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.

• '70 ~ '77 bikes used a single breather, so the right airbox half has a 22mm hole.
• '78 ~ '80 bikes used a dual breather, so each airbox half has a 17mm hole. One example is casting 1 253 719 (L) and 1 253 722 (R) with eight holes and thin sleeves.
• R90S bikes had eight drilled holes in the back of the airbox halves. Originally 13 72 1 259 812/814.
• Some '77-on engine configurations got 8 holes in each half, some got 12, and some got none.
• If the elbow clamped directly on it had an O-ring, with a sleeve it didn't... usually.
• In the final years, the holes may have been eliminated for noise control

Current parts are:

• 13 72 1 253 717/720 no holes /5, /6, 32mm /7 - O-ring? dual breather?
• 13 72 1 262 784/785 8 holes /7-on (should be dual breather, probably no O-ring)
• 13 72 1 265 051/052 12 holes 40mm RS/RT/S
• 13 72 1 335 690/691 8 holes O-ring R80 and R90S, single breather
• 13 72 1 335 772/773 no holes /7-on

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.

• 13 72 1 251 611/612 26mm (sleeve at carb)
• 13 72 1 251 653/654? Early 32mm (silver/gold) NLA
• 13 72 1 258 059/060 32mm (rubber sleeve at carb end)
• 13 72 1 262 787/788 40mm, steel ring
• 13 72 1 335 111/112 R65, two molded rings, steel ring
• 13 72 1 335 700 Late 32mm, steel ring.

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.

• 13 72 1 337 250 Airbox lower without SLS drilling
• 13 72 1 337 188 Airbox lower with SLS drilling

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:

• 13 72 1 264 392 upper sleeve between airbox and elbow (where applicable), and pre-'85 40mm intake manifold.
• 13 72 1 254 200 lower elbow sleeve 26mm
• 13 72 1 257 395 lower elbow sleeve 32mm
• 13 72 1 259 820 lower elbow sleeve R90S
• 13 72 1 254 654 32mm manifold
• 13 72 1 338 360 32mm manifold 1988-on R100
• 13 72 1 338 362 40mm manifold 1988-on R100

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:

• 26mm carbs had a sleeve at the bottom the whole time.
• 32mm carbs started with no sleeve, then got a bottom sleeve.
• R90S has different bottom sleeve, and it got an additional upper sleeve, too.
• Clamps all changed from plated to stainless for '75, but nothing else changed.
  
• Starting with the 40mm carbs (and for every design that followed) BMW put the sleeve at the top.

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	52x12 clamp	silver 13 72 1 251 611/612	13 72 1 254 200 2 48x12 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	392 sleeve 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.

• In the early '70s they were galvanized with flat head screws.
• In the mid '70s they were stainless with recessed flat head screws.
• By the '90s, they were stainless with 6mm hex drive
• Current replacements are 6mm hex but are sized differently.