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Airhead air intake

Airhead air intake systems

The intake elbows and rubber sleeves used to connect the carburators can be a bit confusing, especially since the microfiche diagrams are not accurate for all models. Here's what the different parts are and how they are used.

The systems broadly break down into two types: the '70s bikes with the split "clamshell" airbox and the later ones with the square airbox. The split airbox was used from the /5 through the /7s and uses a cylindrical air filter. Because the outlets have to come from the center of the transversely mounted filter they cannot be staggered to match the cylinders, so the elbows are different from left to right. The square airbox was used from 1980 onward, and allows the outlets to be staggered to match the cylinders. Therefore, the elbows are the same left and right.

Split airbox, 1970~1980

There were several types of this airbox used in production, depending on whether the intake elbows used rubber sleeves at the top (no O-ring) or at the bottom (O-ring), or had holes for the single breather hose or dual breather, or had "fast holes" drilled in the back to let in more air. Note that some of these details are glossed over in the current parts fiche. It's more useful to know what came on what bikes than it is to try to reconcile that within the current replacement parts system.

This airbox system is where the vast majority of the confusion originates.

O-ring outlet
Smooth airbox outlet
Single and dual breather holes in right half

Current parts are:

Elbows

The elbows for the 26mm carbs had rubber sleeves connecting the bottom to the carb.

It's important to understand that the sleeve bridges the elbow and whatever it abuts (either the carb or the airbox, depending on the model). The elbow does not overlap the carb or airbox. To install, you start with the sleeve pushed fully back on the elbow, position the elbow at the end that clamps, and then slide the sleeve over and clamp to complete the conection.

The very first elbows for the 32mm carb (R75/5) used an elbow that clamped directly to both the airbox and the carb; it was the only elbow to do so. These are the silver elbows that turned gold over time.I suspect that was too difficult to assemble those early 32mm elbows without damaging the ends, because before the /5 run ended the elbows were changed to accomodate a rubber sleeve between the elbow and the carb similar to what the 26mm carb elbows had. At the top, all of these 26mm and 32mm elbows clamped directly onto the airbox outlet. R90S models had a similar (or even the same) elbow as the 32mm carb, but there was a rubber sleeve added over the top connection with a larger clamp (the only model to have this).

When the 40mm carbs were introduced, the concept was changed so that the sleeve was at the top and the elbow clamped onto the carb. This may have been so that the connection at the carb did not get too fat, what with the larger carb opening and a rubber sleeve on top. Also, the O-ring was eliminated from the airbox port, allowing the outlet to have a larger ID. THis would have allowed better air flow for the larger carb, since the nominal size of the airbox outlet did not change. This top-sleeve system was continued with the subsequent R65 sleeves, and again with a redesigned 32mm elbow when the square airbox was introduced.

Where elbows are clamped by rubber sleeves there is usually a steel reinforcing ring inside the end of the elbow. Currently sold 26mm and 059/060 32mm elbows do not come with this ring, and it is not available separately. New R65, late 32mm, and 40mm elbows do have it.

Square airbox, 1980-on

This uses a flat, horizontal filter which allows the outlets to be staggered to match the cylinders. There is only one breather system in use, but the airbox was available with and without SLS (Secondary Air) which was an emissions control scheme.

By this point BMW had standardized on the top sleeve configuration, and the previous left side geometry was now in place on both sides. Therefore, the left R65 and the 40mm elbows were used on both sides, and a new 32mm elbow was designed to be used with the sleeve on top. There were no more 26mm carbs in use, so that elbow didn't need to be updated.

Sleeves and manifolds

The rubber sleeves used (including the manifolds between the carb and the cyinder head) are:

392 Upper elbow sleeve
50mm ID x 30 x 4
362 40mm manifold
49mm ID x 30 x 6
360 32mm manifold
42mm ID x 29 x 6
654 32mm manifold
41mm ID x 30 x 5
395 32mm lower elbow (stepped)
48/52mm ID x 23 x 4/6

Yeah whatever, how do I put MY bike together?

Although it seems like there are a bewidering and nonsensical number of configurations, they are the result of a fairly logical progression of design throughout the '70s:

As a result, the sleeves and elbows go together like this, roughly in chronological order:

  Model upper elbow elbow lower elbow manifold

s
p
l
i
t

26mm 52x 12 clamp? 13 72 1 251 611/612 13 72 1 254 200
2 clamps
phenolic bushing
32mm early1 52x12 clamp silver/gold 52x12 clamp 13 72 1 254 654
2 52x12 clamps
32mm to '74 13 72 1 258 059/060 13 72 1 257 395
2 60x9 clamps
'75-on 32mm 37-57 clamp 13 72 1 257 395
2 44-64 clamps
13 72 1 254 654
2 37-57 clamps
'74 R90S 392 sleeve
60x9 clamp

13 72 1 259 820
2 60x9 clamps

plastic bushing
'75, '76 R90S

392 sleeve
44-64 clamp

13 72 1 259 820
2 44-64 clamps
40mm 13 72 1 264 392
2 44-64 clamps
13 72 1 262 787/788 clamp 13 72 1 264 392
R65 13 72 1 335 111/112 37-57 clamp 13 72 1 254 654
s
q
u
a
r
e
32mm 13 72 1 335 700 13 72 1 254 654
2 37-57 clamps
'88-on R100 32mm 13 72 1 338 3602
2 37-57 clamps
R65 13 72 1 335 111 13 72 1 254 654
2 37-57 clamps
Twinshock 40mm 13 72 1 262 787 44-64 clamp 13 72 1 264 392
247E 40mm 13 72 1 338 362

1. Before the end of /5 production this was changed to black elbows with rubber sleeves like the /6

2. A 1990 SIB recommended replacing them with the 654 if there was vibration-induced leakage.

The Norma clamps have changed over the years. Original clamps are described in the table, but current replacements are generally different.

Early 32mm
32mm through '74. 26mm is similar.
'74 R90S
'75 R90S
'77 R100RS
'84 R100RS
R100GS 32mm. R65 similar.