odd tank fitting issue.

[jjwithers]

I recently picked up a R75/5 that I plan to restore. I think it is a '71.

None of the tanks i have fit it properly. The frame shows zero signs of being damaged or bent.

The tanks line up with the upper front bumper, and 2 rear rubber screws.

However, above the airbox, behind the 2 rubber screw mounts, both sides of the tank scrape the "Y" part of the frame.
Typically there is about 1/4" on either side between the tank and the frame tubing back there. but not this bike.

Were there inconsistencies of frames back then?

Any proposed solutions? The frame will eventually get blasted and powder coated so I am open to any ideas.

Thanks in advance

-Josh

[Deleted User 62]

As you mount the tank, start with it tail down and about 6" forward and guide/rotate it into position. Note: it helps to remove the damper knob first, so it's not in the way. I have a couple tanks like that, large and small. I think they probably get squeezed or bumped narrower over time. Try a tight one on another frame to check for variation, or you could spread the tank.

[jjwithers]

It definitely isn't a question of mounting technique. I have a large and toaster tank and both of them must scrape the frame in order to mount on those two screws.
I have a R90/6 frame and the tanks mount on it as expected.
I'm thinking that spreading the tank might be the safest option. I'd rather not compromise the structure of the frame in any way although I thought about various options of heating those tubes.

[Duane Ausherman]

Many tanks scrape the frame, when new, at the "Y" and so we didn't get alarmed. I really don't recommend trying to spread the tank.

[Ken in Oklahoma]

. . . I'm thinking that spreading the tank might be the safest option. I'd rather not compromise the structure of the frame in any way although I thought about various options of heating those tubes.

Spreading the tank doesn't sound very safe to me. (Though I've never tried it.) If you think about it I think you would agree that to spread the tank would require the "in-side" skin of the tank to stretch or the "out-side" skin to compress. I think it would be very unlikely you even could stretch, say, 10" length of skin, let alone compress the same or longer length on the outside. I envision the outer surface of the tank buckling (easier than stretching) were you to make the attempt.

And how would you go about it? Some kind of hydraulic spreader?

The question in my mind devolves to whether you can live with it on your existing tank? Failing that I'm guessing you would need to find and buy a suitable "as is" /5 tank that happens to be a bit wider.

If it will help I'm thinking the bright steel marks on the frame and the tank inner surface can be thought of as period correct character. And the airhead originality police will give you respectful nod for keeping your bike correct.

How'd I do?

Ken, period correct in Oklahoma (My outer skin has naturally stretched as part of the aging process.)

Editing: I see Duane beat me to the same advice. I like to think I gave mine with better style though.

[jjwithers]

I pulled tank #3 off the shelf and to my surprise, it fits with about 1/8" clearance on either side. Luckily I have a few tanks at my disposal.

Given the fact that the other tanks fit perfectly on the other frames I have, I am guessing that there are both inconsistencies in the frames and the tanks. Or perhaps they were modifications made throughout the years?

This is a new one for me that definitely has me scratching my head...

[Jean]

I vote for differences in the both the frames AND the tanks, especially based on the time of manufacture.
If metal-on-metal troubles you, put a little pad of old innertube where the metal touches. OR leather.
Leather would be FANCY!

[Ken in Oklahoma]

I vote for differences in the both the frames AND the tanks, especially based on the time of manufacture. . .

I'm in alignment with Jean's view. Back in the day (as well as nowadays) manufacturers had to be very concerned about production costs, that is unless you were going for the top end niche. I'm thinking here of names like Brough Superior or Vincent, which were unaffordable for all but the well heeled customer. I reckon that back in the day BMW's targeted customers where focused on build quality, reliability, and snob appeal. But focused doesn't mean demanding the finest possible design, materials, and build quality. In today's market no motorcycles come to mind,but the very high end limited sports cars costing hundreds of thousands or even millions of dollars do. That would include the cars highlighted on this website (the first one that popped up on my google search).

http://editorial.autos.msn.com/slidesho ... =1100714#9

If our airheads (and other BMW motorcycles) had been built to this high of a quality level, well here we wouldn't be talking on this forum. But those few heirs and heiresses who did buy the uber airheads would would hear a very satisfying snick as the tank moved smoothly into place on the frame.


Ken

[Duane Ausherman]

I watched them build the frames and then straighten all of them to spec. They were concerned with the head angle, motor mounts and the box section to mount up the swing arm. There were considerable variations in the space between the frame down tubes and the engine case.

Just suck it up and get used to it. We had to, so now after 40 years it is your turn:-) Not all of them would track in a straight line.

[khittner1]

Hmm---I guess that there's more "legend" to the "legendary motorcycles" than I realized. Or maybe a bit more "craft" in the "craftsmanship".