1990 K75RT Gas Tank Weirdness

[Al]

OK, here are some pix I took of plastic on the frame tube under the tank. As can be readily seen the plasticine is crushed right down to the steel on the frame. Hence the rocking motion from side to side.

Martin there is no insulation in that area. That is the red stripe you can see on the under tank photo. That is where I cut the insulation out to see if it would drop down into place.

Plasticine on frame tube in uncrushed state:


PLASTICINE ON FRAME TUBE UNCRUSHED.JPG (177.49 kB . 660x442 - viewed 476 times)

Plasticine on tube with crush marks right down to the steel:


PLASTICINE ON FRAME TUBE IN CRUSHED STATE.JPG (203.68 kB . 660x442 - viewed 472 times)

Close up of crushed plasticine:


PLASTICINE ON FRAME TUBE IN CRUSHED STATE.JPG (203.68 kB . 660x442 - viewed 472 times)

[Laitch]

The time for action has arrived! Please submit the video to YouTube and post the link here.

[Al]

Laitch, I'm sure that would be a good idea and even kind of fun but, to quote our Prime Minister, you are asking for more than I can give. I'm a latter-day Luddite. I thought I had hit a high water mark in technological excellence by managing to get some pictures posted OK. Maybe I can find a 10 year-old to help me with it.

[Laitch]

Laitch,  you are asking for more than I can give.

OK, just tell the tale around the campfire. How about just hooking the tank to the front mounts then sitting on it to induce its coupling into the rear grommets? Something's gotta give. 

[Martin]

Curiouser and curiouser said Alice!! Is there anybody that you can swap tanks with and/or do a comparison.
Regards Martin.

[Al]

It is raining so I can't even tell a campfire story. The wood is too wet.

Martin, I don't know of another K75 in the area let alone an RT. We are not living at the end of the earth but you can see it from here.

Anyone have an old beat up corroded tank they want to donate to science?

What is a used tank worth?

[Martin]

You need to check the parts book but I believe that besides the early and late models some K100 tanks are the same as the K75RT. I also think a lot of the differences were to do with the fairing contours and the underside should be pretty much the same. A mate has swapped tanks between a K75 RT and a K75s with the fairings removed
Regards Martin.

[The Mighty Gryphon]

Since I have a '94 K75RT, tomorrow afternoon I'll pull my tank up and take a look underneath it to see what it looks like. 

I see from the MaxBMW fiche that there are two tanks for the K75RT and the change occurred in January of 1993, so my tank will have a different part number than the one in question.  I don't know if they are the same as far as how they fit on the frame.

It's hard to tell from the photos, but the tabs that hold the bushings for the battery covers don't look the same as the ones on my tank.  Since the frame for our bikes is the same part number, I suspect the underside of the tank will be the same for both bikes as well, and whatever difference there is will be somewhere else.  Tomorrow will tell.

[Al]

Thank ya. Thank ya very much.

[volador]

Plasticine on frame tube in uncrushed state looks exactly like artificial worm I put on my hook on Friday's fishing trip

the Mighty G is on to something. As a 'S' aficionado there is a petrol tank revision sometime as was mentioned 1993. The bottom of the tank changed from a 'V' down the center (or similar) to a flat bottom newer version. The rubber bumper mounts also changed space distance reduced I believe compared to older version of petrol tank. I'm uncertain what change was made to the 'RT' as that tank is interchangeable with certain year K100's.

[Al]

Mine is the flat bottom with ribs running across from side to side. As far as I know it is the original tank from 1990. The bike is low enough mileage that I doubt it was ever replaced. But who knows?

[Laitch]

According to my understanding of what I've read in this thread, the tank on Al's moto has always fit his moto. It was secure; it didn't rock or rattle. He removed it to repair a leak and now it doesn't fit. Something Al did—or failed to do—to that tank or its frame fittings during that process is preventing it from being remounted. We already know one of the forward mounting brackets was inadvertently crushed. We don't know what other misstep hasn't been discovered but it's probably there somewhere.

[Al]

The truth is out there. Laitch you summed it up accurately. This has me completely baffled. I made no changes to the tank other than repairing a small leak under the side cover tab. Nowhere near the frame tube. It should have been a simple remove and replace scenario.

When I tried to put the fairing pocket brackets on they were rubbing on the tank and would have scraped the paint off. Never an issue before.

So I tried to jockey things around a bit to get it to slide into home play with no joy.

I finally pulled the tank off again and then noticed the bent tab. No idea when that happened. I have certainly never put enough pressure on it to bend the tab. I never had a reason to since the tank always went into place easy peasy.

Perhaps today I'll pull the rubber pole off as per Gryph's idea. It would eliminate one variable. Although , I have never had it off since I've owned the bike but what the heck. Nothing ventured- nothing gained.

[Al]

OK. I think I am making some progress. I think this bike has the wrong tank on it. Volador's comment made me look up a 1990 K75 tank. I found this picture and it would definitely solve the problem:


https://pickupmoto.com/gb/tanks/2099-fuel-tank-bmw-k100-k75.html

Notice that nice groove in the bottom that would allow it to straddle the frame tube.

Mine is flat in that area with ribs that run from side to side.

Why it never caused a problem before now, I have no idea. Maybe a PO bent that lip up and that made is sit properly or close to it. Maybe a K100 tank was put on it at some point in its previous life? I am at least the 3rd owner. I have have a Florida registration and it shows an SC code under the heading "Previous Registration" or something like that. I'm guessing SC might have been South Carolina? I bought it from a guy in Alberta who had imported it from the US.

