) and copious amount of self vulcanizing repair tape and it was at least initially working when tested. To make the life easier for the said switch I installed a rear mudguard extension that may or may not do anything but in theory will help the water making it all around the guard to safely exit below the swinging arm/gearbox as opposed to just behind them.



The rear edge got the the same treatment as there were unwanted splashes on the lense of the rear light after my initial journey on the bike. It can be muddy here in Ireland.

The other jobs apart from splines and fluid changes: I reseted the ABS fault code for low voltage (# 5)  and as I now have a new front brake like switch fitted (was not fitted at all) hopefully the system will be operational now. At least the brake light tests now result in the appropriate sequence of the ABS/bulb fault indicators and also the ABS test socket middle pin now reads constant voltage indicating no faults present.

Is 10.8 VDC expected btw on this pin with a battery voltage of 12.8 Volts or do a have a bad connection somewhere? I suspect all is good as the ABS now seems to work without the low voltage indication. Haven't of course had a chance to road test it yet to see if the wheel sensors are gapped correctly but all the systems at least pass the POST.

Now just need to patiently wait for the HTS to arrive...

[samih]

Still no sign of those brazing rods but I contacted the vendor and they were definitely posted. There seem to be a post Brexit related slowdown with the post between UK and Ireland.

As this is moving more on the way of generic maintenace: Checked the valve clearances last night. The #2 and #3 inlets are a little bit tight but decided to leave them be for now and will check again on the next service. The spec in millimetres is 0.15-0.20 mm for the inlets and these both were a very loose 0.10. The others very right at the mimimum at 0.15 (in.) and 0.20 (ex.). In general all is looking great under the valve cover and had I not known better I would have guessed that the lobes were machined about 6 months ago:



I think in general it was just a question of blowing away the cobwebs from this fine motorcycle:



As mentioned the splines were looking great:

[samih]

Bought the basecoat, paint, 2k clearcoat and the epoxy coating for inside the tank so all ready for the brazing rods to arrive (any day now).

While waiting I fitted Oxford Touring heat grips. As the voltage on the standard heated grips connector underneath the tank was somewhat fluctuating I decided to just use it to drive a new relay which I fused with 10 Amp fuse directly from the battery. There was an empty space for the relay inside the relay box. That small fuse holder is part of the loom from Oxford which I re-appropriated as the control circuit for the relay. I guess no harm having that fused also since the holder was built-in. If you look closely you might see that I have packed all the plugs and sockets with my favourite preventive maintenance ingredient, red rubber grease. I have done that on all of my bikes since the alternator circuit of my Honda Africa Twin had a meltdown due to corrosion. And incidentally most of the connectors on the RT has also shown definite signs of water ingress and corrosion so definitely a good thing to do in damp climates like Ireland.



The grips were a perfect fit and a suitable place for the controller was:



Need to fit bar ends of some sort:



I also used the new relay to drive a power socket on the dash, and that dash panel also needed a repair as it was cracked. I fiberglassed the backside and used epoxy glue to fill in the visible crack:



I fitted the socket on location which clears the location for the extra gauges in case I decide to install those. I actually have already inquired motorworks.co.uk for the price of full set of gauges/looms/oil pressure sender and will probably fit them as both fuel and coolant temp gauges are quite essential, imo.

What are these two rubber parts? They must belong somewhere on the bike but I have no idea where they came from.

[John Lang]

Those are the rubber mounts that fit into the front end of the under-seat tool box. BMW #51 16 1 459 064

[samih]

Thanks John, that helps and I can relax with the knowledge that they are not from bowels of rear suspension or something 

[Laitch]

Need to fit bar ends of some sort:. . .

A K75 should run so smoothly that bar end weights would be unnecessary. Plug the ends with long multi-colored streamers and be the envy of your neighborhood. 

[samih]

Still no sign of HTS-2000 but the seller has agreed to post me more. In the meantime I received some other alternative flux filled brazing rods which I just tried and not successfully. My existing propane blow lamp type of device is not hot enough to heat the tank sufficiently hot for the repair to be possible. The results was just blobs of metal that do not stick on the tank.

Ordered a MAPP torch and gas from a local supplier. More to follow when the stuff arrives. Oh yes and also ahead of myself and ordered the fuel and temperature gauges, cabling and temp sensor.

[samih]

Ok, the long awaited HTS-2000 brazing rods as well as the MAPP torch arrived yesterday. Smashing!

This was the starting point after I had opened up previous weak points near the bottom seam of the tank but before actually properly sanding it:



The after image looks much better already. Thanks to the fan I had had running to drive away the petrol/fuel/gas residue there were no dramas about explosions or fire.



They key here I found was to use as much heat as possible and have the area being worked as clean as possible newly filed to remove the corrosion and other crud. I also found out that that if I allowed the molten HTS-2000 to flow and settle on the area being worked at the repair worked out much better. Here is the main damaged area without any post processing. If you look carefully there is another patched hole bit further back the seam which I somewhat ground down to see how it looks. I don't think I'll bother doing it here as the material is real hard compared to the rest of the tank and who cares if the finish is not totally factory as long as as it's strong. This won't be a show bike or anything!



The repaired area looks really solid looking from inside. I was hoping not to have to strip the paint out fully but it seems I'll need to do that as I found many layers of paint and the finish got quite badly scorched at places during the repair. So paint stripper treatment next, then look for other possible weak points (looking at you dark spots towards left on the photo) before it's time to basecoat outside and cleanup + epoxy inside the tank, and finally prepare it for the actual paint and clearcoat. Perhaps this weekend coming...