Post by: Al on June 01, 2020, 12:14:20 AM
Cleaned up between the hanger and the tank.
Cleaned it with acetone.
Plugged it with Permatex fuel tank epoxy sealant.
Put some fuel in the tank to ascertain that there was no leak.
All was sweetness and light until....
I tried to put the tank back on and it wouldn't seat properly. The tank pins at the back would not go down into their respective grommets on the frame. The tank just rocked from side to side.
I finally removed the tank and looked it over. I saw that one of the "cups" that engage the splined rubber cylinders that stick out perpendicular to the frame was bent up so it couldn't cup the rubber cylinder.
I gently straightened it with a vase grip with rubber sheet protecting the lip from the vise grip jaws.
Tried it again with the same result. The two pins on the back of the tank would not go into their grommets. The tank still rocked from side to side.
Called up my BMW buddy to have look. We slid a piece of cardboard between the bottom of the tank and the central frame top tube. With little bit of pressure on the tank it was apparent that the bottom of the tank is rubbing on the frame tube so that the cardboard strip was hard to slide out.
We could see the frame tube had some scuff marks from the inflation on the bottom of the tank. So we cut away a 1" wide strip of the insulation. Still rocks and pins will not seat in the grommets.
We removed the splined rubber cylinders and rolled some 1 1/8" wide rubber sheet around the steel pins projecting out of the frame. Wound them to about 3/4" diameter, thinking that this would allow the front of the tank to rise up and allow clearance betwixt the tank and frame tube thus allowing the rear to seat in the grommets. Retried the tank with the same result- still rocks and pins will not seat.
Nothing has been modified- it is all factory original.
I have had this tank off at least 6 times since I've owned it and never had a problem like this before.
I am trying to sell this bike and do not want to spend a whack of money and time to get it working right. It seems the only way to get this tank down the rear would be to cut a strip out of the bottom center of the tank and weld in a channel to allow it to clear the frame. NOT going to happen.
What the heck is going on here? Anyone come across this before?
Post by: volador on June 03, 2020, 07:54:56 PM
Post by: Al on June 03, 2020, 08:27:52 PM
Post by: Past-my-Prime on June 05, 2020, 11:34:57 AM
Sounds kind of obvious but that was my own experience with removing / reinstalling the fuel tank.
Post by: Laitch on June 05, 2020, 11:59:46 AM
It's an interesting challenge.
Post by: Al on June 05, 2020, 02:55:33 PM
I have gone over the tank very carefully. The cups engage the rubber cylinders just fine. But I can't get the rear pins within 3/16" of the rear grommets. I have removed the front rubber entirely just as a point of comparison and the rear still will not go in because the bottom of the tank hits the frame. The bottom of the tank is flat. It has never been damaged. It worked just fine when I took it off to fix the leak. It worked just fine the previous times I had removed it.
Laitch I might try your ice pack since nothing else presents itself as the problem. Maybe I should post a picture of the front, rear and bottom of the tank to see if some eagle eyed Brick Boy sees something I don't. "An interesting challenge" isn't what comes to my mind. A PIA comes closer to my thinking. After hemming and hawing for the last few years I finally decide to sell it and now I can't because some weird juju has happened to it. It will be worth nothing if the tank doesn't mount properly. The bike only has 78,000 miles on it. If my health and leg strength and bank account would allow I'd be keeping it but it has to go, sad as that makes me. I want it gone.
Post by: The Mighty Gryphon on June 05, 2020, 03:30:17 PM
On occasion I have had trouble with getting the pins in the grommets on my RT, the usual cause is a connector(one of those big ones for the combination switches) not settling down into that huge mass of wire around the frame backbone.
I've also had to reposition that big rubber wishbone shaped pad that runs from the headstock down the two side rails of the frame. That thing can prevent the tank moving forward as well as down. Truth be told, I have no idea what it's for other than to prevent airflow under the tank. As long as you're selling the bike, maybe you can remove it and just include it with the bike.
No matter what, when the tank is finally in place, there isn't any spare clearance under it.
Post by: Al on June 05, 2020, 06:03:10 PM
Yeah, I put some wire ties loosely around the the under seat connector bundles. I can see where the tank is scuffing the frame tube. I'll see if I can post some pix tonight.
Maybe I'll try and remove the wishbone and see if that makes any difference. I doubt it but I'll try. Got nothing to lose at this point. The tank goes into the general position no problem it is the pins at the back that cannot be lowered into the grommets 'cause the steenking frame hit the tank. Or is the steenking tank that hits the frame?
Post by: Martin on June 05, 2020, 06:21:56 PM
Regards Martin.
Post by: champ7fc on June 05, 2020, 08:27:37 PM
Post by: Al on June 05, 2020, 11:11:22 PM
champ, It has never been pressurized. The bottom of the tank is flat and not bulged out at all that I can see. I didn't pressurize it since the leak was quite obvious. Thank you all for the ideas.
