Why doesn't this airbox fit my R90/6?

[CATPart]

I have a 1976 R90/6 (produced 12/75). I bought an R90s airbox from a guy that said it would fit. It fits on the left side of the engine but not on the right side as seen in the pictures. When I was searching for an airbox online, lots of people offered me airboxes from potentially undocumented sources, so I didn't trust that they would fit. Do I need to find an airbox from specifically an R90/6 or will some others fit correctly?
Thank You,
David

[schrader7032]

In checking the microfiche, the R90S part number is different than the /6 or /5 bikes. All /5 and /6 bikes have the same part number - 13 72 1 253 717 and 720. The R90S numbers are 13 72 1 335 690 and 691. Interesting, the /7 part numbers are the same as the /6, but includes two more sets, none of which are the R90S.

Not knowing the differences in the parts, the difference in part numbers suggests the R90S version won't work as your pictures suggest.

[CATPart]

anybody need an R90S airbox? thanks for the help Kurt. These castings have 1 253 719 printed on the inside of the left side, and 1 253 722 on the right half. These numbers don't match your research, very odd. I wonder if a matched pair of castings would have the same numbers instead of different numbers like mine.

[schrader7032]

The casting numbers are just that...casting numbers which only meant something to the people making the molds and using them. Sometimes they're close to the part number but not always.

[CATPart]

I had some emails with the guy that sold it to me. He assures me I am installing it wrong. Well, he doesn't know me, so i can only assure him that it is installed the only way it can be. I am going to take it off this weekend and take some measurements and post them here. Maybe somebody can confirm for me that there is a size issue with either my airbox or my transmission. Oh, Kurt..it dawned on me why the r90s part numbers may be different, since the r90s airboxes had holes in the back of the airbox for better air flow.
david

[schrader7032]

David -

I have heard of this modification to the airboxes, holed drilled in the backside. This was a factory mod on the '77-'78 S/RS models and bikes with the 40mm carbs...possibly it could have been done to the R90S as well, but by a previous owner, not the factory. From what I've heard, about all this mod did was increase the noise of the engine running and allowed more dirty air into the filter area. It might have helped the top end performance of the bike, but few really run their bikes to these levels.

[schrader7032]

David -

I don't think it will matter much based upon the lack of fit you're showing in your pics, but I presume you have all the hardware to install these. It's not much...there's a flat metal "spring" that is bolted to the top of the transmission...the bottoms of each half of the clamshell slip under that. On the right side, there's a nut/washer that fixes that one in place. Of course, the rubber hose from the breather goes out the right side into the carb. The only other thing is the long bolt that comes in from the left side and screws into the right side to pull them together.

Another question...you alluded to having the right transmission. In 1981, BMW changed the flywheel to something they called a clutch-carrier. In addition, the input shaft of the transmission was changed. Generally, '81-on trannies aren't a direct swap into engines prior to '81 without making a bunch of other changes. Do you still have the big heavy flywheel? The R90/6 tranny would also likely not have any ribbing on it...probably beginning in '79 or so, they began to add external ribbing to the bottom area of the transmission.

[gapl1953]

My 1975 R90/6 has the same mismatch, and as far as I can tell it came from the factory that way!
Greg