That is why it is marked in MPH instead of Kilometres as a Canadian bike would have had. I like MPH better anyway. I'm old school.

Now the question: do I buy a new tank for a bike I don't intend to keep? What other option is there?

[volador]

Well at a $1700 discount I won't be changing my mind any time soon. I'll cut and weld a channel in the bottom of the tank for far cheaper than that.

Guess you will be retro-fitting that red tank as you predicated

I think you are right Al you have a newer version petrol tank post 1993 or early 1994. To reiterate I'm coming from the K75 'S' perspective and that flat bottom tank is definitely newer version.
The sleek royal blue tank you have linked is for the pre-1993/94 K75 'S'. All 'S' tanks have the indentation or step on the lower sidewall.

A quick search on the Ebay shows a 1993 police version 'RT' with the 'groove' or 'frame channel' down the bottom center.

https://www.ebay.com/itm/1993-BMW-K75-rt-Police-Bike-Fuel-Gas-Petrol-Tank-K100-K75-Hard-to-Find/224032758339?hash=item3429649a43:g:ETQAAOSwFbFe1bW8

[volador]

If you buy another petrol tank from a K100 make sure not a 1984-85. That version tank completely different & will not fit due to rear mounting tab.

[Al]

Yeah there seem to a fair number of variants. Ideally I'd like to have them side beside but that is unlikely. The police tank is bit spendy in Canuck Bucks. Especially when you add in shipping. Again if I was keeping the bike it might be different. I have a US shipping address since I only  live 30 miles from the border and that saves me a lot of money on shipping but the border is closed because of covid.

[Al]

I have put an add on Kijiji but who knows when that will hit pay dirt? Upside is I can also sell my tank.

[Al]

"The sleek royal blue tank you have linked is for the pre-1993/94 K75 'S'. All 'S' tanks have the indentation or step on the lower sidewall."

Right. My buddy noticed right away that it din't have the side panel mounting tabs. Just because you can get on doesn't mean it fits.

[The Mighty Gryphon]

Okay, I went out and looked at the tanks on my three Bricks.  A '91 K100RS 4V, '88 K100RS, and the '94 K75RT.

The tanks on the the K100's are the same and have the "tunnel" for the frame tube.  The RT has the flat bottom like Al's RT.  The tank on the RT fits, but does need a fair bit of force to push the pins down into the bushings far enough to get the clips on them.

I then went to the parts fiches at MaxBMW.  Turns out the frame has the same part number for all K75 models so as far as the top tube, either version of the K75 tank should work.

As mentioned, there are two different tanks that fit the K75RT.  The one with the flat bottom Like Al and I have appears to be p/n 1 611 2 307 468.  It is also used on the K1, K11LT, and the K11RS.

The earlier K75RT tank with the tunnel looks to be p/n 1 611 1 455 101.  That tank is also fits the K1, K1100RS 4V, K11LT, K100, K100RT, K100LT, K100RS 2V.   

It certainly looks like there is a lot of interchangeability between tanks and bikes.  In looking at my bikes, and with the information from the parts fiches I can't find any reason why Al's tank hits the frame tube and won't go into the bushings.  I'm stumped.

[Al]

Gryph, in my searching i came across another tank- part # 161 100 26098 Can't guarantee it is an RT tank but I'm pretty sure it is. I stand to be corrected.

I can't even get the rear pins to enter into the grommets with a huge amount of force. To push them in far enough to get the clips on the tank would be distorted- and that's with the strip of insulation removed.

Anyone have a tunelled tank for sale?

[Laitch]

  In looking at my bikes, and with the information from the parts fiches I can't find any reason why Al's tank hits the frame tube and won't go into the bushings.  I'm stumped.

I'm stumped to understand how its rear pins were able to be mounted before all this happened. The frame's number in Max's fiche seems to be the same for all those RT years. The front of the tank would have needed to be elevated enough for the rear pins to fit through the rear grommets. If the front of the tank wasn't secure, there would likely have been rattling. I would be obsessed about getting it to fit again but Al seems to have transcended that obsession  and thankfully for both of us, I'm too far away to pop in for a look-see.

[Al]

Laitch, I have pondered many an hour about getting it to fit. I have had other gearheads look at it. I am at my wits end trying to understand how it fit up until now. The bottom line is that at this point it will not go down. Raising the front of the tank with the rubber cylinders removed does not allow it to  into place at the rear. The frame is hitting at the back of the tank no matter how high I raise the front.

I really want the tank to fit as it but I see no way to make that happen apart from big hammers and kilotons of force.

In order to put enough pressure on the tank to make it seat properly will essentially make a crude tunnel in the tank anyway and likely distort the tank in some way. Why BMW made this change is beyond my ken. I can assure you that the tunnel tanks would NOT have this problem. So much for da fine Churman Enchineerink. Perhaps it was a cost saving measure by the bean counters to reduce the number of parts being made. But still- how they got it to work at all mystifies me.

Gryph, did you try a piece of paper under your tank? I am wondering how much clearance your bikes have?

[Laitch]

Laitch, I have pondered many an hour about getting it to fit.

I'm not doubting that. What is odd is that the tank has stayed put since 2017 but won't stay put no more and you can't make it. 

[Al]

Am I hearing, "Na, na, na, na, na" from you?