Post by: The Mighty Gryphon on June 05, 2020, 11:26:20 PM
Post by: Al on June 06, 2020, 09:37:47 AM
I'll be out turning money into noise this morning with my 6.5 Swede so it won't happen until later.
Post by: Laitch on June 06, 2020, 10:24:02 AM
Still, the icing of the tank floor could be fun—dry ice especially. :laughing4-giggles:
Post by: The Mighty Gryphon on June 06, 2020, 04:20:00 PM
Post by: volador on June 07, 2020, 11:40:28 AM
'how much do you want for the moto'?
Post by: Al on June 07, 2020, 11:44:03 AM
Post by: volador on June 07, 2020, 11:53:03 AM
Post by: Al on June 07, 2020, 11:55:46 AM
This tank issue has never been a problem before now.
New York is along way from Manitoba.
Post by: volador on June 07, 2020, 12:05:03 PM
Post by: Al on June 07, 2020, 01:42:00 PM
Post by: Al on June 07, 2020, 03:41:53 PM
Fuel tank Front Cups:
[ Invalid Attachment ]
Fuel Tank Underside:
[ Invalid Attachment ]
Front Rubber Cylinders:
[ Invalid Attachment ]
Frame Scuffing:
[ Invalid Attachment ]
Post by: The Mighty Gryphon on June 07, 2020, 04:26:07 PM
It's pretty obvious to me that the tank is being held up somewhere and you haven't a clue where or what it is. The only thing I can suggest is putting a piece of paper a couple inches wide and 11 inches long between the tank and the frame and trying to pull it out while the tank is putting just a little downward pressure on it. You are using it as a feeler gauge to see where the tank is hitting whatever the F is holding it up. Once you know what the problem is you can address it.
Post by: Al on June 07, 2020, 04:31:44 PM
The tank rocks a bit side to side which also tells me it is contacting the frame and that's what allows it to rock sideways.
I have felt underneath the front and can feel the rubber cylinders go into the cups. I guess I can pull the Rubber wishbone out but I don't that is the problem.
Post by: Martin on June 07, 2020, 04:45:28 PM
Regards Martin.
Post by: Al on June 07, 2020, 05:16:00 PM
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)Post by: Laitch on June 07, 2020, 05:52:30 PM
Post by: Al on June 07, 2020, 05:56:13 PM
Post by: Laitch on June 07, 2020, 06:25:04 PM
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. 112350
Post by: Martin on June 07, 2020, 06:28:33 PM
Regards Martin.
Post by: Al on June 07, 2020, 08:36:57 PM
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?
Post by: Martin on June 07, 2020, 09:34:05 PM
Regards Martin.
Post by: The Mighty Gryphon on June 07, 2020, 10:25:29 PM
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.
Post by: Al on June 08, 2020, 12:24:16 AM
Post by: volador on June 08, 2020, 01:06:29 AM
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.
Post by: Al on June 08, 2020, 01:10:05 AM
Post by: Laitch on June 08, 2020, 06:27:02 AM
Post by: Al on June 08, 2020, 09:28:14 AM
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.
Post by: Al on June 08, 2020, 02:22:53 PM
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?
Post by: volador on June 08, 2020, 07:01:41 PM
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 (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)
(https://i.ebayimg.com/images/g/SNMAAOSwu0Fe1bbH/s-l1600.jpg)
Post by: volador on June 08, 2020, 07:25:48 PM
(http://www.motobrick.com/gallery/2/5332-010320112521-2764652.jpeg)
Post by: Al on June 08, 2020, 08:47:14 PM
Post by: Al on June 08, 2020, 08:49:08 PM
Post by: Al on June 08, 2020, 11:03:27 PM
"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.
Post by: The Mighty Gryphon on June 08, 2020, 11:24:18 PM
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.
Post by: Al on June 08, 2020, 11:35:32 PM
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?
Post by: Laitch on June 09, 2020, 08:20:05 AM
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 :laughing4-giggles: and thankfully for both of us, I'm too far away to pop in for a look-see.
Post by: Al on June 09, 2020, 11:54:41 AM
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?
Post by: Laitch on June 09, 2020, 03:19:00 PM
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. :laughing4-giggles:
Post by: Al on June 09, 2020, 04:20:20 PM
Post by: Al on June 09, 2020, 04:27:23 PM
Maybe I'll go full redneck and just bungee strap a 5 gallon gas can to it.
Post by: Laitch on June 09, 2020, 04:38:54 PM
Post by: Al on June 09, 2020, 11:13:12 PM
Post by: volador on June 10, 2020, 02:39:19 PM
If searching for a new used tank I recently purchased parts from the Old Motorcycle Shop in Calgary AB
Post by: Al on June 10, 2020, 10:52:51 PM
I guess the time has come.
It still seems weird that after all these years the problem rears its head now with no way of overcoming it. But the system is pretty simple.
Post by: Laitch on June 11, 2020, 11:15:04 AM
It still seems weird that after all these years the problem rears its head now with no way of overcoming it. But the system is pretty simple.:thisplacewhack